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PRACTICALRADIO KINKS

AND

SHORT CUTSA Compendium of Practical,

Time -saving Methods for the

Constructicii, Operation and

Repair of Radio Receivers

By B. BAKER BRYANT

pLIE31-11SHED BYC R F T PUBLICATIONS

99 HUDSON St-NEWYoR

UM

L

1,800PAGES

Servicing Radios

is Made Easier

with the New

GernsbackManual

RADIO'S newest of service manuals-published by theHouse of Gernsback. VOLUME 7 OFFICIAL RA-

DIO SERVICE MANUAL contains many featuresnever previously published in any service guide. Thisedition is the most outstanding GERNSBACK Manualpublished since 1931.

Features of Volume 7 Official Radio Service Manual Over 1,3000 Pages Over 3,000 Illustrations Over 1,600 Sets Represented Complete Service Data

Includes 1938 Models Not Found in Other ManualsAll New Material-No Reprint or Rehash

Operating Voltages for Over 85% of the Sets Intermediate Frequencies of All Superheterodynes

Full Alignment Procedure for Over 75% of theSuperheterodyne Receivers Listed

Full Cathode -Ray Oscilloscope I. F. Alignment Pro-cedure for Many of the Complicated Superhetero-dyne Receivers

Simple Mechanism Permits Pages to Be Easily Re-moved and Re-inserted

Stiff, looseleaf covers-size, 9 x 12 inches A Master Index of 74 Pages Includes Sets Listed

in All the Gernsback Manuals Since the Publica-tion of Volume I in 1931.

Send remittance of $10.00 by check or money orderfor your copy of VOLUME 7 GERNSBACK OF-FICIAL RADIO SERVICE MANUAL. If yousend cash, or unused U. S. Postage stamps, registeryour letter. YOUR MANUAL IS SENT TO YOU

POSTPAID.

RADCRAFT PUBLICATIONS, INC.99 -PR HUDSON ST. NEW YORK, N. Y.

PRACTICAL

RADIO KINKSAND SHORT CUTS

The contents of this book have been planned with the

view to help Radio Dealers, Servicemen, Experimenters

and Set Builders overcome many of the common, simple

obstacles found in everyday radio work.

By B. BAKER BRYANT

RADCRAFT PUBLICATIONS, Inc.

PUBLISHERS

99 HUDSON STREET NEW YORK. N. Y.

Copyright 1939 by Hugo Gernsback Printed in U. S. A.

Contents ofPRACTICAL RADIO KINKS

AND SHORT CUTS

Pages

Chapter I. Introduction 3

Chapter II. Servicing Short Cuts 4

Chapter III. Testing Methods 13

Chapter IV. Testing Equipment and Meters 17

Chapter V. Vacuum Tubes and Circuits 33

Chapter VI. Volume Control Methods 36

Chapter VII. Amplifiers and Phonograph Reproducers 38

Chapter VIII. Power Supply Equipment 39

Chapter IX. Coils and Tuning Circuits 46

Chapter X. Short Waves 53

Chapter XI. Loudspeakers and Phonograph Pickup, 56

Chapter XII. Tools and Accessories 59

PRACTICAL RADIO KINKS AND SHORT OUTS :1

CHAPTER I

introduction

)VERY

Service Man and radio experimenter has his own"pet" way of accomplishing certain details of set repairor construction, whether it is the way in which he hangs

his soldering iron on the bench or some individual way of test-ting a circuit or piece of apparatus. Many of these "Kinks"have a distinct advantage over the accepted ways and natur-ally will interest the serious radio man.

It is the purpose of this book to present some of the mostuseful of these "Kinks" and Short Cuts in a form that willbe of interest to everyone. Naturally, we can all learn froththe experience of others and even the most clever technicianwill profit by a careful study of this little volume.

The financial return, in servicing radio equipment to-day, depends to a great extent on the speed with which theService Man eliminates the trouble and places the receiveror equipment in correct running order. The man who canhandle the greatest number of service calls a day is the onewho will gain most, either directly in service fees or in-directly in increased business. A number of the "Kinks"are directly applicable to service work and may be instru-mental in reducing the time required to locate an obscuredifficulty in some set.

We all know the peculiarities of short-wave sets andthe fact that some work much better than others for noapparent reason. The Short -Wave Fan will note that hehas not been slighted as a generous number of short-wavehints will be found in the chapter on this subject.

In compiling the volume, the editor found that a largepercentage of the available material could be classified undergeneral headings such as Servicing Short Cuts, TestingEquipment, Vacuum Tubes, Speakers, etc. However, someof the "Kinks" were rather stubborn and refused to be un-tangled into the headings selected. It was necessary to placethese unruly items in the topic most nearly associated withthem. The headings in the Contents can, therefore, only beused to supply a general idea of the details included.

CHAPTER II

SERVICING SHORT CUTS

A PILOT -LAMP ADAPTERBy Audie Robertson

FOR the Service Man, the deviceshown in Fig. 1 is very convenientwhen working in the dark corners ofthe set. A miniature socket, to fit adial -light bulb, and a burnt -out bulb,with two flexible insulated wires, arethe only requisites.

F G. I-The dial -light is converted Into a trouble-shooting lamp by the use of this simple adapter.

Take the pilot lamp out of the set,put it in the socket of the extensionlead; and screw the plug of the latterinto the pilot lamp's socket. The lampcontinues to operate from the set's cur-rent. Two -foot leads are usuallyenough.

ILLUMINATED VERNIER DIALBy Louis B. Sklar

A HIGHLY efficient vernier illumi-nated dial can be made from an RCAvernier drive, Part No. 6675. The com-plete assembly of parts is illustratedin Fig. 2.

Secure a piece of light -gauge cellu-loid, and shape it as shown in Fig. 2A.Make a mark with a needle or othersharp -pointed instrument in the centerof the celluloid strip to form a "hairline." Glue strip to the outer shaft ofthe RCA drive as indicated. With a jig-saw or other suitable tool make a semi-circular slot in panel. The dial mark-

ings are drawn on transparent paper ortracing cloth and pasted on the backof panel. The dial markings can bedrawn 0 to 100 or in kc. if you know theexact position of the variable conden-ser for the various frequencies.

CELLULOID .NDILATOR GtuEo ONSHAFT

DIALEIGHT

SOCKETSOICKRIO

ToSHAFT

RCA DRIVE

SEMICIRCULARGROOVE IN PANEL

TRANSPARENTPAPER DIALIN RACE or

PANEL

METALGLUE

- SIDEVIEW -

CELLULOIDINDICATOR

FRONTslew -

FIG. 2-Sketch of the dial shown In Fig. 2 A.

FIG. 2A-Homemade illuminated vernier dial

A miniature base socket, radio type,can be soldered on at the shaft shownso that the light will always be directlyin back of the "hair line" indicator.

This vernier dial can be built forabout 25c, yet, it will surpass in serviceand smoothness of operation, dials cost-ing about 6 times that amount.

PRACTICAL RADIO KINKS AND SHORT CUTS 5

HOME-MADE SET DIALSBy D. P. Hartley

IN MAKING dials for receivers, me-ters, etc., the idea shown in Fig. 3 willenable you to have a neater job thancan be secured in the ordinary way.

DRAWDIALON

BRISTOL

BOARDWITHINDIAINK

PI HAVE NEGATIVE MADE TO DESIREDSIZE AND PRINT ON HEAVY

PORTRAIT PAPER II

1114., ..011111 _

FIG. 3-Make your own all -wave dial to suit.

Make a master dial, calibrating asdesired (this may be merely a sketch),then enlarge to scale, on Bristol board,to 12 ins. or more, and draw as care-fully as possible in India ink. Takethe drawing to a photographer, havinghim make a negative; and then prints,with the portrait camera, to the sizeyou want, on heavy paper. A sampledial I made this way for an all -wavereceiver is illustrated; the original was101/2 ins. in dia.

(A simpler scheme than using a pho-tograph would be to get a "glossy pho-tostat."-Editor)

ELIMINATING HUM FROM

NEARBY LIGHT LINESBy J. H. Mills

\V I -I EN your antenna must be placedparallel and close to high-tension lines,a great deal of hum is picked up by it.

To eliminate the biggest proportionof this hum, put up a two -wire antenna,as shown in the drawing above. SeeFig. 5.

FIG. 5-"Phasing" an antenna, to reduce the affectsof Interference pick-up.

The wires cross in the center of thespan and they must not come in con-tact with each other.

The principle on which this aerialworks is as follows: The pick-up ofthe interfering current is equal in eachend of the wire and as the positions ofthe wires are reversed at the centralpoint, the current picked up in one ofthe wires opposes that picked up inthe other.

A HUM KILLERBy George W. Brown, A. I. R. E.MANY A.C. sets hum even though

the filter system is quite efficient. I

have found that, in sets using push-pullaudio stages, a 100,000 -ohm resistor(R1) connected across the secondaryof the input transformer will reducethe hum considerably. In extreme cases,another 100,000 -ohm resistor (R2) maybe connected across the secondary ofthe first audio transformer. This sec-ond resistor may make a very slightchange in the volume; but it will cer-tainly kill whatever hum may be left.See Fig. 4.

Rz

Tot100,000 ,00,000OHMS OHMS 8+

cc

4

us

FIG. 4-These resistors give a solution of the per-sistent hum problem.

6 PRACTICAL RADIO KINKS AND SHORT CUTS

CAR -RADIO IMPROVEMENTSBy John E. Mason

A CONSIDERABLE improvementin performance of car -radio sets maybe had by retuning the antenna trim-mer after the set has been installedand connected to the car antenna.

In some cases it is also necessary toslightly retune the first -detector trim-mer.

CLOSED-CIRCUIT TIP -JACKBy Charles Horvath, Jr.

THIS handy jack may be made froman ordinary tip -jack. All the neededinstructions are given in Fig. 6. Whenthe phone tip is inserted, the two

SONNECTICP4MAO( ..EAC

ConnieTwee

OP1 WI

00010wee SyrIONS L

\,......... F... ONININAATAIG wASNENANO SENO AS SNONTA

TIP JACK

INSmATINSWAAINCAS

COPPIA SIAMEye'SOSPINIED Nett

ANTIS WNW IS MNr10 SNAP! SPAS AO SOLNA

FIG. 6. above-Re-making a tip jack

lengthened prongs spread apar andopen the circuit with the heavy wirethat is to be added. An ideal jack foruse in test sets and many other appli-cations.

PLUGS AND JACKS IN THE LAB.By Joseph Riley

SINCE the convenience and adapt-ability of plugs and jacks in the labora-tory are not known to every experi-menter, the writer ventures to call at-tention to the numerous arrangementsillustrated in Fig 7A. Although notnew (having appeared originally in anissue of the "General Radio Experi-menter") it is believed they are ex-cellent examples of the versatility ofplugs and jacks.

The following lines are quoted fromthis interesting number of the "Experi-menter": "In the laboratory the spaceprovided under the bench too often he -

FIG. 7A-A number of plug and jack arrangementsthat facilitate experimental work. Clean-cut appara-tus like that illustrated above makes experimenting

easy.

comes the resting place for discardedbread boards, a procedure which soonproves both unsightly and uneconomic-al. A satisfactory solution is a universalbreadboard provided with the neces-sary jacks for plugging in various cir-cuit elements. This sort of device isillustrated in Fig. 7B.

FIG. 78-A laboratory breadboard, illustrating on.of the many uses for which plugs and jacks may be

used.

"To meet this need for a flexible sys-tem of connection, G. R. units havebeen designed around the Type 274plugs and jacks. A spacing of 44 -in.has been adopted.

"This manner of arranging test equip-ment makes it extremely convenient tomake comparisons between units of agiven type, such as audio transformers;for that matter, even systems of opera-tion may he compared, such as the rela-tive performance of transformer or re-sistance audio -amplification, providedthe voltages, etc., are correctly bal-anced."


SCREEN GRID CLIPBy Fred Erdos

THIS "kink" used by the writer mayhe of interest to the readers of RadioKinks. Fig. 8A shows the adaptation ofa Falmestock clip to fit the control -

FIG. 8-A clip for this control -grid cap of a screengrid tube is quickly and effectively made as shown.

grid cap connection on the top of ascreen -grid tube. At 8B we have theclip before the little middle springhas been broken off and a larger, out-er one, bent to fit

AN EFFECTIVE INTERFERENCEELIMINATOR

By Ernest V. AmyMOST Service Men are familiar with

the use of a choke coil for the sup-pression of artificial "static" radiations.However, it is generally believed thata successful unit must he purchased.The construction and application of theunit pictured in these columns will ex-plode that fallacy.

In practically every instance of in-terference from motors it is usual toapply a palliative at the point wherethe motor line connects to the powerline. This materially reduces interfer-ence conduction into the light lines;but, it does not prevent interference

FIG. 9A-An effective "static" radiation reducer,for use on sparking devices, wired as described. Itis quickly and inexpensively constructed and easilyapplied: and will give relief from the "man-madestatic" generated by so many installations. The

closer It Is to the sparking device, the better.FIG. 911-"Skeleton" and schematic circuit of the

radio -frequency choke coll.

radiation from the current lead betweensocket and motor.

The design of this device is based onthe fact that all motors of any real sizehave "fused switches" close to them.

To install this air -core choke. (Fig.9C) a fuse is removed from this switchbox, the choke inserted, and the fusescrewed into the choke. The oppositeside of the line is tried; one in eachside may be necessary-in extremecases.

Fig. 9A illustrates the series circuitso formed. This schematically indi-cated in Fig. 9B.

A brass shell is ar-ranged in one end ofa fiber tube. This isthe receptacle shell,R.S. for the fuse. Inthe opposite end isfastened the plug -in -shell P.S. On the tubeare two windings;each of which con-sists of 140 turns ofNo. 18 D.C.C. wire,plain layer -wound.They are spaced asshown to reduceself -capacity whichwould act as a high -frequency by-passand nullify the reac-tive effect of thechoke. FIG. 9C

CONVENIENT WORKBENCHLIGHT

By Boris S. NaimarkTIIIS handy extension light for theworkbench takes the place of severallamps permanently installed; it is avail-able at any point of the work -benchand can be shifted from point to pointwith ease; and it requires nothing, thatcan not be found in the junk -box, forits installation.

Stretch a length of steel or "stove-pipe" wire from one end of the benchto the other, preferably over the centerof the bench and approximately threefeet above it. Slip over it a piece ofinsulating tubing which may be be-tween thrqe and five inches long. Se-cure the electric -light socket, as show,'


FIG. 10-The method of suspending a lamp abovea work -bench is safe and convenient to a high

degree.

in the accompanying drawing, at theheight considered most convenient,

The lamp can now be moved theentire length of the bench, and thusmakes light instantly available whereit is needed most. (Fig. 10.)

POPULAR RADIO ACCESSORIESBy J. G. Sperling

IN spite of many opportunities, theaverage Service Man does not availhimself of the possibilities of sellingvarious radio accessories in the home.

The writer has carried a few itemsin his kit for the past year, all ofwhich have proven very successful. Itis a rare home, indeed, in which atleast one of these accessories couldnot be sold.

Noise ReducerThe first of these accessories is a

noise or static reducer. As seen fromFig. 11, it consists of a neon glow -lampin series with a variable resistance.This device is connected across thevoice -coil terminals of the loud -speakerIts operation is relatively simple, it be-ing a form of an automatic volume-control. First, the manual volume-con-trol is set at some definite level. Itwill be necessary to mark this point onthe dial, for successful operation ofthis device depends upon the correctposition of this volume control. Thenthe variable resistance in the unit isadjusted till the lamp starts to flicker.

Therefore, if there are any extrane-ous noises such as static or electricalinterference it will be shunted or by-passed through this device. There willnot be any loud crackling such as pre-viously present, but only a low -pitchnoise or "plop" whenever there is alarge amount of static. Whenever thisoccurs, the neon lamp will glow.

FIG. 11. left-Circuit of the static reducer.FIG. 12. right-Hum-control circuit.

The parts used in this device are aG.E. 1 -watt neon glow lamp with asmall Edison base, and a 100,000 -ohmvariable resistor. This unit is housedin a small container and sold to thecustomer for $2.50. After a free dem-onstration on a bad night, the customerwill always buy this device.

Hum EliminatorMany of the early type as well as

some of the later model A.C. sets hada very had hum. Different methodshave been tried to combat this evil butonly one device seems to be the panaceafor all our hum troubles. This is anadaptation of the hum -bucking unit de-signed by Miessner and used by Loftin-White in their amplifier. Almost every-body knows what a success it has beenin the above units. It is simple to con -

FIG. 13, r ght-Another hum -control connection.FIG. 14. left-Tone-control unit.

PRACTICAL RADIO RINKS AND SHORT CUTS 9

struct and adjust. It promises an in-viting field of revenue for the wide-awake Service Man. As seen in Figs.12 and 13, it consists of two .5-mf. con-densers of 400 -volt rating and a 5000-

ohm variable resistor. When installedit is only necessary to turn the arm ofthe resistance to a point where no humis heard.

This hum -bucker has been used withsuccess in such sets as the Majestic"70" and "90," Temple "8-80," Victor"RE -32" and RCA "16," "18," "33" and"60." It should be connected in thelast radio -frequency stage.

Tone ControlIn spite of the great popularity of

dynamic speakers, many sets are stillfound employing the magnetic speaker.Very often the owner complains of in-sufficient bass and a superabundance oftones in the middle register. This sit-uation can be remedied by the use ofa device called the equalizer.

The constants for the trap for usewith magnetic speakers are one .1-mf.condenser, one 80 mh. choke such asSamson, and a 0 -50,000 -ohm resistor.The device is connected across tlwterminals of the magnetic speaker. Theresistor is then adjusted until a pleas-ing response is obtained. The schema-tic is shown in Fig. 14.

DEMAGNETIZING PHONE POLESBy L. B. Johnson

HEAD -PHONES that give a lowsignal and appear worn-out often maybe rejuvenated by removing the coilsfrom the phone (the magnet may beremoved instead) and connecting 5

FIG. I6-A 6 -volt supplyfor dry shaver from a

car radio.

FIG. IS--Citimagnetizing phone polies

volts of A. C. across the coils for about5 minutes. This eliminates residualmagnetism in the polepieces. Head-phones "treated" in this manner haveoutperformed previously more sensi-tive ones. Figure 15 indicates how thisis done.

ELECTRIC RAZOR POWERSUPPLY

By G. E. Weniger

MANY people who possess 110 -volt"dry shavers" would like to be ableto use them on camping trips when110 -volt electricity is not available. Asthese electric razors operate on eitherA.C. or D.C. and draw only very littlecurrent, they can be run from thepower supply of an auto -radio set andmay be easily connected as shown inthe diagram, Fig. 16. The switch dis-connects thie filaments of all the tubesexcept the rectifier (if any), thus re-lieving the power supply of the platecurrent. The resistance, R, must he

POWERSUPPLY

RAZOR

CKET

FILAMENTS

4-* (TO SET)

OP DT.SwITZ.H ALL WIRING

I CHANGESI ARE SHOWN

INSIDE 00T-TEO LINE.


of the correct value to reduce thevoltage delivered by the power sup-ply to approximately 110 volts whenthe razor is running. This value mustbe found by experimentation.

PANEL ENGRAVINGBy Erick Erickson

FOR the experimenter who has nofacilities for the regular type of en-graving, it is possible to get good re-sults at practically no cost as follows:take ordinary painter's white lead andmix white shellac with it to the properthickness. Using a sharp pointed in-trument, apply the mixture to thepanel as required. If you desire to re -engrave, simply apply some plain shel-lac and wipe the panel clean. Thistype of "engraving" gives a very goodappearance on test sets and the likeand best of all, the mixture will notcome off, even under hard use. Thehandier the builder is at engraving, thenicer the finished job will look. Itmay be necessary to experiment withthe mixture to get a consistency whichwill flow properly and yet not be toothin.

FINE RESISTOR ADJUSTMENTSBy J. E. Noble

WHEN a fine adjustment of resistorvalues was required for some experi-mental work, I made up the vernierslider, for a "Truvolt" wire -wound re-sistor, which is illustrated herewith.As may be seen, it consists of a slidermade slightly wider at its midpoint,with a distinct semi -globular indentation impressed or stamped therein

FIG. 17-This slider obtains very fine adjustments ona standard wire wound resistor for calibration

purpose.

this indentation is made to ride thethreaded channels of the resistors, al-lowing almost a micrometric adjust-ment to be made. Rough adjustmentsare first made in the regular up-and-down manner; then a turn to the rightor left does the trick.

Those who desire an easier methodof constructing the slider can simplycut two small slots in a standard slider;filing away the part which is not re-quired, and giving the remaining smallportion a slight inward bend with a pairof pliers. Be sure to file off any sharpcorners which remain, to prevent cut-ting the resistance wire when makingthe adjustment.

(See Fig. 17.)

FURNITURE SCRATCH REMOVERBy Morris Dorsey

THIS idea is a remover that willeradicate the worst scratch, yet .costsonly a few cents to make. Grind upabout a dozen pecan kernels, takenfresh from the shells and rub theminto a piece of cheese cloth. You nowhave one of the finest scratch remov-ers it is possible to make. The clothwill last for a year or more beforeneeding renewal of the pecan oil. Sim-ply rub the cloth over the scratch, letthe oil dry a few moments, and polishwith a clean, dry cloth.

MEASURING PLATE RESISTORSBy J. E. Kitchen

With the increased use of resistorsin plate circuits, it may be sometimesdesirable to determine their value,without pulling the chassis, and usingan ohmmeter.

Plug a set tester into the receiver inthe usual manner and place the tube inoperation in the tester. Note the platevoltage reading, say 130 volts, and theplate current reading, say 0.2 ma. Nowtake a lead from the "B" tap concernedand plug it into the unused socket ofthe analyzer (which is connected inparallel with the socket being used).Note the new plate voltage reading,say 180 volts. The voltage drop acrossresistor is seen to be 50 volts. As a

PRACTICAL RADIO KINKS AN!) SHORT CUTS 11

ANALYZERSOCKETS ,

TEST -

WIRE L --

F G. I8-This simple method of measuring voltageon both sides of a resistor, when the current isknown, quickly gives its ohmage. (A high -resistance

meter is needed.)

current of .0002 -ampere is flowing, theresistance will therefore be 50/.0002, or250,000 ohms.

The method shows up high -resistancejoints in the plate circuit and is quickerand slightly more accurate, than goingover the set with an ohmmeter. It alsoaffords an easy way to try new valuesof resistors. (See Fig. 18.)

MIDGET RADIO SENSITIVITYBy G. H. BAIRD

MANY of the popular 4 -tube mid-get sets with a single stage of R.F.are somewhat lacking in selectivityand sensitivity, and may be im-proved by the addition of a tuned an-tenna. The size of the coil will de-pend on the set and antenna used, butas a rule 70 T. of No. 30 D.C.C. wire ona 1% in. form will be about right whentuned by a 350 mull. condenser. Thecoupling coil consists of 10 or 15 turnsof the same size wire wound on top ofthe other coil. "A" and "G" of Fig. 19go to the antenna and ground connec-tions of the set. If the set has noground connection, the "G" post maybe connected to chassis. In Fig. 19 B,

-A-A

-s- DU T.

FIG. 19-Increasing sensitivity of midgets.

the same circuit is shown with a switchadded to cut out the tuner if desired.

Service Men should find this gad-get an excellent demonstrator to aidin showing the need for modernizingolder sets.

TIME SWITCHBy A. A. Schmitt

HERE is an idea of mine which Iwish to enter in the Short -Cut ideascontest. It is a simple, inexpensivemethod of turning on a radio set atany predetermined tune and can beassembled in a few hours. As the dia-grams herewith slow the only partsneeded are an alarm clock, a phonog-raph stop switch and a thin strip ofspring brass, bent as per drawing, itssize depending on the type of alarmclock used.

orc-ALARM -LEVER

FASTENCLOCK TO BASE BOARDWITH WASHER AND SCREW

REMOVE SCREW ANDSTOP -ARM ON

THIS LEVER.

RADIO RECEIVER

BREAK HEREAND CONNECT

SWITCHLEADS

110V PHONO-

SWITCHGRAPH STOP

ARM 'A'SWITCH

TO owla LINEAND SET

WOOD SCREWTO MOUNT THINBRASS BRACKET

SOLDER TO SHAFT -id

SWITCHARM 'A -

TOPwRLINE

INSU-LATE',STAPLE

THIN SPRING -BRASS BRACKET

BOLT SWITCH TO BRASSBRACKET ANO SOLDER.AFTER SWITCH ISADJUSTED

TO CLOCK.

RILE ENDS ALARMSQUARE WINDING

HANDLE

FIG. 20-Alarm clock and phonograph stop make apractical electrical time switch.


The operation is as follows:-Turnon your radio set and tune in the sta-tion you will want to hear next morn-ing. Then set switch arm "A" (Fig.20) in the "off" position and rest it onthe alarm clock winding key in theposition shown in Fig. 8B. Now setyour alarm clock in the usual way andforget about it all. When the alarmgoes off the alarm winding handle willunwind or turn, pushing up switch arm"A" which locks mechanically andturns on the radio receiver, lights, orwhat -have -you.

This little device can be made forthe small sum of 60c (cost of switch)provided you already have the alarmclock. The switch assembly, for ap-pearance's sake, can be enclosed in asmall box.

A NOVEL TEST LAMP UNITBy W. B. Sanford

THIS little unit is made, as Fig. 21shows, by re -assembling a standardcube -tap. In drilling out the rivetsholding the receptacle springs to theplug prongs, the hole is made largeenough to admit a fibre bushing aroundthe small screw used to reassemble theparts as shown in the self-explanatorydiagram. The front spring of A is

SMALL SCREW -REPLACES

RIVET

FRONTSPRING -4

INSULATED wASHERSNUT

(20 k

FIBREBUSHING

NUTSIDE SPRINGS

INSULATED

-A- REASSEMBLY

/AHALF OF CASE

al- REMOVED,SHOWING

B JUMPER

FIBREWASHERS

BUSHING ----171FIBRE

SMALL SCREWS REASSEMBLY

FIG. 2I -The fast -lamp adapter unit.

insulated from the rest, as are theside springs of B. The connection be-tween these two insulated springs, as inthe diagram with the cover removed,completes a unit in which the side re-ceptacles are in parallel with each otherand in series with the front. Completethe equipment by adding a plug-inadapter with screw -in socket, a plugbase, plug fuse, and test prods leadingfrom a plug.

A list of applications would entailconsiderable space, so a few are men-tioned and the rest left to the creationof the reader. A continuity tester ishad by putting the adapter with a lampinto the front receptacle and the testprods in the side, or vice -versa, withthe whole plugged into the 110 -voltservice. To use the lamp in series withthe line as a current limiting device,when working on a shorted applianceor radio set, plug radio receiver infront and lamp in side. \Vhen theshort is remedied shunt the lamp byjoining the prods on the other side orreplace the lamp with a fuse. Discon-nected from the "110," with the lampand prods in the sides, it can be usedas for checking circuits other than thosewith standard plug connections. \Vhenfused, 110 -volt flexible leads are needed,plug in adapter with a fuse on one sideand the leads on front. When thewriter does low -voltage testing-as ondoorbells and auto systems-he uses alow -voltage lamp and drycell in placeof the 110-V. supply. By fastening aplug receptacle on the battery, it makesan ideal portable set-up to use where"110" is not available.

HOME-MADE PHOTO -VOLTAICCELL

By Donald Roberts

MY contribution is a home-madephoto -voltaic cell for the experimenter.This cell changes varying light inten-sities into electrical impulses by vary-ing the potential between the elec-trodes of a primary cell. The solutionis made by adding 1 oz. of lead nitratecrystals to 1 gill of distilled water (4gills = 1 pt.). An ordinary pickle bot-


- PHOTO VOLTAIC CELL -

BACK WALEOF BOTTLEANO BACKSIDE OFCOPPERPLATE PAINTED

BLACK

COPPERPLATE rx4"CONTACTEDWITH CUPORUS

OxIDE

COPPER LEAD 01 -ms

.

TERMINAL

FROM CAP

R,1 O0 . 0 0

HMS

LEAD STRIP'/2"X 6"

SOLUTIONOF LEADNITRATE

FIG. 22-Home-made photo -voltaic cell.

tle is used for a container. See dia-gram A. The electrodes used are alead strip 1/2 by 6 ins. and a copperplate I by 4 ins. Heat the copper platein a flame until the entire surface iscoated with a black flaky substancecalled cupric oxide. (Use a blue flameinstead of a yellow one, to avoid a de-posit of soot.-Editor.) Now wash theplate in a weak solution of ammoniawater to dissolve the cupric oxide. Thisleaves a golden brown coating of cu-prous oxide, which is light-sensitive.The back half of the pickle bottle andthe back side of the copper plate shouldbe painted with black lacquer.

See Fig. 22.

CHAPTER III

TESTING METHODS

FREQUENCY -DOUBLING IDEA

By John E. ClarkII ERE is a frequency -doubling idea

which I have found useful when a rela-tively small amount of power is re-quired at a frequency higher than thatof the available supply voltage. Thecircuit shown in Fig. 1 was used whena frequency of 120 cycles was desired.The arrangement shown readily adaptsitself to multi -stage doubling but re-quires the addition of a power ampli-fier of some sort if other than a fewmilliwatts of power output is desired.The doubling arrangement is nothingmore than a full -wave rectifier withoutput filter purposely omitted and asuitable A.C. output coupling devicesubstituted in its place.

Standard audio components are en-tirely satisfactory for the circuit and

the output coupling can be of any typewhich will be suitable to feed into thefollowing amplifier.

FIG. -Circuit of frequency -doubling Idea Lowerright, frequency quadrupled.


FINDING ELIMINATOR"REGULATION"

By Samuel Eidensohn

IF the experimenter knows just howthe voltage delivered by his power sup-ply varies with the load, lie is in pos-session of some important information.

He knows, for instance, whether hecan change the audio circuit to includepush-pull operation without alteringhis "B" supply; perhaps by purchasinga new power transformer.

The simplest method of determiningthe "regulation" of the "B" output isto connect a 0-100 milliammeter MAa 0-500 voltmeter VM, and a 0 -25,000 -ohm variable resistor R3 (capable ofcarrying 100 milliamps without burn-ing) in accordance with the diagram(Fig. 2) RI and R2 are portions of thevoltage divider in the "B" unit.

FIG. 2-It is standard practice now to test the A.0input for fluctuations. It should be equally so, to

test the D.C. input of the voltage divider.

As indicated, only one wire in theunit, the "B Max.", is broken. The leadfrom the rectifier and filter net -workis Ito be connected to the "+" post onthe milliammeter.

The resistance of the voltmeter doesnot matter; because the current itconsumes is a relatively slight propor-tion of the total amount indicated bythe milliameter. However, if the re-sistance of the voltmeter VM is wantedthe following procedure is followed:Read MA with VM disconnected; read-ing equals Io. Re -connect VM, andread both meters; current in milliampsIa, and voltage equals V. To find theresistance of the voltmeter, these val-ues are to be interpolated in the fol-lowing formula:

V-1000

Ia - To

With the instruments connected asshown in the diagram, resistor R.3 isvaried and the readings on the metersare plotted.

This method is applicable to all pow-er supplies. It is advisable to keep thevoltage -divider unit, R1 -R2, and themilliammeter permanently in circuit toavoid the strain on the filter conden-sers that would result if this portion ofthe filter circuit were open while thecurrent is on.

RAPID METHODS OF LOCATINGFAULTS IN RADIO RECEIVERS

By Delbert Myers

FAULTS occurring in radio receiversare all similar in characteristic mani-festation.

One common trouble or fault in setsis tube failure. For rapid work we pro-ceed by the process of elimination. Alltubes are tested for emission, and pos-sible shorts, and all faulty tubes arereplaced.

To test, put receiver in operation. Re-move detector tube and replace. If thisaction produces a click, we can con-sider that the audio system is 0. K.Tapping the detector tube lightly withthe finger should produce a ring in thespeaker if the detector and audio sys-tem are 0. K. We, therefore, eliminatethis part of our circuit. Next pull outthe tube in the radio -frequency ampli-fier preceding the detector and replace.If click is heard the trouble is in pre-ceding stages. Take each succeedingR. F. tube out of socket and replace.If one of the tubes does not produce aclick the fault is in that stage or in oneof the preceding stages. This processwill work for any part of the circuit.

METHOD OF DETERMININGPOWER (WATTS)

CONSUMPTIONBy A. E. French

FOR the Service Man, experimenteror amateur a means of accurately de-termining the power in watts consumedby any electrical apparatus, radio re-ceiving set or transmitter, which de-rives its power from the usual alter-nating current power lines, is very oft-


en beyond the range of available testequipment, due mainly to the expenseinvolved in securing an accurate A. C.wattmeter.

In order to determine the power orwatts input to any electrical equipmentwhere a wattmeter is not available, theservice kilowatt-hour meter which isprovided for utility customers can beused. Connect the unit to be checkedto the source of supply, being very surethat it is the only equipment connectedto the load side of the meter. Nextcount the number of revolutions madeby the meter disc. Next determine themeter disc constant. This will be foundon the nameplate.

After determining the meter disc con-stant and the number of revolutionsper minute, it is only necessary to ap-ply the following formula: Revolutionsper minute x basic constant x 60=Load in watts.

FIG. 3-How to figure wattage drain by use of theregular light -line power meter.

The basic constant is usually foundon the name plate of the meter. Someof the constants watt -hours -per -revolu-tion of the more commonly used me-ters follows:General Electric -0.3 Duncan-%Westinghouse -1/3 Sangamo-5/24These are all 60 -cycle, single-phase

meters of the basic 5 A., 115 V. 2 -wiretype. For 220 volt meters, the constantis doubled, while if it has a higher cur-rent rating, such as 10, 15, or 25 am-peres, the rating is doubled, tripled,etc. Where instrument current trans-formers are used with a meter on largerinstallations, the result, after solvingthe formula must be multiplied by thetransformer ratio.

A power rating in D. C. watts is fig-ured the same way. (See Fig. 3.)

METHOD OF DETERMININGPARALLEL RESISTANCE

By Al Robinson

TIME is money, so this time-saver isa moneymaker, when no resistance chartis handy and the resultant resistance ofparallel resistors must be computed. Itavoids the confusing "reciprocals of re-ciprocals" which must be used in com-puting values by means of the formula.

From any two points on a horizontalline, erect the perpendiculars, AB andCD (see Fig. 4A), proportional to therespective known resistances, R1 andR2, using any convenient scale such as,1000 ohms to each 1/16th of an inch,etc. Join AD and BC. From F. thepoint where AD and BC intersect, dropa perpendicular (FE) to AC. The lengthof this perpendicular, FE, representsthe total resistance (Rx) desired to thesame scale. If more than 2 resistorsare to be used in parallel, take any 2and work out as before. Then applythe same method again, using the resultof the first 2 with the next, and so on.

If the required resistance is knownand you have none of the right value,you can select from your stock thosewhich, in combination, will fill yourneed, by using the following method,shown in Fig. 48.

On a horizontal straight line erectAB representing the required value (for

E

R z 12.000R 2 16.000

A C

FIG. 4-Simple graphic calculating of parallelresistances.


example, 12,000 ohms, R; the scalemight be, as above, 1/16 -inch per 1,000ohms). If you have, among others, a16,000 -ohm resistor (R2), erect theperpendicular CD to the same scale.Run a straight line from D through Bto a point, E, on the horizontal line,and erect a perpendicular there. Thenrun a line from C through B to thisperpendicular, intersecting it at a point,F. Measuring EF, you will find thatyou require an additional resistor ofapproximately 54,000 ohms.

A SIMPLE METHOD FOR MEAS-URING A.C. RIPPLE IN FILTERS

By Clifford E. Denton

THERE are many times when theexperimenter or Service Man wants toknow the exact ripple voltage froma high voltage power system or motor -generator.

A simple method which has beenused by the author for this purpose em-ploys a rectifier -type A.C. voltmeter,which will measure the average (0.636)value of the A.C. or "ripple" voltage.See Fig. 5.

To isolate the meter M from theDC., but allow the A.C. to pass throughit, condenser C is used; a 4-mf. high -voltage type unit will be satisfactory.

It is important that the "working"voltage of the condenser be equal to,or greater than, the power supply's"peak" voltage; which is equal to the"R.M.S." value of the voltage appliedto the plate of the rectifier tube, multi-plied by 1.41.

OUTPUT OFPOWERSUPPLY

ANDFILTER

FIG. 5-Measuring "ripple." A.C. Voltmeter M,D.C.Insulated by C, Indicates potential when switch

Sw. is depressed.

The switch Sw. should be of theclosed-circuit type, which remainsclosed except when pressed; this pre-vents the initial charging current drawnby the condenser C from passingthrough the meter. The meter shouldbe connected into the circuit only afterthe receiver is in operation. This placesthe actual working load on the filtersystem and reduces the chances ofruining the meter.

If we have a power supply which de-livers, say, 500 volts under load, andthe meter range is 50 volts, then a full-scale reading would indicate that theripple is 10% of the applied voltage.It can readily be seen that this methodrequires a minimum of parts; and sat-isfactory approximations of the ripplevoltage can be obtained.

TESTING TRANSFORMERSBy G. H. Nakas

ONE of the most frequent causes ofnoisy or scratchy reception in receiversis a defective winding of an A.F. trans-former. It is very difficult to test forsuch a condition, since the winding isnot actually open. A 4.5 V. battery

FIG. 6-Testing for noisy audio transformers.

may be connected across either wind-nig of a suspected transformer, and apair of phones across the other wind-ing. If the transformer is faulty, aloud scratching sound will develop ina few minutes. In case no noise showsup, reverse the phones and battery,connecting each where the other pre-viously was, so as to test both windings.

(See Fig. 6.)v

CHAPTER IV

TESTING EQUIPMENT AND METERS

CALIBRATEDAUDIO -FREQUENCY

ALTERNATORBy Cal Brainard

HAVING need for a source of A.C.of known frequency, I made a simplemachine from a portable phonograph,the general layout being shown in Fig.1. A 9 -in. gear wheel is mounted on theturntable, and a (Brandes) headphoneunit is used as a pickup. The phone isused without the cap and diaphragm,and is mounted on a heavy lead blockso that it may be moved for best posi-tion. The pole pieces are set so thatthey are parallel -to, and just clear, thegear teeth, and they must be even withany 2 of the teeth.

FIG. I-Calibrated A.F. Alternator.

The speed of the turntable is 80r.p.m., so 12,000 teeth pass the polepieces per minute, or 200 per second,giving a 200 -cycle note. The outputmay be amplified to any level.

THE "HOWLER" AS A SERVICEOSCILLATOR

By B. FoxTHE old idea of putting a microphone

transmitter against a receiver may nowbe put to more practical use than an-noying the party at the other end ofthe wire, as illustrated in Fig. 2.

Transformers T1 and T2 are anyhandy telephone or high -radio audiotransformers; while condenser C (or -

F G. 2-A telephone transmitter, receiver, andtransformer, are the essentials of this "audio howl-

er" designed for use In radio adjustment.

dinarily about .01-mf.) may be variedfor different tones. The telephone re-ceiver is designated as H; the micro-phone, M; the voltage supply for themicrophone, A (which will vary witheach mike-although its average valueis 3 to 41/2 volts).

If no microphone is available, onemay be conveniently made by mount-ing on the diaphragm of the receiver a"microphone button," obtainable forabout a dollar. The assembly is to betaped together and placed in a box,padded with felt or cotton to preventthe audio howl being heard in the room.

JUNK -BOX CONDENSER TESTERBy L. H. C. Smout

FIG. 3 shows a circuit diagram ofa useful radio servicing device. Thejob can be constructed very cheaply,in fact most of the components will befound in the Service Man's junk box.Owing to the advent of the "tuningeye" tube, the older tuning meters arebeing discarded and the device has beendesigned around a Weston Tune -a -Lightmeter. The plate voltage and grid biasresistor are selected to suit the tube

"WESTON-TUNINrMETER

INPUT

10 PILOTON METER

SW

110 V,

FIG. 3-Circuit of Junk -Box condenser tester.


used, so that with a free grid, the platecurrent will have a value of about 5ma. which, with a Weston tuning me-ter, will cause the light beam to prac-tically disappear. However, when thebias is applied to the grid by shortingthe input terminals, the plate currentwill be reduced to about 0.5 -ma, whichwill give nearly a full-scale light beam.Thus when the device is switched on,the light beam will disappear, but byconnecting a high resistance betweeninput terminals the bias is applied witha consequent appearance of the lightbeam. The job will detect leakagesabove 10 inegs., and I have found itvery useful in detecting inter -elementleaks in tubes and small -value couplingcondensers. Condensers of a value of.0I-mf. and over show a full light beamwhich gradually disappears as conden-ser charges up, the time varying froma second or two to several minutes withlarge -capacity condensers. The job willnot give quantitative measurements, andcare must be taken not to touch any-thing connected to the grid terminalwhile operating, as body capacity willgive a false reading; also the grid term-inal must be insulated with a good qual-ity insulator as any high -resistance leakin the material will nullify results.

A LOW -CAPACITY CONDENSERTESTER

By Arthur ZagonTHIS novel circuit will test conden-

sers as low as 50 mmf. Enough platevoltage is used on V2 to cause the meterto swing to maximum with no voltageapplied to VI. The latter tube acts asa rectifier, and when A.C. is applied to

FIG. 4-Low-capacity condenser tester.

it, a varying voltage will be applied inturn to the grid of V2 This voltagewill vary in accordance with the sizeof the condenser being tested. By clos-ing the switch, resistance or A.C. voltsmay be read at the proper bindingposts. Inductance can also be meas-ured. Calibration is made by compari-son with known values.

(See Fig. 4.)

"NEON" CONDENSER TESTERBy Alfred Kafoury

THE diagram Fig. 5, shows how theService Man may construct a conden-ser tester using a neon tube, that is"suitable for testing all types of conden-sers.

FIG. 5-A Neon -Tube cond tette .

A S.P.D.T. switch, Sw. 1, is placedin position 1 when testing paper con-densers, and in position 2 when test-ing electrolytics The latter must betested according to polarity. SwitchSw. 2 is closed when first testing elec-trolytics, for protection of the meter.then opened and a reading taken. Ifthe condenser passes more than 1/2 -ma.per microfarad, it should be discarded.

In testing paper condensers, with Sw.1 in position 1, the voltage can be reg-ulated to the working voltage of theunit being tested. The 500 V. D.C.meter is for this purpose, and while notabsolutely necessary, it is very handy.

A SIMPLE WATTMETERBy Eugene Kingrey

HERE is a simple method of con-structing a wattmeter, which is as ac-curate as needed for ordinary measure-ments. The device may be calibrated


FIG. 6-Circuit of a simple wattmeter.

by the use of bulbs, electric irons, orany other apparatus of known (notethe emphasis) drain.

There is only one precaution to ob-serve-have the meter shunt fastenedin place firmly so that it cannot be re-moved unintentionally, as removal whilethe apparatus is in operation will ruinthe meter. Incidentally, various rangesmay be had by using different sizes ofshunts for the meter, but the switchused to change the range must be ofthe shorting type.

The line voltage across A -B shouldalways be adjusted to read 100 V. he -tore taking a measurement of wattsdrain.

initial calibration should be madewith a S0 -W. lamp. Then other appli-ances may be connected to the outputsocket, and meter readings noted untilenough are obtained to make a cali-bration chart.

I believe Service Men will find thisapparatus of some use.

See Fig. 6.

FIG. 7-Circuit of ElectricalUltra -Micrometer.

ELECTRICAL MICROMETERBy Bell Labs. Record

MANY requests have been receivedfor a circuit of an "electrical micro-meter." Figure 7 shows such a circuit,while Fig. 8 gives a detail of the micro-meter "head." The arrangement con-sists of 2 oscillators, one of which isfixed. The variable oscillator is con-nected to, and tuned by, the plates ofthe micrometer. Moving the top plateand thus changing the capacity, changesthe frequency of oscillator V1, thischange causing galvanometer G tomove. This particular equipment workson 2,000 kc. No constants are givensince the technician will doubtless havehis own ideas about them.

The principle is that which is com-mon to all ultra -micrometers --the va-rious types differing only in their meth-od of measuring the change of fre-quency produced by the displacement.

By making the spacing between theplates very small the frequency changewill be relatively large for small platedisplacement. By substituting this fre-quency change for a displacementchange, the instrument gains greatlyin sensitivity.

The potentiometer F must be set sothat the galvanometer reads on -scale,and for large displacements the poten-tiometer must be constantly adjusted,while for very small displacements, thechange can be watched on the meter.

CL

V (TYRE VT 1 TRIODES) V2

tilC2

A IMILL1014114LENTITAFTER MOYFM MToftRATE OF -A -MAY PE MEASURED

OSTJUATORWAVE -METER

POTEN-TIOMETER


FIG. 8-Ultra-Micrometer head.

It must be emphasized that the ap-paratus here is of the highest preci-sion type, but the principle is very in-teresting, and might be Put to use bythe home mechanic.

INCREASE THE METER'S RANGEBy John J. Nothelfer

MANY good Weston or Jewell me-ters, designed for the old RCA battery -model receivers, can now be boughtvery cheap in some of the salvagestores. The fan who cannot afford tobuy a new voltmeter, or milliammeter,can make use of an old instrument bya few changes and convert it into a volt-ammeter of all ranges with great sav-ing to his purse, since these meters canbe bought for around a dollar to adollar and a half.

As Eig. 9C shows, the case is re-moved' from the meter and a smallpiece of insulated stranded wire is sol-dered to the resistance terminal whichleads to the armature at the bottom ofthe meter. (Care should be taken notto solder to the terminal that leads tothe terminals of the meter). A smallhole is drilled in the bakelite back, andthe wire is drawn through. (Fig. 9A.)The case is then replaced, and themeter is ready to he mounted on asmall box.

RESISTANCEI SCALE

HOLE

X \VIRESOLDER

-A- HERE

5 50 250-B- MILLS. VOLTS

FIG. 9-Connections to the meter Illustrated in Fig9C. With the action used. R3 is 5625 ohms; R4

61,875.

Various meters have different inter-nal resistances and draw more or lessohms per volts, so the correct resist-ance values cannot be given. However,the manufacturers of most instrumentswill he willing to give the rated re-sistance, upon request. The Jewell andWeston meters mentioned, however,have a resistance of 125 ohms per volt.So, for each volt to be added, a resist-ance of 125 ohms should be used. (Fig.913.)

FIG. 9C-A voltmeter, with its case removed, un-dergoing the operation described by Mr. Nothelfer,which gives access to the basic mIlliammeter unit.

The meter has already a 5 -volt scale,with a 625 -ohm resistance built in. Toincrease the voltage of this meter to50, a total resistance is required of50 x 125 = 6250. The meter alreadyhas 625 ohms resistance; subtract thisfron 6250, and the additional resist-ance required is found to be 5625 ohms.A fixed resistor of this size is quite hardto obtain; and a good substitute is oneof variable type, with 6000 ohms max-imum, adjusted to the required length.

To obtain milliampere readings fromthe meter, the wire which was solderedto the armature is used with the term-inal of the meter which connects di-rectly to the other winding of the arma-ture. Resistance wire from old, heavyrheostats will answer nicely as a shunt.

UTILIZING A GALVANOMETERIN TESTING WORK

ELECTRICAL meters are the eyesof the radio experimenter and ServiceMan, for spotting trouble, and I am aclose adherent and zealous user of thesevaluable electrical devices. I know of


F G. 10-A galvanometer (or milliammeter) takehe guesswork out of servicing. All data sheets arebased on the assumption that the Service Man ha

means of measurement.

many a Service Man who still dependsupon guesswork, a pair of phones witha battery, or 110 -volt line with a lampin series, as the only means of locatingfaults, disregarding other factors andignoring the actual cause of the trouble.For most Service Men are interestedonly in shorts and breaks in the wir-ing; and anything else is immaterial tothem.

A continuity test is all that is re-quired in most cases, but there are in-stances where partial breaks or shortsoccur and a phone or lamp test yieldsvery little information, if any.

A galvanometer (or a low -readingmilliammeter-which is practically thesame instrument, in principle) is theideal device for testing; and very valu-able information may be gained throughits use. See Fig. 10.

With a small "C" battery and a va-riable high resistor in series with theinstrument a number of different testsmay be performed.

This combination is exceedingly help-ful for testing windings in audio -fre-quency transformers and, by takingreadings, we may determine the ap-proximate transformation -ratio of thewindings and establish the identity ofthe primary and secondary when theseare not marked. Other comparativetests of resistance may be performedas one becomes more familiar with thepossibilities of this valuable instrumentfor radio testing.

Care should he taken, when makingthe necessary connections to the instru-ment, that the resistance is at a max-imum and of a value of at least 100,000

ohms, when first closing the circuitwith the battery in series. These in-struments are very sensitive and del-icate, and may be damaged even witha small run down battery.

By varying the resistor, we may read-ily adjust the pointer to any desirableplace on ;the scale, and the instrumentis then ready for use. The same pro-cedure as in the case of phone testingis followed; but instead of hearing clickswe watch the deflection of the pointer.

ADDITIONAL METER SCALESBy J. Christine

THE trend in service equipment, forthe man who "rolls his own," is touse one instrument for a multiplicity ofpurposes. If a single meter is to beused as a voltmeter, ammeter, milli -ammeter, ohmmeter, capacity meter,etc., the numerous scales that are nec-essary complicate the reading of themeter to such an extent as to make theinstrument impractical. If separatescales are to be used, then we are facedwith the problem of removing the glassfrom the meter every time a change inthe scale is to be effected. The novelscheme illustrated in Fig. 11 overcomesthis difficulty.

The zero and top mark lines of themeter scale are drawn on the secondscale to facilitate lining it up when it isplaced on the instrument. The meteris then calibrated and the markingsplaced on the new scale.

With this arrangement it is possible

(A)BRASS THIN SPRING -PLATE

FIG. 11-The desired paper scale Is pasted over removable non-magnetic plate (A).


to use as many extra scales as is de-sired, without, at the same time, open-ing the actual instrument itself.

EXTENDING RANGE OF A.C.VOLTMETERS

By D. V. Chambers

SERVICE Men who have old-styleA.C. voltmeters with very low voltage,full-scale, such as 0-3 volts or 0-4 volts,find very little use for them in presentday A.C. receivers. However, by theaddition of a shop -made transformer,they may be made to read low, me-dium or high voltages. Such a transfor-mer is herewith described and is suffi-ciently accurate for all set measure-ments, even up to 1,000 volts or moreby the use of series resistors. See Fig.12.

Take an old audio transformer (thosein the RCA catacombs of old, are agood size), the smaller the better. Re-move all old windings and use original

FIG. 12-Circuit for extending range of ow-vo tageA.C. voltmeters, as used in instrument pictured.

cardboard core, with side pieces cutto fit, and cemented in place to just fitinside window of laminations.

If a 3-V. A.C. Weston 476 is used,which I have, the primary will requireabout No. 24 or No. 26 enameled wire,45 turns, and 4 -volt --60 turns. (Corecross-section squared and divided by6, gives volts -per -turn available foryour particular laminations. If it is3/4 -in. wide and 3/4 -in. thick, then 3/4

± 6 = between 10 and 11 turns -per -volt. 10 is OK, for it isn't usedfor any great length of time, and sowill not overheat.)

Several layers of tape, varnished

cloth or heavy brown paper are shel-lacked in place, for insulation. Then,start winding the secondary, which iscontinuous and tapped. For the 1stsection, 8 volts, wind 120 turns in evenlayers, using about No. 28 enameledwire; for the next tap, wind 120 moreturns (for the 16-V. tap), using No. 32E. wire; then, 360 turns of about No. 34for the 40 -volt winding. If there is suf-ficient room and you want to includea 150- or 160 -volt tap, then (for a 150 -volt tap) add 1,650 turns of very finewire, such as No. 38 or No. 40 enameled,in layers with a thin sheet of paper be-tween each layer.

Having finished the coil, shellac andimmediately wind several layers of tapeor paper around and shellac. Put thelaminations in place and, if a small me-tal box such as coil shield with lugsfor mounting is handy, place in can andfill with pitch or wax; this makes aneat job.

Having only wound to 40 volts, I

used resistors (1 -watt carbons), forhigher voltages, namely, 150 and 750.Use several 1 -watt resistors in seriesfor 750 -volt reading, as it should beabout a 5 -watt resistor to drop this 600volts. It requires about 5,2000 ohmsfor the 150 -volt reading and about 21,000ohms, 5 -watt, for the 750-V. reading.

This whole affair can be put in asmall box size about 4 x 7 ins., and 3to 4 inches deep. An aluminum panelmakes for an attractive -looking prod-uct. This arrangement, to avoid shocksor fireworks, must be well insulated!By the simple addition of a right -sizeresistor to make the meter read full-scale with 110 V. circuit, condenserscan be checked, chokes measured, etc.It is the next handiest thing to theohmmeter, which in my case it matchesin size and shape.

TUBE TESTER AS V.T.VOLTMETER

By William WilsonALMOST any emission -type tube

tester may be used as a fairly accuratevacuum -tube voltmeter. With the useof a 1,000 -ohm potentiometer and a4% V. "C" battery, a wide variety ofdifferent tube types may be used. Anadapter is made for the tube to he used

PRACTICAL RADIO KINKS AND SHORT CUTS

r60

TUBETES TERSOCKE r

PLUGti

NO El euTTONCLOSED TOTOP CONTACTFOR TEST

a.22O'. 111111

ti SO-L-MAAV-1 -V",ESSENTiAL INTERNAL CONNECTIONS

OF 'EMISSION' TUBE - TESTER

0/1000.

-CABLE

6F5

TEST

BATTERY AND POTENTIOMETERIN CORO COMPARTMENT

1,000POTENTIOMETER

',CALIBRATEWITH KNOWNAC OR D.0VOLTAGE ti

FIG. I3-Any emission -type tube tester may be used as a vacuum -tube voltmeter.

with the grid circuit left open. if gridconnects to top cap, no adapter is need-ed. I used a type 6F5 since it has atop cap which can be used as a probeand the socket can be connected to aplug and cable for high -frequency meas-urements. My instrument is a Supreme89D and the circuit is shown in the(diagram of Fig. 13. Calibration curvescan be made to suit the instrument used.

V.T. VOLTMETERBy William Chaney

WISH to present an A.C.-operated,flexible, vacuum -tube voltmeter of en-tirely original design, simplicity andproven worth. T use this meter in ser-vice work, and it was used as an out-put meter "on the line" in one of thelocal radio manufacturing companies.comparing favorably with the special-ized equipment used for this purpose.The 6J7 metal tube is used because theresponse is linear from 2 to 5 ma. (withcircuit constants used) so that the meas-ured voltages may he read directly fromthe meter scale. The ranges are 10.

100, 500 and 1,000 V. A.C. and 0-13 V.D.C. (with use of the 9 meg. potentio-meter, the D.C. range may he extendedmuch higher and still have linear res-ponse). Frequency error is small upto 200 or 300 kc. T have used it tomeasure output of an all -wave oscilla-tor. Some of the uses of this meterare: measuring A.C. voltages fromtransformer; A.V.C. voltages; bias onaudio tubes at the grid, and when tap-ped high -resistance voltage dividers areused; output meter (from plate to

of

EIEG.

ground or across voice coil); gain infinal stages of a receiver; etc. In somecases an A.C. operated V. -T. voltmeterwill induce a terrific hum in a receiverwhen A.V.C. voltages are being meas-ured. There are a number of thingswhich can be done to cure this: (1)change the power transformer of theV.T. voltmeter (which may have leak-age or too high a capacity between pri-mary and secondary, etc.); (2) insulateall grounds from the chassis, includinginsulated mounting of the filter con-densers of V.T. voltmeter; (3) insert aresistance and capacity filter in theground lead of the D.C. section of theV.T. voltmeter (see diagram); (4)reverse line plug; (5) remove externalground from receiver and line bypasscon( enser (if then: is one). See Fig. 14.

(.10

2 MF.

s v '10000OHMS

61-451

9(14( 000M5

0 MF

17

cm) soo t000

0.4- 0.5-`MEG.igulEG

0 1-14EG. (EACH

0-S MA.

24,0130OHMS3.000

16

8.012s,0016:_ms..

D

9ME

al-s2-w7 11574-

/ /5044

MEG.Pi°5-(toyC clprtinA

FIG. 14-A Ver stile V.T. Voltmeter.


A DYNATRON VACUUM -TUBEVOLTMETER

By C. W. Melotte

IN Fig. 15 is shown the wiring dia-gram of a vacuum -tube voltmeter I use.Its advantage is that very small volt-ages can be measured with it withoutthe use of an ultra -sensitive meter.

FIG. 15-Circuit of a Dynatron V.T. Voltmeter

An ordinary 0-1. milliammeter is sen-sitive enough to measure such smallvoltages as would not operate an or-dinary V.T. voltmeter using the morestandard three -element tube. An inputof 0.05 -volt gives a reading of approx-imately 0.2 -ma.

The potentiometer is used to accur-ately adjust the plate voltage to thecorrect operating point.

"MAGIC EYE" LEAKAGE TESTERBy Ray Jefferson

I RECENTLY finished building a"Magic Eye Leakage Tester" and havefound it can be made very useful bymaking the additional changes as in-dicated in the diagram, Fig. 16.

As an output meter it is very sensi-tive, as it will operate on a signal whichis barely audible. This is a great helpin aligning sets with A.V.C. and sets

with poor L.F. gain. This sensitivitycan be controlled by the 0.5-meg. po-tentiometer.

As may be noted in the diagram, the6.3 V. filament circuit is used to testthe capacity of small condensers.

I used only one filter condenser, asan input condenser was found unnec-essary; it raised the voltage too high.

A 50,000 -ohm variable cathode resis-tor was found to he too large, as all ofthe action took place on one end. A10,000 -ohm tapered unit was found tobe satisfactory. (Switch Sw. 6 is of the"skip" type in order to prevent short-ing the "B" units.)

The 6 input terminals are connectedas follows:1 and 2- to voice coil of speaker.3 and 5- to plate of output tube.4 and 5- to test leakage (open Sw. 1).4 and 6- small condenser capacity test

(open Sw. 1).4 and 5-A.V.C. line (open Sw. 2).

SIMPLE OUTPUT METERBy Andrew Frevert

A SMAI.I. output meter, that is madeup to use in conjunction with a serviceoscillator, is shown in the sketch.(Fig. 17.)

The combination (of jacks A, B, C,D; switches SI, S2; and transformerT) makes a variable input to meter Mand detector CD which forms the out-put indicator; depending on types ofsets.

With connections to set on jacks Aand B and switches in No. 2 position,primary of transformer is in serieswith speaker: secondary in series withmeter and detector. With switches inNo. 1 position, input with primary inparallel is fed direct to meter andcrystal.

THIS RESISTORIS USED TOCUT VOLTAGETO 250.

egica.

FIG. I6-Here is an im-proved version of the"Magic Eye" leakage test-er described in March.1936. Radio -Craft. Severalfeatures have been added.which make the appara-tus much more adaptable.


FIG. I7-This set-up is flexible for measurements onvarious sets: and also for continuity tests.

With input leads in jacks C and Dand switches in No. 1 position, second-ary is in series with input, primaryfeeding to meter. With switches in No.2 position, input is in parallel with sec-ondary and feeding to meter.

With input leads in A and C jacks,Si on No. 1 position and S2 on No. 2position, meter is connected direct toinput signal.

For use as voltmeter: Jack E plus,G minus 5 volts; Jack E plus, H minus25 volts; Jack E plus, I minus 100 volts.

Jacks E and J are for continuitytesting or, if scale is calibrated, for useas ohmmeter.

A.C.-D.C. VISUAL OUTPUTINDICATOR

By P. J. DonneauA VISUAL output indicator can be

made with nothing more than a 6E5visual tuning indicator tube and socket,a 30 -watt electric light bulb and socket,a line plug, a 0.1-mf. fixed condenser,some wire, a 1-megohm fixed resistor,and 3 battery clips. The 6 -prong socketand the ten -cent -store light socket maybe screwed to a small baseboard.

The connections (see Fig. 18) areas follows:-One side of the line plugis connected to the filament and cathodeprongs of the tube socket; the otherside to one terminal of the light socketand the plate prong of the tube socket.The remaining terminal of the lightsocket connects to the other filamentprong of the tube socket. A large bat-tery clip for grounding the unit is con-nected to the cathode through the fixedcondenser; 2 small battery clips are

directly connected to the control -gridand cathode terminals of the socket.These two clips connect to the outputtransformer of the receiver, in place ofthe voice coil. The tube is placed inits socket and the 30 -watt bulb (pre-ferably painted black to avoid glare)is screwed -in, and the unit is ready tooperate. In some cases it works bet-ter if a 1-meg, resistor is externally con-nected between the plate and targetprongs of the socket.

CDT"CLIPS TO SPKR

6E5 -

30 WATT LAMP

110 V. A.C.

1 MEG

0.1_ innMF.

LARGE GROUNDCLIP

FIG. le-Efficient output meter, Improvised from 6E5 tub* and parts found around any workshop.

The unit may he used on A.C. or D.C.,and if care is taken to make sure thecathode side of the plug is always onthe grounded side of the line, the fixedcondenser may he omitted.

MEASURING A.V.C. VOLTAGESBy Willard Moody

FUNDAMENTAL Circuit: (See Fig.19A.) When opposing voltages areequal there is a zero current and there-fore infinite resistance; the voltage asread on V. is then equal to the sourcebeing measured. In the diagram (Fig.19A) T are the terminals for the volt-age to be measured; J is the jack forheadphones or micro -ammeter and Bthe battery supplying the buckingvoltage.

Practical Circuit: (See Fig. 19B.)


0SW oa

alcaoga.4mETER

J

FIG. I9-Measu es A.V.C. circuits accurately.

Important: Proper polarity must beobserved in connecting the infinite re-sistance voltmeter in the circuit undertest.

A PHONOGRAPH -CASEOSCILLATOR

By R. Douglas Clerk

THIS novel and useful oscillator, to-tally shielded and fitting into a port-able phonograph case, is an audio -modulated R.F. oscillator covering thebroadcast band. The R.F. coil used inthis job was taken from an AtwaterKent "Model 35" receiver; coupled tothis coil LI is a 10 -turn winding I2in variable inductive relation. Thispick-up coil is connected to the anten-na and ground binding posts of thereceiver under test.

it is shown by diagram in Fig. 20.The electrical characteristics of an

average antenna are simulated in theelectrical values of a "dummy anten-na" arrangement of the complete inputconnections to the radio set being test -

r - 71

i

f V 1

LI

1

I

I

I

I

i

iTI 00025 MF _ii,L

1--. :TO-.-GR-OUNO"ON SET SHIELD-. TO ANT. -ON SET

L2ioTURNS

- It1000 OHMS 45t).

FIG. 20-The "portable broadcaster" is a very cornpact oscillator, built into a portable phonograph

which gives modulation.

ed; its components being L2, R, and C.The entire set-up is shielded, as in-

dicated in the diagram, and the pho-nograph turntable motor and the pho-nograph pick-up are grounded to thisshield. In this particular unit, tube Vwas a Northern Electric "peanut" tube(which has a 1.1 -volt filament), and thepick-up was a Canadian Marconi unitof the high -impedance type; although,of course, any make of apparatus hav-ing equivalent characteristics may heused without the least bit of trouble.

AN A.C.-OPERATED SERVICEOSCILLATOR

By W. R. WheatleyNO doubt, every radio experimenter

and Service Man has many times wishedfor a small portable generator of sign-als of various frequencies, either modu-lated or unmodulated. I have con-structed one which is exceptionallycompact, obtains its power supply bysimply plugging into the light socket,

FIG. 21-This oscillator, with home-made powertransformer, is an extremely -compact and conve-nient device, modulated by the A.C. hum of thelight line. A jumper is put across the lower tip

jacks when this meter is not in circuit.

and covers a wide range of frequen-cies with three plug-in coils. It com-prises a '27 tube used as an oscillator,in the conventional tuned -grid circuit,and an '01A tube with grid and platetied together as rectifier. (See Fig. 21.)

One 30 -henry choke is used in thefilter circuit, which is conventional; thevoltage divider is a 50,000 -ohm poten-tiometer, with the plate of the '27 con-nected to the slider, so that a variablevoltage is provided for the plate of theoscillator. The filament supply of thetwo tubes is from a transformer, and


the plate voltage is taken direct fromthe 110 -volt A.C. line; as this suppliesvoltage high enough for the purposeafter it is rectified. Tip jacks are pro-vided in the grid -return cicuit, so thata meter may be plugged in to be usedin lining up gang condenser or testingthe resonance of circuits. A smallvariable condenser is provided, withone side connected to the grid and theother to a tip jack; so that the oscil-lator is easily coupled to other circuits.

This oscillator is very handy for lin-ing up gang condensers and neutraliz-ing sets; I have used it as the oscilla-tor in a superhet. An ordinary set canhe converted to a super by connectingthe grid of the first tube to an externaltuned circuit coupled to the oscillator.Although a very slight A.C. ripple re-mains in the voltage supply of the os-cillator, when the signal of the oscil-lator is tuned in on a sensitive receiverit appears about the same as the A.C.hum in ordinary receivers.

In order to modulate the signal suf-ficiently, so that it may be heard dis-tinctly, a switch is connected acrossthe 30 -henry choke. When closed, thismodulates the signal with the 60 -cyclehum which is very distinct. The sche-matic diagram shows all details.

However, when I looked for a fila-ment transformer I was unable to findone small enough; so I constructed one.I used the core iron from a 30 -henrychoke, and also the form on which thewire was wound; on this I wound 1200turns of No. 28 enameled wire for theprimary. Over this were 28 turns ofNo. 18 D.C.C. wire for the 2/ -volt sec-ondary, and over this 55 turns of No.20 D.C.C. wire for the 5 -volt secondary.Although this transformer becomeswarm when in operation, I have oper-ated several hours without undue heat-ing.

To illustrate the compactness of thisoscillator, the panel is 7 inches wide by9 inches long, and the entire appara-tus is housed in a box 31/2 inches deep.

A BRIDGE -TYPE TESTERBy Lowell Slack

THIS unit, which operates on the

principle of a simplified Wheatstonebridge, is very useful for Service Menand experimenters in testing for resist-ance, capacity, plate resistance of vacu-um tubes, ratio of transformer wind-ings and a comparison of inductive re-actance.

The unit consists of a type 56 or 27triode tube and a 3 -to -1 A.F. trans-former connected as an audio oscilla-tor, feeding a potentiometer which isequipped with a scale calibrated inequal parts (of the resistance of thepot. See Fig. 22A).

1.500oR2 Coo-oum

PO,EMI.O.E OR

FIG. 22A-Simplified A.G. Bridge unit. B, methodof calculation at Tower right.

In operation, for resistances, con-densers, and inductances, the unknownunit is connected to one side of thebridge and a corresponding unit ofknown value is connected in the otherbranch. Then the potentiometer isshifted for minimum tone from the os-cillator and the ratio of units on thescale of the potentiometer will give theratio of unknown to known. In theexample shown in Fig. 22B the un-known capacity is 3/2 of the knowncapacity which equals 11/2 mf.; and, inthe instance of checking an unknownresistance, when compared to a 10 -ohmresistor it is 15 ohms.

To arrive at the plate resistance ofa tube, the tube should be connectedwith "A," "B" and "C" batteries ofcorrect size for the particular tubeoperation desired and the negative leadfom the "B" battery to cathode shouldbe opened and connected to the "un-known" terminals of the bridge. Thenby inserting a suitable known value ofresistance in the "known" side, the ra-tion of plate resistance to known resist-ance can be determined.

28 PRACTICAL RADIO RINKS AND SHORT CUTS

In the comparison of inductive re-actances, an inductance having a knownreactance is inserted in the "known"side of the bridge and the unknown in-ductance in the "unknown" side. Thebridge is then balanced and the reac-tances are thus compared on the poten-tiometer scale.

IMPROVING THE OHMMETERBy J. E. Kitchin

THE ordinary method of measuringresistances with a 4,500 -ohm resistorand 0 -1 -scale milliammeter will not gobelow 50 ohms with a shunt; and theslide -wire bridge is not suitable forcarrying around.

A better idea can be incorporated inmany set testers by adding one bindingpost to the existing circuit, as shown.The writer used two positions on hisWeston bi-polar switch; instead of add-ing a D.P.-D.T. switch. (See Fig. 23.)

METER -A-

1-1O

0 HIGH

COMMON

O LOW

HIGH

Rx

COM

- B- c -COM.

FIG. 23 --In this ohmmeter the D.P.D.T. switch maybe part of a larger unit.

To operate it, close the meter switchon the low side, and adjust the currentto one milliampere Then, if the un-known resistor Rx is of high value,switch the meter to the high side, andconnect Rx across H and C (as at B).The higher the resistance, the less willbe the current indicated by the meter,as is well known.

If Rx has a low value, leave the meteron low, and connect Rx across termin-als L and C (as at C). The meterreading will be low in proportion as thevalue of Rx is low; for Rx is a shuntaround the meter.

Calibration, by means of Ohm's Law,is performed in the usual way; the lowscale should be calibrated with knownresistance values, since the internal re-sistance of the meter must be taken in-to consideration.

A RESISTANCE -CAPACITY BOXBy E. A. Marchant

I GIVE a diagram of what I termone of the handiest pieces of equip-ment around our shop. As can be seenfrom the diagram any resistance valuefrom 200 ohms to 2 megs. may be hadby merely operating the switch; but inaddition to this, any one of these valuesmay be varied to any desired value bythe use of the variable control in thenegative leg of the circuit. The 3 decksmake it possible to name a deck forlow values of resistance, a deck forhigh values, and a deck for condensers;which means that the whole thing isin one box, with only one switch. SeeFig. 24.

A neat, 3 scale dial plate can be drawnfor the switch and the volume controlsmay be calibrated or the regular platespurchased for them.

When repairing a set I manipulate mycondenser switch for different values ofcondensers at different points of thereceiver. You will be surprised at howmuch improvement you can make in thereceiver with a few extra condensers.

Another place where the condenserresistor box saves time is in cases ofopen high -value resistors. If you arein doubt about a resistor of high value

SOD., LOCIO

Wir2.000.

110,101t"

( 10.000.

15000.

(up-00vft3 DE,.,R.° u3) argo,

on

Th000.n.

01-10CG

4_,PeSISTANCIS

o-cseec.r.

FIG. 24-A resistance -capacity box which savestime.


and your ohmmeter will not measurethat high, then set your resistor switchto the value needed and go directlyacross the resistor in doubt, and alsosometimes you will find an open re-sistor which is not color -coded and theservice manuals do not give the value.From your tube characteristics chartor other data determine what voltageor current you should have, connect avolt or current meter to the circuit andplace the test leads from your condenserresistor box across the open resistor andvary the resistor switch until you find aresistor that gives nearly the voltageor current wanted, if necessary you maycut in the volume control and bring theexact amount of resistance in. Alsoyou can experiment with resistors inload circuits and greatly improve areceiver.

You will find many additional usesfo this condenser -resistor box.

CONSTRUCTING A RESISTANCEMETER

By Malvern H. Berry

ALMOST every radio and electricalexperimenter has need of an efficientand reliable resistance meter. With thishe can design his own resistors, chokecoils, and many other things. The in-strument mentioned in this article wasconstructed from a potentiometer, agalvanometer, two binding posts, onedial, and a small box (Fig. 25).

Assemble and wire in accordance withthe diagrams; Fig. 25A is the schematiccircuit. The potentiometer R should beone of about 1000 ohms. The galvan-ometer G may be replaced by a high-range milliammcter and the results willbe the same. The battery B is just atwo -cell flashlight battery, which canbe purchased from the ten -cent store.

After everything is assembled comesthe calibration of the potentiometer R.This can be done with a Wheatstonebridge. (If the constructor does nothave a Wheatstone bridge, one may hehad for the asking at your local highschool. In the event that the construc-tor is not familiar with the Wheatstonebridge, the physics instructor at thehigh school would be glad to explainit.) If you can use the bridge, proceed

29

FIG. 25-Layout of resistance meter; the graph inthe lid is the reference calibration.

as follows: attach to the potentiome-ter a dial, (vernier preferred) and ad-just the potentiometer for a reading of5 on the dial. With this fractional partof the potentiometer in the circuit, con-nect it to the Wheatstone bridge andfind what the resistance of that partis. Get a piece of "graph" squared pa-per and graph the resistance in ohms,for every five marks or degrees on thedial, across the paper; and graph thereading or degrees on the dial up anddown. Where the two intersect on thegraph page, place a dot. After the re-sistances have been calibrated fromzero to the full value of the dial forevery five degrees, draw a line throughall of the dots. This will be your cali-brated curve for the resistance meter.

To operate the meter, place an un-known resistance Rx across at the bind-ing posts, and note the reading of thegalvanometer G when the unknown re-sistance is placed in the circuit. Switchon to the calibrated potentiometer Rand adjust until the galvanometer readsthe same as before. The value is then

FIG. 25A-A good wire -wound potentiometer, agalvanometer, some calibrated resistance standards,and a few odds and ends, compose this handy

instrument.


the same in both resistances. Take thereading of the dial in degrees and lookthat reading up on the graph, and thevalue of the unknown resistance can beread direct from there.

By ganging several variable resistorsof assorted ranges at R, and tappingthem to a selector switch, the resistancerange may thus be greatly increased.

BUILDING A RESISTANCECALCULATORBy S. H. Burns

FEW experimenters are fortunateenough to have an ohmmeter or otherinstrument for the measurement of re-sistance. There is no end to the oc-casions that call for the use of somesuch device, even while carrying on thesimplest of experiments.

With the current and the voltageknown, the resistance can be calculatedby applying the formula for resistancein Ohm's Law. A voltmeter and amilliammeter, when used in connectionwith a battery, will give these values.The disadvantage of this method is inhaving a voltage supply that is con-stant while the current that must flowthrough the resistance being measured,is drawn from it. Then too, a consider-able variation in the voltage must beavailable to accommodate the measure-ment of greatly different resistancevalues with any degree of accuracy. Fora low resistance measurement, it is notpossible to use a high voltage; on theother hand, when dealing with highervalues the voltage should be increased

Where the work can be done quickly

FIG. 26A, left-Theore Ica, circuitca culator.

FIG. 2611, right-Final circuit diagram of the cal-culator.

of the resistance

batteries are satisfactory, but often-times the voltage required for accuracymay be as high as 100 volts. In thesedays of battery eliminators, it is some-what of a problem to secure this bat-tery voltage.

A Reliable Voltage SourceVarious schemes were tried out while

searching for something that would sup-ply any reasonable voltage for as longa time as was necessary to completethe work at hand. It was decidedthat 100 volts would be sufficient forall requirements. The A.C. lighting cir-cuit seemed to offer an unfailing sourceof energy. Now to convert this intothe direct current required. After dis-carding several ideas as altogether toocomplicated, the scheme illustrated inFig. 26A was adopted.

The only things needed are tube forrectifying and a variable resistance toregulate the voltage output supply.Several tubes were tried and a '26 wasselected since the rectified voltage wasplenty high enough and the current out-put sufficient. Then also, most experi-menters will have several of these tubesnot in use since they have been re-placed by other types.

The 110 -volt line has one side tiedto the grid and plate terminals of thetube while the other side of the line isnot connected to the rectifying tubedirectly. A variable resistance of 25,-000 ohms is in series with the tube fila-ment and the other side of the line.The 2-mf. condenser shown across thisresistance levels out the rectified volt-age just as in any rectifying circuit.It can be of the low -voltage bypasstype since the voltage is not great.

The filament of a '26 operates on 1.5volts and consumes 1.05 amps. An idlefilament transformer having such awinding can be ;used for this; or, ifone is not handy and the device is to bemade more or less permanent, a heatertransformer can be quickly made. Forthe core, remove the laminations froma burned -out audio transformer. Thatportion of the core inside the coil is inmost cases about /2 -in. square.

If the dismantled transformer had abobbin in which its coil was wound, re-move the wire and use the bobbin for


the new coil. Otherwise, a form can beeasily made of cardboard. The totalcurrent is low, therefore a primarywound with No. 32 B&S wire willcarry it. Using this wire with ena-mel insulation, or better still, enameland silk, wind 1800 turns on the coiland insulate it with tape. As the wireis quite thin, flexible leads had best besoldered to the start and finish of thisprimary. The 1.5 -volt winding consistsof 24 turns of No. 20 B&S enameledwire. Cover the coil with tape to pro-tect the wire. The laminations shouldnow be put back in place in the newcoil. To eliminate any tendency tohum, dip the transformer into a pot ofmelted wax; this when cool will holdeverything firmly.

The Variable Voltage FeatureIn Fig. 26B, the parts are shown con-

nected diagramatically. It is the con-nection from the movable arm on the25,000 -ohm resistor that gives the volt-age and current used for our purposeWith the arm at the end nearest thefilament connection, the voltage ob-tained will be 100 volts when the maxi-mum of 10 ma. is being used. The drainthrough the resistance will be about 4ma., making the total less than 15 ma.at maximum.

The meter connections are shown inthis figure also. The voltmeter shouldhave a 0 -100 -volt scale and preferablymarked in 10 -volt divisions. The 25000ohm resistance in series with the milliammeter is only used when measuringlow resistance, and can be cut in orout of the circuit at will, with the singlepole switch shown.

The Resistance CurveTo eliminate the necessity of work-

ing out each resistance problem, thecurve given in Fig. 26C is used. Alongthe lower edge appear the current val-ues in milliamperes. The resistance inohms is at the left, vertically. Thiscurve gives the resistance value di-rectly when the voltage used in meas-uring is 10.

To make a measurement, proceed asfollows: referring again to Fig. 26B theunknown resistance is connected to the-terminals at 3 and 4. There is no needfor haste in taking the readings as thecurrent used will have no effect what -

25.000

20000 1

I I i

UFORDOTTED

15,000 ),PART

14000 'N......s...................,..................._

9,000

a000v)

7.000IX

6P00° 10 VOLT CURVE

5.000

4,000

3.000

2,000

1.000

.5 1. 2. 3. 4. 5. 6. 7. 8. t. 10.MILLIAMPERES

FIG. 26C-The 10 -volt calibration curve. The dottedportion It for an applied voltage of 100.

ever upon the voltage. Assuming thatthe resistance is not known, have theswitch at the left open thus cutting the25000 -ohm resistance into the circuit.Move the pot. arm to the right as faras possible, thus decreasing the volt-age to a minimum. Plug into the 110 -volt lighting circuit. Next move thevoltage adjustment to the left until thevoltmeter indicates 10 volts. If thereading on the milliammeter is low,close the switch and forget about the25000 ohms. The most accurate con-clusions are arrived at when using thatpart of the curve between 2 and 5 ma.Therefore, should the meter show lessthan 2 ma., move the voltage up untilit comes within these limits.

Assume That it requires a potentialof, say, 40 volts to produce the desiredcurrent flow, and again for purpose ofexplanation, assume that this current is3 ma. Following the vertical 3 -ma. lineto the point where it intersects thecurve, and looking left along the hori-zontal line also intersected at this point,it is found that the resistance value liesbetween 3000 and 4000 ohms. And aseach horizontal line represents 1000ohms, the exact value is 3,330. Thiswould be true if the voltage used was10; however, as 40 volts were used sim-ply multiply the result by 4, giving13,320 ohms as the resistance.


In this manner, one curve is used forany multiple of 10 volts by simply mul-tiplying the result by the multiple usedUsing 50 volts, multiply by 5; or using90 volts, multiply by 9. Any value canbe measured with 10 volts between 1000and 10,000 ohms and taken directly fromthe curve.

TIME -DELAY CIRCUITBy J. E. Templier

MANY experimenters want a meansof delaying the closing of a circuit byan accurately predetermined length oftime; this would he useful in the con-struction of robots, selector systems.burglar alarms, etc. The following de-scription of one may prove useful (SeeFig. 27) Unit LI-L2 is a 2 -coil relay;TA has a high impedance, to match theinput signal, and L2, which is woundover LI, a low impedance. The signalapplied to LI closes switch SI andcompletes the secondary circuit of thefilament transformer T. which includes12. The coil 12 holds the circuit closeduntil the cathode becomes heated andcurrent flows through L3; this closesswitch S2 and, a fraction of a second,later opens S3 and cuts off the currentfrom 12; this releases SI and the circuitis again ready for operation, as soonas the cathode has cooled.

FIG. 27-A V. -T. type time -delay relay circuit whichis variable from 10 to 60 seconds.

By using the older, type 27 tube andnormal filament voltage, about 30 sec.is required to close S2. However, byusing a transformer, T, with a 5-V. sec-ondary, and a rheostat, R, of about 7ohms, the time may be adjusted from10 to 60 seconds. By using the fila-ment -type tube, this time may be great-ly decreased.

Details for relays L1-12 and 1.3, arenot given; as most experimenters haverelays on hand or know how to build

them. Note that 12 must have a lowresistance. The arm of S2 should beabout one-half the length and weightof that of S3, so that it will close soon-er and have time to operate the outputrelay (not shown).

Notice that S2 is closed only mo-mentarily, so that another relay mustbe added to keep the circuit closed;also that only a momentary signal needhe applied to TA. However, by chang-ing the transformer connection, fromX-Y to X-Z, S2 will remain closedas long as the signal is applied to LI.

FORD CUT-OUTS MAKE RELAYSFOR BURGLAR ALARMS

By J. TheriaultRADIO Service Men who are called

upon to install burglar alarm systemscan oftentimes get a used 110 V. A.C.telephone signal hell and with the ad-dition of two old Ford cut-outs, makethe required relays as shown in Fig. 28.

V

12V

MASTER

My.Ac

SWITCH

RELAY NE 2

tlomozzl.

ALARM SUTTO'S

t.21

FIG. 28-Burglar alarm system using Ford cut-outs.

Remove the heavy outside windingsfrom both cut-outs. For relay No. 2be sure to remove the first layer, nextto the core. This can be easily doneby catching the outside turn, next tothe core, and pulling it out.

When any of the alarm buttons haveonce made contact, the bell will keep onringing until the master switch isopened, as the two relays form a "lock-ing" circuit.

LOW-COST TEST OSCILLATORBy Louis B. Sklar

THIS simple instrument will givegood results on either A.C. or D.C.Since it is intended for such universal


FIG. 29-Here is a simpleA.C. - D.C. test oscillatorwith several novel features.

PLUG INCOIL

350MMF.

MIN.LAMP

OUT PUT

250MMF

110V )AC 0C-)

ELECT RIC ;

LINE GNU(RES. IN LINE CORO) 350011MS _L

use, the 5-W miniature lamp is usedas shown in Fig. 29. If the lamp lightswhen the plug is put in the socket, theplug should be reversed. Of course, aground is needed on the oscillator forthis test. This test should always bemade; since if the plug is not insertedin the correct manner, the tuning con-denser and some of the other partsof the unit will be "hot" and unlessshielded may cause injury or damage.The rest of the circuit is very simpleand may be followed from the diagram.Either a line cord resistor or a 30-W.lamp may be used as the series resist-

ance to feed the heater of the 12A7.Standard plug-in coils of the desiredband ranges are used for tuning.

The use of the line cord resistancewill enable the builder to construct amore compact unit, while the 30-W.lamp will be cheaper and will likely beeasy to obtain. The cord should havea resistance of 350 ohms. The calibra-tion chart may be fastened to the coverof the box, and the extra plug-in coilsmay be mounted there also. This oscil-lator is so simple that it cannot get outof order and it will be found extremelyconvenient to use.

"17

CHAPTER V

VACUUM TUBES AND CIRCUITS

A PHOTO -TUBE RELAY FORUNIVERSAL OPERATION

By C. H. W. NasonONE of the writer's friends is a stage

designer. He builds pretty models ofstage sets with trick lighting effects.The other night he suggested that aphoto -electric relay would be a niceadjunct to one of his display sets. Thetrouble was that the "gadget" had tooperate on either A.C. or D.C., re-gardless.

A few moments of thought followedby a half-hour or more of intensive sol-dering resulted in the arrangementshown in Fig 1. It will be noted thatthe triode tube may be of any type thatuses a heater voltage of 6.3 V., anddraws 0.3 ampere. Types 37, 76, or6C5 are suitable.

The filament is lighted through a se-ries resistance and the device may thusbe operated on either A.C. or D.C.110 -volt supply circuits.


The grid bias is made variable bymeans of a potentiometer connected be-tween cathode and ground. This shouldbe adjusted so that the relay does nottrip under normal light conditions.This permits the room to be illuminat-ed without such indirect illuminationof the photo -cell affecting the opera-tion of the device. The relay may beso adjusted as to operate either to turnthe controlled circuits on or off withthe application of light.

This same device may be used sothat automobile head -lamps control theopening of the garage door-so thatpersons intercepting a beam which nor-mally keeps the relay closed will causeit to open and thus sound an alarm. Nolong-winded description is necessary,

PHOTO EtECTR.0CELL

si

Nr. VA,. a 'I10 000Co4t..4

RE v

P 1 M II Igo ))

FIG. I-A P.E. tube relay for A.C. or D.C. mains.

however, for a thousand uses for thedevice will immediately suggest them-selves.

The parts are as follows: RI, 350ohms, 5 watts; R2, 3500 ohms, Elec-tra(' potentiometer; R3, 1- to 5-meg-ohm grid leak; P.E.C., a caesium typegas -filled photoelectric cell; Relay,Yaxley 10,000 -ohm relay or device ofsimilar sensitivity.

BATTERY TYPE TUBE TESTERBy Floyd M. Glass

THIS tester is designed particularlyfor the rural Service Man, or anyonewho must test tubes without the useof power from the light lines. It willtest the most widely -used battery tubes.The equipment may be assembled in asmall box, and small -size batteries maybe included, if desired, to make the unitentirely self-contained. Socket No. 1

FIG. 2-A self-contained tube tester.

is for 01A, OIAA, 71A, 30, and 31

tubes. Socket No. 2 takes the 32 and34s, while Socket No. 3 is used for33s. The tester should be calibratedwith new, good tubes. All tubes aretested first with switch Sw. 1 closed andSw. 2 open. A good tube will show afluctuation upon opening and closingSw. 1. The screen -grids are tested thesame way-but by opening and closingSw. 2 (See Fig. 2.)

MATCHING PUSH-PULL TUBES

By E. J. Christians

PUSH-PULL tubes must be matched,in order to obtain good quality, lackof hum, and plenty of pep. A perfect

FIG. 3-Illustrating procedure for matching push-pull tubes.

match will be shown by zero readingwhen the voltmeter is connected be-tween the plate prongs of the outputtubes. If meter reads backwards, re-verse the connections. A reading ofmore than one volt indicates that a newtube should be put in. (Fig. 3.)


A TUBE SHORT TESTERBy A. E. Ellison

THIS is a worthy addition to anytube testing equipment, and is very sim-ple to make and use. As may be seenfrom Fig. 4, the switch is the heart ofthe tester and will have to be home-made. Any 7 -point switch will serve,but the arm must be replaced with a7 -contact fan, as shown. The seventharm is of bakelite, with a contact onthe end, which is connected to the cir-cuit with a flexible lead.

eakwiloiclWarneMEmmNWIIM=MIMs, 11141

(DOOM CONUCI TASTETNOSaVe TO

OK NTT!

WICK WM. LL( StetTeorTNO.4 h$ soolTert T,Tom.. Oops VaotIC.

FIG. 4-A tube -short tester.

In operation, the switch arm is re-volved rapidly over the contacts, ashort showing up by a flash of the neonbulb.

USE FOR OLD TUNING EYETUBES

By Roland T. SchaeferOLD tuning eye tubes that no longer

can be used as indicators but in whichthe triode section still checks O.K.Most Servicemen have a few suchtubes in their shops. Types 6E5, 6G5,6T5 and 6U5 are more common.

These tubes can be used as audio am-plifiers with quite a bit of gain. Thecircuit in Fig. 5A is an example. The

INPuT e4R.

/Low

MEG.G

0

A V CVOLTAGE

2500 MMF.

OUTPUT

01-MF.

0.25-MEG.

1'

0.25-MEG.

552.0004%.

MEG PAP T. TARGET10

FIG. 5-Uses for old "tuning eye" tubes.target should be grounded when notused.

Another example, Fig. 5B, is forusing the tube as a 2nd -detector in asuperhet. In this case the target isused as a diode. This works almostas well as a type 75 tube.

INDICATOR FOR CONDENSERANALYZER

By H. A. WirschingI SHOULD like to contribute the

following to the Service Men who builtor bought the Thordarson CondenserAnalyzer as I know that the phonemethod of minimum hum was, and is,tiresome to use.

I conceived the idea that the "eye"tube (6E5 or 6G5) might take the placeof the phones and found that it worksvery well, and is very compact andlight as I use a 25Z5 in a voltage -doubling (transformerless) circuit. Itrust that others may find this an easierway to balance the bridge. See Fig. 6.

NEONTUBE

EXTERNALSTANDARD

a POLARIZING 115V.VOLTAGES AL

FORE LEC TROLY TICS

°°,,AF5- 155-MF

5 MF(4 +

1 MEG.UNKNOWN

TEST

ORS6E5

- - -PHONES

101-MF

TOP VIEWOF

SOCKETSRESISTOR

F

Lti

115vACDC

8MF

200v

STANDARD 25Z5 VOLTAGE -DOUBLING CIRCUIT "-

FIG. 6-Circuit for use of "tuning eye" as indicator for condenser analyzer.

CHAPTER VI

VOLUME CONTROL METHODS

A VOLUME -CONTROL METHOD --By Vincent Campbell

RECENTLY I decided to build afour -tube regenerative receiver which Iknew would give me excellent volumeand tone. But here arose the problemof obtaining a real volume control. Ofcourse I tried the antenna control and"B" lead control, but all I could obtainwas a continuous whistle. After sometime, I finally hit upon the method tobe described which gives control rang-ing from the merest whisper to terrificvolume.

The section outlined in dotted linesin Fig. 1 is the control. I have used aCentralab 500,000 -ohm modulator anda .02-mf. fixed condenser. The positive"A" and negative "B" lead connects to

FIG. I-Design for a good volume control.

the center post of the resistor; the con-denser on the end of the resistor to thegrid of the R.F. coil, as shown.

You can pass this on to anyone whowants a good volume control. (SeeFig. 1.)

AN IMPROVED VOLUMECONTROL

By Russell L. Wooley

AUTOMOTIVE radio receivers have,usually, a 50,000 -ohm potentiometer,shunted across the 67/2 -volt section ofthe "B" supply, to vary the screen -grid

30,000OHMS

90.10Frua

TO SCREENGRIDS

Istiolour

-1111.111111181111111-4111011111-7.-

23.000OHMS

-WET

TO SCREENGRIDS je

15.000,"OHMS -IMPROVED NIETN00-

- USUAL METHOD - TO POWERTUBES,asty.A. RANGe

LAUD'TO R3WER.

TUBES

111111111-`"

FIG. 2-Just as tuning is spread by adding anothercondenser. so volume control may be made less

critical by a series fixed resistor.

voltage. This is a good method ofvolume control; but the usefulis limited to about half the arc of theslider's movement. The result is thatthe change in volume from "soft" to"loud" is very abrupt.

This method of volume control maybe improved by using a fixed resistorof, say, 25,000 ohms value, connectedin series with a variable potentiometerof 25,000 ohms; the useful range isthereby spread over the entire arc.

These particular values do not, ofcourse, hold true in every case; condi-tions, such as the sensitivity of the re-ceiving set, the field strength of localstations, the number of screen -grid R.F.stages, and the screen -grid voltage, de-termine the value of the potentiometer.The total resistance, however, should he50,000 ohms.

Inasmuch as a volume control of thistype is shunted directly across the "B"supply, a switch should be included, todisconnect it from one terminal of the"B" battery when it is not being used;for otherwise it wastes current. (SeeFig. 2.)


TONE CONTROL BOX

THIS may be made as illustrated inFig. 3. The pictorial schematic indi-cates a fixed condenser of .02 mf. ca-pacity in series with a variable re-sistance. This resistance must he noise-less.

Many dynamic loudspeakers soundharsh, due to the presence of verystrong high frequencies. The moderndynamics have this tendency the least,as they ordinarily have a high frequen-cy cut-off obtained either mechanicallyor electrically. The Tone Control Boxillustrates one way of doing it electric-ally when the loudspeaker does notcontain such compensation.

The exact dimensions of the com-pleted unit will depend upon the partic-ular parts used.

o 500 000SottOICE

FIG. 3-The circuit of the portable tone -control box.

As the resistance is lowered, the highnotes are by-passed more and more,with the final results-very drummy re-production.

The unit connects between the radioset and the loudspeaker. The loud-speaker plugs into the jack and theTone Control Box plug is plugged intothe jack on the radio set; of course, theplug is not necessary if the receiverhas binding post provision for theloudspeaker cord tips. Two bindingposts may he used on the Tone ControlBox in lieu of a jack.

FIG. 4-A novel tons -control circuit.

NOVEL TONE -CONTROL CIRCUITBy Lansing L. Post

FIG. 4 shows a circuit of a variabletone control. The value of the poten-tiometer is 0.5-meg. while the condenservalues can be arrived at by experiment,to suit the builder.

I find that shifting from "highs" tobass is gradual instead of suddenly aswith a single condenser and variable re-sistor in series.

REMOTE VOLUME CONTROLBy Quentin Achzehner

IT is very handy to have a remotecontrol for the radio receiver situatedat the telephone or any other point,so that the control on the set may becut out at will and the remote con-trol substituted. This is easily done by

FIG. 5-A remote volume -control arrangement.

the use of the circuit in Fig. 5. A vol-ume control of the same type as thatused on the set is needed, as well asa D.P.D.T. switch (to select either vol-ume control).

rT,

CHAPTER VII

AMPLIFIERS AND PHONOGRAPH REPRODUCERS

EMERGENCY CLASS ABTRANSFORMER

By Lando K. Meyer

THE power transformer from almostany make of old "B" eliminator, willserve as a very efficient input transform-er for a class AB amplifier. The trans-former must, of course, be one that has

FIG. I-Emergency transformer for Class AB ampli-fiers.

a center -tapped high -voltage second-ary. Those taken from a Majestic Su-per "B" work very well. The second-ary has ample carrying capacity forthe grid current on positive peaks.(See Fig. 1.)

A MOVIE ADDRESS SYSTEMBy Russell L. Woolley

A "TA LKI E" operator uses the meth-od shown herewith, to make short an -

MIKE

GVD.0

AMPL FIER OF L SETA F.T. 1 '27 A.F.T. a '4

DOUBLECIRCUITJACK

'8+PWR

SPEAKER

PiC K UP...1%W-UNIT

MIKE 1RAOR 2 AUDIO

TO THEATRE TRANS INSOUND AMPLIFIERS SERIES

\ BRACE

` 2"K4"SUPPORT

FIG. 2-The mechanical coupling ofmagnetic speaker gives an input

amplifier.

a pickup to afor an audio

nouncements over his Western Electricsound equipment. All that is needed todo this is a portable broadcast set, with adouble -circuit jack connected across thegrid -circuit of the first audio stage ofthe receiver; a good "mike," a center -tapped transformer; and four dry -cellbatteries. Two old audio transformers,with their windings connected in se-ries, may be used in place of the mi-crophone transformer. The broadcastreceiver acts as a speech amplifier; thespeaker-a magnetic one-is mountednear the pick-up unit of the "talkie"system. When it is desired to make anannouncement, the needle of the pick-up is centered on the diaphragm of thespeaker unit. (Fig. 2.)

A RADIO -PHONOGRAPH KINKBy Louis B. Sklar

HERE is a very novel scheme ofplaying the radio and phonograph si-multaneously. Anyone having an elec-tric pick-up and a phonograph can per-form this stunt without difficulty, asshown above. (Fig. 3.)

Dow ioN of ItymE MMROTATION NEEDLE

ritr-uP .EERIE

1RELI RECORD

FIG. 3-This us. of the same disc for radio andphonograph at once is not only amusing, but a test

of quality.

Looking at the layout, you see thatthe pick-up needle is at one side of therecord; while the tone -arm needle isexactly opposite. When the recordstarts to rotate, music will be heardcoming from the phonograph as wellas from the radio speaker. The musicfrom the two speakers v*;11 be slightlyout of synchronism, because the twoneedles are not on the same point ofthe record; even though they are placedin the same groove. This produces aneffect as if one instrument were playingand the other accompanying it.


A LOW-COST MIXERBy C. M. Dibrell

BY this means, several sources ofinput may be mixed and any may bevaried without disturbing the others.Ordinary 0.5-meg. potentiometers areused for the individual volume controls.The cost of the extra tubes and asso-ciated equipment is less than that of theexpensive, constant - impedance typemixers that are ordinarily used. (SeeFig. 4.)

FIG. 4-Circuit of the low-cost mixer.

HOME-MADE PANEL RACKBy Robert Duncan

IN building some radio equipment Ihad need for a relay rack. As com-mercial racks were so expensive, I de-cided to build my own.

Two old bed rails were obtained fromthe junk yard and cut to the desiredlengths. They can either be bolted orwelded together in the form of a rack.The finished product makes a very stur-dy and efficient rack.

The general idea is illustrated in

Fig. 5.

FIG. 5-A trip to the junkyard, and the use of thikink, saves dollars.

CHAPTER VIII

POWER SUPPLY EQUIPMENT

AN EMERGENCY BATTERYBy John J. Nothelfer

RECENTLY, the writer was calledout of town to service a battery -modelconsole radio set. Upon arriving, aday ahead of the promised date, I foundthat the storage battery had been takenaway to be recharged, and it would bereturned early the next day. The ideaof coming back the next day over therough country road was unpleasant;and that of using the car battery seemedthe solution.

Upon trying to loosen the clamps onthe battery, it was found that they weretoo tight; the pliers would never loosenthem, and the required wrench had beenleft home. Having a roll of No. 14rubber covered lead-in wire, I drovethe car as close as possible to the win-dow nearest the set; and the wires wereconnected to the battery terminals. Inthis manner six -volt direct current wasobtained, and the set was tested andrepaired in the usual manner.


EDISON "B" BATTERIESBy Chester Rector, W9BFW

REBUILDING a storage "B" bat-tery of the Edison type has several ad-vantages: first, the investment is small;second, no destructive acid to spill;third, the nickel steel elements are non -sulphating and last indefinitely; and,fourth, the Edison type is lighter thanmost others. These batteries hold thecharge for a long time when not inuse. If one is not at hand, it can beobtained from a radio store very cheap;and can be made to give very good ser-vice by the following method:

Carefully remove all of the elementsand test tubes from the rack, and cleanthoroughly. After they are completelycleaned, lay them out to dry. Fromthe drug store purchase two pounds ofpotassium hydroxide, 5 lbs. distilledwater, and 1 oz. mineral oil.

Mix in an earthenware or glass bowlthe distilled water and potassium hy-droxide until there is a reading of 1,250in a clean hydrometer, one that has notbeen used for testing acid batteries. Atthis stage the solution will become quitewarm; so it should sit until cool, beingtested again for correct reading. If thereading is higher or lower than 1,250,distilled water or potassium hydroxideshould be added as required. Leave thesolution to settle, and then filter.

000epoi3c3c0oos0000000000000,000000e000000.e000000,1000000000et)avoev00100 0E9)410

000000

B 50 100

POS.

SEP.

TUBE- NEG.

FIG. I-The storage "B" bakery Is much favored InEurope OM. Many old ones may be picked up here.

Next assemble the battery, as in Fig.1. Fill each cell to within one inch ofthe top, by using the hydrometer. Theplates will soak up some of the solu-tion; so the process should be repeat-

ed. With a clean medicine dropper,place about three to four drops of themineral oil (liquid paraffin) upon thetop of the solution in each cell.

The charging can be done from a 110 -volt A.C. line, by the use of a tricklecharger. However, if a 32 -volt D.C.lighting system is available, the batterycan be charged this way, after it isdivided into 25 -volt sections; of course,no rectifier is necessary.

The battery must be tested with avoltmeter since the specific gravity re-mains the same whether it is chargedor discharged. The solution should hechanged about once every year when inuse; and the level should be kept upby adding distilled water.

Two of these units make a very goodplate supply for the beginner's ama-teur transmitter. Also, this type ofbattery works very well on any radioreceiver.

A 6 -VOLT BATTERY FOR 2 -VOLTTUBES

By Audie RobersonNO doubt the best way to furnish

power for the 2 -volt tubes is by usingthe Air -Cell battery, but a great manypeople have an old 6 -volt storage bat-tery and are reluctant to throw it away.They may easily be converted to 2 -voltbatteries and I believe that it is econo-mical to do so.

The first operation is to saw the con-necting bars, as shown in Fig. 2. Thecenter cell is then raised and its posi-tion reversed; when placed back, it will

DRILL HOLE FOR BOLTBEFORE SAWING 's'

w- NUT

., diiiiit.sVIEW AT A", VIEW AT 11-

SHOWING HOW SHOWING NowTO BOLT STRIP SCREW IS PLACED

TO TERMINAL IN HOLE

FIG. 2-A conversion that certainly makes for econo-my. Old 6 -volt storage batteries may be rebuilt asdescribed, for use with new 2 -volt battery tubes


appear as shown. Now procure twostrips of lead connectors that will justreach across the battery and bend oneend so that it will fit as shown. A holeis then drilled through the connectorand the battery post so they may hesecurely fastened.

Now as near as possible to the endsof each cut connector bars, drill a hole.The bars are then bent upward untila bolt can be inserted and then bentdown with the end of the connectorbar fastened to the battery post asshown in the sketch. Two more holesarc drilled to correspond with the holesin the cut connectors which arc thenbolted securely as shown. It is wellto sand -paper each connection beforetightening so that the very best con-nections can be obtained.

HOME-MADE BATTERY CABLEBy M. W. Johnson

MOST battery sets employ for thebattery leads a cable, one end of whichis soldered to the receiver. This ar-rangement therefore presents severaldisadvantages.

For one thing, it is not convenient tomove the set to a position beyond thelimit of the cable's slack, without firstremoving all the battery connections.Also, it is inconvenient to make testson the receiver without the battery po-tentials, unless it is desired to removebattery connections.

UN' TUBSOCKET-

BelS%

F G. 3-The set builder who is working over an oldbattery set can use the connecting method shown toprovide a 5 -lead cable which will facilitate connect-

ing and disconnecting.

Electric sets and the more expensivebattery sets overcome this situation byarranging the current -supply cable toplug into a receptacle on the receiver.

This idea may be applied to any re-

cciver requiring not more than five leadsin a single cable, as shown by Fig. 3.A 5 -prong UY socket is mounted onthe rear of the set cabinet, and the cab-inet ends are soldered to the prongs;while the battery cable is connected tothose of the old tube -base. The cablemay be held in place with sealing waxpoured into the ex -base. If desired, an-other plug and socket may be "riggedup" for the opposite end of the cable.

CONNECTIONS FOR 110 V., D.C.By Oscar Block

IN those districts where the "'lam"is accursed with D.C. mains and at thesame time uses all .25 or all .3 -amp.type tubes, either one of two methodsof connecting the filaments may beused, as shown at A and B, Fig. 4.

Circuit A has the advantage of lowcurrent consumption, but has the dis-advantage that the "B" battery poten-tial on each tube will be low. It isalso not adaptable to push-pull circuitssince this type of hookup requires aparallel connection of the filaments inorder to use conventional push-pulltransformers.

LFIG. 4-"DC"-tuba filament connections. Circuit Aconsumes little filament current, but "8" potentialis low: that of 8, results in high "A" current and

"6" potential, and; C, a safer arrangement.


Circuit B illustrates a series -parallelconnection of filaments. This mode ofconnection has none of the disadvan-tages inherent in the connections of Fig.4A, although the current drain is great-er. It has the distinct disadvantagethat if one of the tubes is removed, theincrease of current through the tubewith which it is in parallel is apt toburn it out. Figure 4C shows a circuitthat eliminates this latter difficulty andat the same time retains all of the ad-vantages offered by the connections ofFig. 4B.

In this case, since each branch ofthe filament circuit and its regulatingresistance is independent of the otherbranch, any fluctuation in one will notmanifest itself in the other. A sug-gested layout for the power unit of aD.C. set is depicted in Fig. 5. This lay-out is designed for .25 -amp. tubes andis intended for those of '71A type. Thepilot light should consume 25 -ampereat 2.5 V.

R.FI DET DUSK -PULL AF

PILOT'01 A '01 A 71 A

LIGHT

R2t10OHMS

GRIDRE-

TURNA.F.RF 1RE 2 R F.2

'01 A

BRANCH 1

350 20OHMS OHMS /

R

350 20OHMS OHMS/

BRANCH 2/

155.A.F. PUSH-PULL'01 A A.F 2 '71 A 6V., S -AMP

DYNAMICSPEAKER

FIELD110 V., D CLINE

FIG. 5-Final connection of the series -parallel circult C, Fig. 4.

If tubes are used with characteristicsother than those assumed, then the val-ues of R1, R3, and the grid returns ofthe various tubes, must be changed.The variable 20 ohm resistors areplaced in the circuit to compensate forline voltage irregularities. They are tobe adjusted until the voltage across thetubes is at its rated value.

It would be interesting to determinejust how the various values of resist-ance used in this connection are de-termined.

First, it is known that the line volt-age is 110, and second, that the term-inal voltage of each tube is 5.0. Thepilot lamp requires 2.5 volts for oper-ation. Each branch of the circuit hasthree tubes in series, and since eachtube requires 5.0 volts, the three tubesmust have 15 volts. This voltage, whenadded to the 2.5 volts of the pilot lamp,gives a total of 17.5 volts, which, whensubtracted from the 110 volts of thesupply line, leaves a remainder of 92.5volts across both R1 and R3.

Now the current through eitherbranch 1 or 2 is the filament currentof the tubes, which in this case is .25 -ampere. The resistance of both RI andR3 is then computed from the formula

E 92.5R =-=---= 370 ohms. In order

I .25to provide a means of adjusting thefilament voltage, 350 of the 370 ohmsare made fixed and the remaining 20ohms variable. The computation is ex-actly the same for either branch.

"MODERNIZING" WITH ATRICKLE CHARGER

By John J. NothelferYOU may be interested to learn how

I adapted the transformer of an oldPhilco trickle charger as the filamentsupply unit for a type 45 power tube.Naturally, any tube having a 21A -voltfilament may be heated in the samemanner; and, by changing the numberof secondary turns, a PA- or 5 -voltfilament.

The case is removed from the trans-former and the laminations pulled out.The secondary windings are then re-moved, and the core rewound with No.16 S.C.C. wire (As different makes oftransformers vary in wattage output itis difficult to state the exact number ofturns to be wound.)

A simple test of correct number ofsecondary turns is as follows: afterwinding what is thought to be the cor-rect number of turns for the secondary,(perhaps 10 to 15 turns, for a 2,/2 -voltwinding; other sizes in the roughly ap-proximate ratio of 5 turns per volt),replace the laminations and connect thetransformer primary to the 110 -volt


/

43

FIG. 6A and 68-Left, rewinding a power transformer. Right, first replacing the laminations and finally,the completed instrument, looking as good as new.

A.C., line. Now, connect an A.C. volt-meter across the new secondary, andnote the output.

If an A.C. voltmeter is not available,an old '99 -type tube may be broughtinto service as a visual indicator of theapproximate output of the supposed21/2 -volt secondary. If it glows dullred when shunted across the secondaryleads, add turns; if very bright, removeturns. The '45 tube filament burns witha dull red glow on 2/2 volts.

Before connecting the transformer in-to the operating circuit, check both pri-mary and secondary for shorts and,more particularly, grounds to each otherand to the core. The latter should beinsulated from the secondaries andgrounded.

Three of the stages, winding the sec-ondary, replacing the laminations, andthe finished assembly, are illustratedin Figs. 6A and 6B.

D.C. FROM YOUR AUTOBy Joseph Riley

A RECENT news item stated that ayoung man interested in radio had mar-ried a young woman also interested inradio. They made a special five -tubeset, for use during their honey -moon,to be operated by the storage "A" bat-tery in the automobile they owned.

Anyone can operate their receiverthis way by following the idea illus-trated in Fig. 7. Any burnt -out "bay-onet -base" lamp may be used. Theglass part is broken out and two leadsare soldered to the contact points. There

are two types of base; single -contactand double -contact; use one which prop-erly fits the particular outlet you wantto take the six -volt supply from.Caution: Examine one of the autolamps and make certain that the D.C.supply is not 12 volts.

A wooden handle is fastened to thelamp shell; this makes it easy to re-move the current tap and replace thelamp. It is usually most convenient totap the current at the instrument board.

The amount of current consumed bythe average five -tube set in an hour isabout one and one-half amperes; theautomobile lamps probably consumethree to ten amperes. So, the compari-

F G. 7-A simple tap fo the automobile storagebattery will operate a receiver efficiently.


son indicates, there is no objection onthe score of undue battery drain. Infact, the starting motor will probablydraw as high as 300 amperes (instan-taneous value) for the few seconds itis on during starting.

If a two-color cord is used for con-necting, it will be easy to distinguish"A" positive from "A" negative, byusing a red lead for the former and ablack wire for the latter.

Of course, it is necessary to have theright connection when the plug is madeup; but, as the sockets will probablyall be connected the same way, the "A"polarities to the set will not be reversedif another socket should be tapped atanother time. Usually, the shell of thesingle -contact base will be negative andthe contact positive; a simple test forthe double -contact base is to connect theplug "A" leads to the set. If it works,the connections are correct; if it does-n't, the "A" connections are reversed.

A clever arrangement would be to in-stall a set of "B" batteries in the carand wire them to an outlet on the in-strument board, to be tapped with an-other plug. Be sure these "B" volt-ages are by-passed inside the set.

A TEMPORARY FUSEBy J. B. McGirt

TH E writer ran across this "kink"when the new Crosley screen -grid mod-els first came out. The sets were equip-ped with a 1 -amp. fuse and frequently,when the radio was first put in use,the fuse would blow; whereas the fusewould hold if the heater -type tubescould be brought up to operating tem-perature. To keep from blowing somany fuses, I resorted to a tinfoil fuse.

Take a piece of tinfoil on paper (suchas ,you find around chewing gum and

FIG. 8-Emergency repair of old fuse.

cigarettes) and cut it to a size that willjust about go around the old fuse tube.Next, cut away a portion of the paperand tinfoil; and your fuse then shouldlook like the one illustrated. (Fig. 8.)

Ordinarily, the least amount of tin-foil that can be used is about right; al-though it can be made to fuse at high-er current levels by making the con-ducting strip wider.

CHOICE OF RECTIFIERSBy Edward J. Arnold

HAVING use in the shop for apower -supply unit capable of usingeither an '80 or a "BH" gaseous recti-fier tube, the writer evolved the circuitshown in Fig. 9. It may be of interestto others who can utilize the idea.

FIG. 9-Two different type of rectifier tubes can beused in this way.

The sockets should be labeled plainly:as it will be exceedingly deleterious toan '80 tube to insert it in the socketintended for the Raytheon.

A.C.-D.C. "B" POWER SUPPLYBy Oliver H. Smith

"NECESSITY is the mother of in-vention." A small amount of 90 V."B" was needed. An inventory of the

FIG. 10-A novel A.C.-D.C. "8" power supply, usinga filamenttype tube as rectifier.

PRACTICAL RADIO KINKS AND SHORT CUTS as

available junk box showed no trans-formers, not even for filaments. Theaccompanying diagram shows the exact"B" eliminator that was rigged up. An01A, 71A, or similar tube that draws1/4-A. filament current would give moreoutput. The less said about the effi-ciency of this circuit the better, but itis simple and served the purpose. (SeeFig. 10.)

1 have never seen a similar hook-upusing a filament -type tube.

HIGH -VOLTAGE POWER SUPPLYBy Hubert Bussard

"HAMS" and experimenters oftenwant high plate or D.C. voltages. Thiscan be supplied by using ordinary goodbroadcast -receiver power transfomers,wired in a bridge circuit which, in con-junction with 83s or some other mer-cury-vapor rectifier tube, give a D.C.output nearly equal to the A.C. voltageof the power transformer. Now, tofurther increase the voltage, wire thehigh -voltage filament of the bridge rec-tifier to center -tap of next power trans-former. You will then have the volt-age of No. 2 added to No. 1. You canmake two bridge circuits like No. 1 andget still higher voltages, wiring themin series, of course. All filament wind-ings to rectifiers must be separate.They can be part of the same powertransformer, but separate filament trans-formers would be less likely to haveany breakdown trouble. By these cir-cuits the voltages are raised but thetransformers are not overloaded. (SeeFig. 11.)

'-110YA.C.

k BLEEDER-,RESISTOR1Pi kilL

0 -T100A C.

FIG. 11-Circuit of novel high -voltage power supply.

INCREASING VOLTAGE OF FULL-WAVE RECTIFIER

By Roy Hockin

THE usual full -wave rectifier circuitis shown at Fig. 12A. Occasionally ahigher voltage than this affords will beneeded while at the same time retain-ing the advantages of full -wave rectifi-cation. This may be accomplished byusing the circuit at Fig. 12B, where theoriginal equipment is all used, thechanges allowing an increase in voltageof about 90 V. The resistors addedshould be of at least 5-W. rating, andmay have a slightly different resistancevalue than shown, as for example 10,000or 15,000 ohms, but both must be of thesame value.

110 U,A C

k.SLOIOSISLQIL

.-110V., A C

Pill

0 0 0 ,

(NOTUSED)

12.000,OHMS5 w.

or

12,000ONMS5W

.5.FIG. I?. left-Usual full -wave rectification circuit is at left: improved version at the right.

CHAPTER IX

COILS AND TUNING CIRCUITS

MEASURING THE FUNDAMEN-TAL WAVELENGTH OF A COIL

OR A COIL AND CONDENSERBy Melvern H. Berry

EVERY Radio Fan has known of atime when he would give a portion ofhis anatomy to have some means totest the fundamental wavelength ofhis coils before placing them in his setto see what they would do. Of course,some approximate idea can be obtainedfrom tables and the use of formulas,but for real accurate results it is im-possible to obtain the information with-out some laboratory testing apparatus.

Most every radio fan has an oscilla-tor and a wave meter. If you have acalibrated oscillator, it will be muchbetter.

A neat and accurate galvanometercan be made for a few cents by wind-ing a few turns of fine wire on a formone inch square. Hang this solenoidbetween the poles of a horseshoe mag-net, allowing the ends of the coil tosupport it.

Place the galvanometer across theends of the coil to be measured (Fig. 1)and bring the coil in close proximityto the coil of the oscillator. Rotate thedial of the oscillator until there is re-sonance between the two coils. Whenthe wavelength of the oscillator is thesame as that of the coil to be measureda current will be noted to flow in the

OSE IL L 01012

COIL AND COND.TO ISE MEASURED

FIG.2

galvanometer. The indication may bevery slight. The amount of deflectionof the galvanometer depends upon itssensitivity and the coupling of the twocoils. If your oscillator is calibratedthe fundamental wavelength can beread right from the curve.

If your oscillator is not calibrated itwill be necessary to employ a wave -meter to get the wavelength of yourcoil. This can be done by bringingyour wavemeter (absorption type) closeto the coil to be measured, and when theresonance of the meter is adjusted tocoincide with the frequency of the os-cillator there will be a second deflec-tion of the galvanometer towards zero.The actual maximum fundamental canbe read directly from the meter or fromthe curve.

When using as a test for the wave-length of a coil and condenser combina-tion, refer to Fig. 2.

It will be necessary to put a radiofrequency choke coil in the lead from"B" plus to plate. Otherwise, the bat-tery resistance would probably be solow there would not be sufficient radiofrequency voltage drop to cause cir-cuit oscillation. This choke may beof any convenient size. On an ordin-ary thread spool, 250 turns of 30 to 36gauge wire with almost any kind ofinsulation would work well.

It must be noted that incorrect coup-ling of the absorption circuit to the cir-

WAVE METER

COIL 10 NEMEASURED

CO2 vcD

25 OHMS

OSCILLATOR. A

FIG.1.

FIG. I and 2-The two schematic arrangements above indicate proper connections and coil relations to beobserved when determining wavelength rang* of a particular coil, or cell -and -condenser.


cuit under test will result in doublehump resonance,-the indicating de-vice will register at two different wavelengths quite close together. To rem-edy this it is only necessary to loosenthe coupling until the two resonancepoints merge into one. (It might be ofinterest to point out that this phenom-enon is harnessed to furnish the cir-cuit selectivity of the HamtnarlundHi -Q2 receiver, where the coupling hasbeen reduced to one per cent.)

Instead of the 25 -ohm potentiometerdiagrammed, it is suggested that a300- or 400 -ohm one be used if a bat-tery type tube is used rather than oneof the A.C. type.

Also, the oscillator may work moresmoothly if the grid leak connects tofilament of the tube. The negative "A"is the correct side for an oscillator.

The .002 mf. fixed condenser must beof high grade as the "B" potential isimpressed on it at all times.

It must be remembered that loosecoupling must be maintained betweencoils, if maximum accuracy is to berealized. As it is necessary to have afrequency or wavelength standard fromwhich to work, such a standard mustbe built or purchased. A simple meth-od for calibrating a home -constructedunit is to use the tuned circuit as anabsorption trap in conjunction with aregular radio set, setting the eventualwavemeter so that it "tunes out" thecarrier of a broadcast station usingcrystal control; these stations can bedepended upon to be adjusted to theirassigned frequency. Once the dial read-ings for these frequencies have beendetermined, the rough calibration of a"wavemeter" has been accomplished.Plotting these values on graph paperwill supply intermediate values.

A SEMI -BAND PASS FILTER FORTHE BROAD TUNING RECEIVER

By R. William Tanner

MANY times the Service Man is con-fronted with the problem of increasingthe selectivity of a receiver and, partic-ularly, of one which was manufacturedwhen distance was the main qualifica-tion of a set. He, generally, eithercuts down the length of the antenna

FIG. 3-Two band-pass arrangements which may beapplied to give greater selectivity to a receiver.

system or installs a small -capacity con-denser in the antenna lead. Both ofthese methods increase the selectivitybut reduce the strength of signals aswell.

It was at a time when the writer wasworking as a Service Man that he de-vised a rather unique means of increas-ing selectivity and at the same time,improving the quality. This was ap-plied in the form of a semi -band passfilter. No tiresome mathematics are re-quired to determine inductance andcapacity values. The only parts need-ed are a few feet of No. 26 to 20 cot-ton -covered wire, a small battery clip,and one of the old style variometers(200 to 600 -meter types).

Referring to Fig. 3A, it will be seenthat the variometer is connected in se-ries with the antenna and ground. Acoil L, coupled to the grounded end ofthe variometer, and another coil LIcoupled to the filament end of the firstR.F. transformer, compose a link cir-cuit through which the antenna cur-rents are applied to the grid of the R.F.or detector tube, whichever the casemay be. L consists of 6 turns, fastenedin place in any manner which comes tomind. LI should have 10 turns, tappedevery turn down to 3 (less than threeturns will result in poor quality due toclipping of side bands). This is whatthe clip is needed for; to vary thenumber of turns in order to secure asatisfactory band-pass action. Before


LI is wound, the regular primary orantenna coil (if one is used) shouldbe removed, to eliminate the losseswhich would result from the "dead"coil being in close inductive relation tothe grid coil.

In operation, the number of turns inLI is decreased until selectivity is atits best with a good quality of repro-duction; always remembering that, themore turns in circuit, the less will bethe selectivity and the better the qual-ity; and vice versa. If no shielding isemployed in the receiver, the band-pass effect will not be very pronounced.

If the variometer is not available, acoil and a variable condenser similarto those used in the receiver may beemployed; this arrangement is depictedin Fig. 3B. A variometer is recom-mended; as then the tuning is not socritical as with a coil and condenser.However, when tuning, either may beset at minimum and, after the signalhas been regularly tuned in, varied forbest results.

A NOVEL COUPLING IDEABy Alan Hamilton

THE problem of adding an antennawinding to a single -winding short-wavecoil of the "tube -base" type has been"solved" by most amateurs, who dowithout this coil; with consequent lossof the qualities obtained by this ar-rangement.Fig 5.)

ORDINARY'13- BATTERY

CLIPS

BINDINGPOST -

BASEMOUNTINGSCREW

FIG. 4-A tube -base plug-in coil is easily equippedwith an adjustable winding in this way.

Another solution of the problem isillustrated in Fig. 4; a coil, larger thanthe "tube -base" coil, is mounted slid -ably on two vertical rods, which maybe bus bar. The two clips are of the"Fahnestock" type and may be loos-enned from the rods by pressing; thecoil thus being easily adjusted to anyposition. As the experimenter may de-sire to use this newly -applied coil as aregenerative (tickler) winding insteadof an antenna coil, the desirability ofeasy adjustment is apparent.

The coil when used as a tickler iswired into circuit with an external con-trol of circuit oscillation.

TUBE -BASE TICKLER COILBy Louis E. Fay

IN winding tube -base coils for 80meters or above, it is impossible to windboth the secondary and the tickler onthe outside. A method that I use putsthe tickler on a small tube -base, in-stead of the usual jumble -wound coil.

FIG. 5-A tickler for a UX tube -base coll, with fixedcoupling, is obtained with a smaller (UV) base.

Wind the secondary on the largetube -base as usual. Then take an oldtube, having a smaller diameter base,remove the glass, and contact prongs.This base will fit inside the large basenicely. Wind the approximate numberof turns, place inside the large base andsolder leads to the prongs of the largebase. Vary either the number of turnsor coupling until proper regeneration isobtained. Then pour in melted waxto hold tickler in place. (See Fig. 5.)


HOW TO MAKE FORMS FORWINDING YOUR OWN COILS

By Charles P. Hansen

THE tyro, the fellow just breakinginto the game, can seldom find the in-formation as to just what to do toobtain a certain result, in the best way.It is obtainable only in the "School ofHard Knocks." Occasionally, a bit ofdata will be found which lightens thework of becoming an experienced tech-nician.

For example, below is described themanner in which the writer fabricateshis own coils (generally referred to asbeing of the "solenoid, low -loss" type).Ten "forms," of various standard sizes,comprise the kit of the author.

Shaping the Wood

When making coil forms the first re-quirement is a rectangular piece of wood(Fig. 6). On each end of this blocka circle is scribed. Just how muchoversize this should be, depends uponthe wood -working ability of the con-structor; because these two circles de-termine the resultant size of the form,since the wood is to be worked downto the diameter these circles indicate.Sandpaper the form to the final size.All sharp corners are rounded off withknife and sandpaper.

Dimension A is two inches longerthan the desired length of the windingspace of the form; and dimension Bis, approximately, the desired diameterof the form. Lines C are drawn acrossone end of the block, to show the small-est thickness of the wedge which willresult (as described below); dimensionD is the largest thickness of this wedge.Lines E are then drawn; these being thelines to follow with a saw, to producethe wedge. Holes F, for machinescrews, are drilled now. At this point,saw, knife and sandpaper are calledinto use to obtain the shape shown inFig. 7.

Fig. 8 illustrates the end and sideappearance of the finished form. Flat-head machine screws drop flush withthe surface, and thread into nuts sunkin the form.

FIG. 6, 7 and 6-The various steps for form makingare evident in this drawing.

Making the CoilTo start the coil, wind a piece of

writing paper twice around the form.Hold with rubber bands. Press thumbtacks at desired start and finish pointsof coil. Fasten wire by threadingthrough holes G, and wind.

When wound, coat with collodion,(being careful not to slop it on thewood form). When dry, remove thetwo screws, tap wedge with hammer,and disassemble; and finish by gentlypulling the writing paper from the in-side of the coil.

Coils may be mounted on paraffin -dipped wood strips (w) as illustratedin Fig. 9.

The photograph (Fig. 10) shows analmost -completed space -wound short-wave coil, with holding strips cementedacross the outside. One at a time, dup-licate strips are cemented to the inside

F G. 9--A convenient way of mounting "low loss"coils constructed with the aid of the forms

of Figs. 6-7-8.

50 PRACTICAL RADIO KINKS AND 511027' CUTS

FIG. 10-Coils completed and in the course of con-struction are illustrated above.

when the coil is removed. Use of toomuch cement at this time will cause thecoil to come apart. The "5 -and -10"stores stock tubes of this (transparent)cement.

A UY PLUG-IN COILBy W. G. Ruppenthal

PERHAPS this method for con-structing a UY plug-in coil will inter-est other readers.

The base is cut as indicated in Fig.11 so that it is a tight fit in the fibertube used for the coil form. A circlewith a 34 -in. radius is drawn in thecenter. A horizontal diameter is thendrawn, and where it intersects eachside of the circumference of this circle,a mark is made. From each of thesetwo points, arcs are drawn with 34 -in.radii in order to intersect the circleabove the diameter.

Front the center, a vertical line (90)is dropped until it also intersects the

COIL FORM

3/iA"(BAKELITE

WOOD/ PINMACHINE

SCREW 90*

-UY- INSIOE DI'A

COIL FORM °;OR)1.1L

HOLES FOR COILLEADS

R

FIG. II-Mounting coil -tubes to a plug-in base.

circle. These five points of intersec-tion are drilled for either 6/32 or 4/32machine screws; if 6/32 screws areused, the threads will have to be filedoff a little where they go into thesocket. The manner of winding theforms is left to the constructor.

Small holes are drilled in the baseto let the coil leads out. Holes 1/16 -in, in diameter are drilled through thefiber into the base, 'and wooden pinsarc then driven in and cut off flush tohold the base and coil rigid if thereshould be any tendency for them toseparate.

A FORM FOR COILS

WITH a hack -saw, cut lengthwisethrough a piece of bakelite of the de-sired diameter, as shown in Fig. 12.

The next step is to wind a pieceof stout paper around the (now slotted)tube, and paste it firmly, using care toprevent the paste from sticking thepaper to the tube. (The paper shouldhe spaced about 1V2 inches from eachend of the tube.)

FIG. I2-The method shown makes it possible toprepare coils with lessened dialectric losses.

Wind the wire on the paper, underwhich is the hakelite tube, being care-ful not to wind so tightly as to causethe slot in the tube to close completely.

When dry, the coil is easily removedby sliding it off the bakelite tube afterpressing the tube until the slot hasclosed.

The finished coil may be mounted inany convenient manner; the writerusually bolts two strips together, oneinside and one out, and then fastensthe mounting in the position dictatedby the circuit.


TESTING I.F. TRANSFORMERSBy H. E. Wessel

SOMETIMES a transformer is sus-pected of having insufficient tuningrange to reach a certain I.F. This maybe due to shorted turns or any othercause. Testing such units is very simpleif a workable receiver having the sameI.F. as the suspected unit is at hand.The doubtful transformer is simply con

FIG. 13-Determining inductance of I.F. transformers.

netted in the control -grid circuit ofthe I.F. stage of the receiver as shownin Fig. 13. If the transformer is ingood condition a decrease in signalstrength will be noted as its trimmercondenser is turned. If no change isnoticed, the transformer is not capableof tuning to the desired range, or it isdefective.

NOVEL 3 -CIRCUIT TUNERBy Louis B. Sklar

A DESIGN for a coil of the tunedradio -frequency type and having an ad-justable primary to compensate forvarious lengths of aerials.

That single sentence tells almost theentire story.

For those who want details it maybe mentioned that the construction ofthe coil is extremely simple and it mayeasily be made from odds and endsabout the work -shop. The sketch showsthe coil assembly, but it does not showany specific dimensions, as they arenot necessary. It is only necessarythat the primary coil be small enoughin size to slip easily into and out ofthe secondary.

The number of turns on the secondarydepends of course on the size of thetube on which it is wound and the fre-

quencies which it is desired to cover;these data may be obtained from vari-ous sources. Usually, with these datafor the secondary, the number of turnsfor the primary is also given where theprimary ,is to be wound on the sametube. In this case, however, the pri-mary is wound on a smaller sized tubeand may also be moved away from thesecondary, both conditions reducingthe inductive coupling between primaryand secondary. To offset these condi-tions it is therefore recommended thatabout twice as many turns be woundon the primary as would be the caseif the primary were wound on the sametube as the secondary. (Fig. 14.)

The primary coil is supported asshown by a piece of ordinary bus -bar,which is bent into the shape of a curveand operates through a binding post ofthe thumbscrew type. When the pri-mary is moved as far as possible awayfrom the secondary, it is also then atright angles to the secondary, becauseof the curve in the bus -bar. Thus itis adjustable through the entire rangefrom maximum to minimum coupling,and provides a gradual variation incoupling impossible to obtain with pri-maries of the tapped variety.

The author worked out this designwhen winding the secondary on a 2 -inch tube for the broadcast frequen-cies. A coil similar to the primary wasmounted on the opposite end of thesecondary as a tickler coil, the feed-back control being a midget condenser.The result was a home-made 3 -circuittuner which, in tests, proved to be su-perior to a high priced and well knownfactory made tuner.

THUMB SCREW IN SINGING POST PERMITS CLAMPING THEADJUSTABLE PRIMARY IN ITS MOST EFFICIENT POSITION

SOLDER LUGLOWER SIDE OF SCREW

BENT BUS BAR----.

PRIMARY TERMINALS

go

MOUNTING SCREWS MERLETERM wot.ES

SPACERS CONSISTING OF BINDING POST THUMB RUTSSERVE TO ELEVATE THE COIL SUFFICIENTLY FOR THEWINDINGS TO CLEAR TIER SUB- PANEL

FIG. I4-A novel 3 -circuit tuner.


A SIMPLE WAVETRAPBy C. H. Nason

SERVICE Men at times face the ne-cessity of providing a wavetrap of sim-ple but effective characteristics. Al-though in years past many such deviceswere on the market, they have nowceased to be a standard commodity;and the radio fan or the Service Manmust shift for himself in the construc-tion of a suitable circuit arrangement.

"Just anything" will not serve thispurpose. It is often desirable to elim-inate the signal from a station whichis but a few kilocycles removed from adesired carrier. If this is to be doneit is essential that the losses in thewavetrap be small, so that a sharp ef-fect may be obtained. Figure 15 showsthe schematic circuit of a simple wave -trap employing a Hammarlund "MC"Midget condenser in conjunction withsuitable coil, the specifications of whichare given below. The degree of ef-fectiveness of the device is determinedby the tapped connection to which theantenna is connected; it is necessaryto find the best arrangement in eachcase.

HAMMARLUND HQC - 29 COIL

HAMMARLUNDMC -M

322 MMF

FIG. I5-The combination of low -loss tapped inducfence and variable condenser makes this wavetrap

adjustable for varying conditions.If the entire winding is connected in

the circuit, the maximum effect is ob-tained; but with the possibility of re-moving the desired signal also, if theinterference is from a station on an

adjacent channel. With the antennaconnected so that but a small portionof the wavetrap winding is in serieswith the antenna lead, the sharpness ofthe effect is greatly increased; althoughits magnitude is somewhat reduced.This will allow us to separate stationshaving dial readings quite close together.

In some localities it may be desirable(to prevent picking up the signals ofpowerful locals) to shield this wave -trap; and perhaps ground the shield, asindicated in dotted lines.

When the selector switch is set ontap 1, the wavetrap is entirely out ofthe circuit-except for a slight "dead-end" effect (which cannot be detectedunless the wavetrap is in close induc-tive relation to unshielded coils in theradio set). When the switch is on tap2, only one turn is in use and thewavetrap action is very slight; a goodcondition when trying for "distance,"with just a little cross -talk observablebetween two weak, distant stations.

The maximum effect in the use ofthe wavetrap is observed when theswitch is placed on tap 6; in which po-sition primary LI is not in use andhas a practically negligible action onthe circuit.

Although the coil in Fig. 15 may beused with the old 350-mmf. Hammar-lund variable condenser, it may be usedto even greater advantage with themore efficient "Type MC -M" 322-mmf.unit recommended. This tuning capac-ity shunts a coil L2 having 60 turns ofNo. 22 D.C.C. wire, spaced 36 turns tothe inch, on a form 3 in. in diameter;while in inductive relation to it (insideand at the filament end) is the pri-mary winding LI, consisting of 15 turnsof No. 24 D.C.C. wire, spaced 39 turnsto the inch, on a form of 2-13/16 inchesin diameter. The leads are broughtfrom the 1st, 4th, 8th, and 15th turns.(Of course, these coil and condenserproportions may be varied to suit indi-vidual preference as to parts or con-nections).

CHAPTER X

SHORT WAVES

ADAPTER FOR POLICEBROADCASTS

By P. L. PennockMANY set owners have asked me,

time again, if it were possible to getthe short-wave broadcasts of the localpolice stations on their present broad-cast receivers. This may be done some-times, when the receiver is very closeto the transmitter, which works only alittle ways below the broadcast band;but, as a rule, a short-wave converteror adapter will be required. However,the method described here involvespractically no cost and very little time;though it is true that the arrangementis not very efficient.

1111111111111

WOUND FULL ZI3ENAMELED WIRE

-B-

HOOK-UP OF SPOOL COIL

FIG. I-An additional inductance in shunt across acoil lowers the wavelength, if there is not too much

self -ca pacify.

Procure sonic empty thread spools(of the ordinary type) and, at each end,drill a small hole to the center, just in-side the flange. (Fig. 1A.) Then in-sert one end of a No. 28 enamelledwire through this hole, leaving at leastthree inches for a lead; and wind alayer completely to the other end, pass-ing the lead through the other hole (asat Fig. 1B). One of these inductorswill be needed for each tuned R.F. cir-cuit in the broadcast receiver, whetherR.F. or detector; but none for the an-tenna coupler, if it is untuned.

If the receiver has screen -grid stages,lift the cap of the connecting lead fromeach tube until you can slip a term-

inal lead, from one of these chokes,under the cap and make an electricalcontact with both tube and grid lead.Connect the other lead to some ground-ed point on the chassis or tube shields(See Fig. 1C) and set the spool on topof the regular coil; do this with eachtuned stage. In any stage which doesnot use a screen -grid tube, the spoolis connected between the grid prongof the tube socket and the filament sideof the tuned circuit. These terminalwires should be polished with very finesandpaper, until they will make goodmetallic contacts.

It will be found, when this has beendone, that broadcast stations will tunemuch higher on the dial; and the short-wave stations on police, amateur andexperimental waves will have come upamong the lower readings of the scale.On a Crossley "42S" located here (Can-ton, Ohio), Louisville, Cincinnati,Akron, Cleveland, Richmond, Youngs-town, Detroit and Buffalo have beenheard. I would suggest that the localstation be expected, but the distantones merely hoped for.

A HINT TO SHORT-WAVE FANSBy Wayne Starch

MANY short-wave fans, like myself,may have short-wave sets which tuneup about 150 meters. Probably attimes, they wished that they couldtune a little higher in order that theymight receive broadcasts when theshort-wave stations are not on the air.In my case, it happened that I wanted

FIG. 2-At A, standard short-wave connection. AtII, circuit changes for longer wave reception.


a friend of mine to hear the dynamicspeaker that I was using, but was un-able to do so in view of the lack ofshort-wave stations at the time. 1 de-cided then and there to fix up my re-ceiver so that I would be able to tunein a few of the higher wave broadcaststations.

Instead of winding a new R.F. coil.I obtained an old one from my junkbox (most radio experimenters havejunk boxes) and used its secondary asthe secondary of a new plug-in coil,and the primary as the tickler. Thisidea is shown in Fig. 2. I found itnecessary to reverse the tickler con-nections on the new coil in order thatregeneration might be secured. Theantenna was connected to the "P" ter-minal of the tickler rather than the an-tenna coil as shown.

The type of plug-in system to usedepends on the type that the short-wavereceiver uses, and obviously should bemade so as to fit.

With the size tuning condensers asshown and using a standard R.F. coil,the range of the set was extended upto 345 meters.

A SHORT-WAVE COIL FORMBy Herbert L. DeWolf

HAVING built a number of short-wave coils and giving them a thoroughtest, the writer is of the opinion thatcoils made in accordance with the ar-rangement shown in Fig. 3 work bet-ter than any others.

The idea is to hack -saw supporting

FIG. 3-Another version of the home-made plug -Incoil, with low -loss construction.

strips from an old coil; and fasten themwith machine -screws to a tube -base, andto the R.F. coil by means of top clamps.

The wire for the tuned coil should beabout No. 16 enameled, spaced 1/16 -in.;on the regular celluloid -acetone form.The tickler may be wound at the bot-tom end of the tuned coil; outside ofthe straps; or formed first and cementedinside the coil.

SIMPLE CODE -PRACTICE SETBy Richard Grey

TWO people may practice code inthe same room using a single buzzerand two separate telegraph keys. Nobatteries are required. An o,rdinarybell -ringing transformer is employed.

op KEYS .,

I OOP>A c.110V,

FIG. 4-Simple cods -practice at.When one party is sending, the otherparty must throw his switch in orderto short out his key, and vice versa.Works from the 110 V. 60 cycle houselighting circuits. (See Fig. 4.)

TUNING IN SHORT WAVESBy John C. Heberger

IF one has a modern broadcast re-ceiver equipped with a power amplifiertube and a short-wave set with at leastone stage of audio amplification, foreignshort-wave broadcast stations can betuned in on the loudspeaker of thebroadcast receiver if the two receiversare connected together according tothe simple diagram shown in Fig. 5.

The writer tunes in daily, by meansof this combination, the afternoon pro-grams from G5SW at Chelmsford, Eng-land, with volume and quality equal toa local station. Three stages of ampli-fication are none too many because thelevel of background noise is usuallyvery low on the short waves. Howlingcaused by mechanical feed -back from


FIG. 5-Short or broadcast waves on the L. S. byswitching the S. W. set to the A.F. of the B. C. set.

the speaker may be avoided by using alonger speaker cord or, if necessary,placing the speaker in another room.

Referring to Fig. 5, the lamp cord"A" joining the two receivers can beof any length, and if the sets are lo-cated in different rooms the phones"B." which are left connected all thetime, can he used to find the station be-fore putting it on the speaker.

The switch SW is placed inside thecabinet of the broadcast receiver in anyconvenient position. One side of theswitch connects the output of the short-wave set to the audio amplifier of thebroadcast set; the other side is used fornormal operation of the broadcast receiver. Care should be taken in connect-ing the leads to the switch so that theconnections to the transformer are notreversed when reconnected to the de-tector of the broadcast receiver throughthe switch.

INCREASING SHORT-WAVE SETEFFICIENCY

By W. H. Nilsson

THE writer has been using the sim-ple switching circuit illustrated in Fig.6 for changing from a long aerial to ashort one to obtain better results witha short-wave receiver, the long aerialbeing used only with the broadcasthand coil.

FIG. 7A-Finished ap-pearance of the multi-

ple switch.FIG. 76-Schematic ofconnections for switch

shown in Fig. 7A.

catNOW INk

A0005Mal ANT.

D.T.

LONG UT.

TICKLER

PLUM. Of

BLUE IONSCOIL

6ND.

6

.0004

FIG. 6-With a set covering several wavebands, thisswitch makes it more convenient to change the aerial

coupling.

This arrangement has the merit ofconvenience, and the better perform-ance of the receiver makes it well worthwhile.

A MULTIPLE SWITCHBy W. A. Guider

THE arrangement in Figs. 7A and7B may be made with any number ofprongs from 4 to 7, depending uponthe number of circuits it is desired tobreak. The two sockets are the sameand are fastened firmly together. Theparticular unit shown was made for usein a short-wave set, two sets of coilsbeing made available. The 6 -prongtube base has every other prong cut offfor this use, so that to shift from onecircuit to the other it is only necessaryto pull the plug out and turn it 1/6threvolution before replacing it. Whenused simply to break a number of cir-cuits at once, no prong need be cut off.

SELECTOR SWITCHBy T. H. Caserta

THE switch shown in Fig. 8 may bemade in any type from 4 to 8 contactsby use of the proper type of socket.The top lamination and the washer areremoved from the socket, then the lat-ter is replaced to hold the prongs in


FIG 8-The novel selector switch made from awafer socket.

place. The shaft and bearing plate ofan old volume control are then boltedonto the socket, using spacers betweenthe two. The contact part of theprongs should be filed slightly in thecenter to provide a small groove forthe arm contact to drop into.

A MULTI -TAP SWITCH FROM AVOLUME CONTROL

By Fred BoettscherA GOOD switch may be easily made

from an old Frost potentiometer byremoving the resistance element anddrilling as many circumference holes asneeded for taps.

OLD FROSTPOTENTIO-METER WITH

.4 RESISTANCESTRIP RE-MOVED.

DRILL AS MANY CUT BACKARMASHOLES AS TAPS INDICATED

NEEDED. ROUND-BY DOTTEDHEAD SCREWS GROUND LINES

FLAT, AND CONCAVE.

FIG. 9-A selector switch madecontrol.

from a volume

Remove the contact arm and cut itback as indicated in the sketch, Fig. 9.This will allow the contact point toslide back further to clear the headsof the screws. Stops may be made bybending the small pieces of metal usedin the original control to hold the endsof the resistance element.

Roundhead screws ground flat on thetop (and concave if possible) are usedas taps.

CHAPTER XI

LOUDSPEAKERS AND PHONOGRAPH PICKUPS

MULTIPLE SPEAKERSBy Victor Trad

A SIMPLE and economical methodof wiring every room for a radio -speaker, illustrated here, obviates drill-ing holes through the ceiling or floor;because one wire may be taken up-stairs or downstairs from the outsideof the building. I have used the ideaquite successfully in making such in-

stallations in my neighborhood, andthey are working quite well. The ar-rangement shown lessens losses causedby the use of two wires. (Fig. 1.)

A volume control, and a switch also,may be used in each room, if desired bythe set owner; convenience outlets areobtainable, or ordinary electrical wallThat attached to the receiver's outputreceptacles may be made to serve. Thecondensers used are of 2-mf. capacity.That attached to the receiver's output


RECEIVER I/

RECEIVER -4-. 4OUTPUTNEXTTO ROOMS

TORADIATORIN ROOMST

CI

FIG. I-This method of wiring I speakers to areceiver is not only simple, but profitable.

is connected to the ground post of theset, when the latter runs directly to theradiator.

(The method shown will be of valueto many Service Men who are preparedto add a profitable extra to installationwork. It is desirable, perhaps, to adda caution that the use of speakers inparallel reduces the impedance and,thereby, the total effective output ofthe set to all of them. The pentode,particularly, requires a large outputimpedance. However, if more than twospeakers are required, the use of a

series -parallel connection will restorethe matching of the load to the outputtubes. On this subject, it will be ofinterest to refer to the paragraph"Matching Reproducer Impedance," inthe article on page 727 of Radio -Craftfor June, 1931; while a very large in-stallation is described therein, the prin-ciple is the same.-Editor)

A UNIVERSAL TEST SPEAKERBy R. R. Smith

I FIND this idea useful for auto -radio work.

It eliminates the removal of over-head or separate speaker from the car.

FIG. 2 -A universal testspeaker circuit and me-

chanical arrangement.

The diagram is self-explanatory. (SeeFig. 2.)

The jumpers indicated are equippedwith phone tips.

Several 24 -in. leads are made up usingphone tips on one end and a large orsmall tube prong on the other to con-nect speaker to radio set.

Most speaker sockets have large andsmall holes.

The cable -and -plug shown is usedfor the '35 and '36 Ford radios. Thesemodels predominate in this locality andI use this arrangement to speed upconnections.

The long lead with clip is used on themodels having the "A" lead connectedto speaker plug.

The speaker is a 6 -in. auto -radio mod-el with output transformer and ishoused in a wooden case, 12 x 8 x 6 ins.wide. The back cover is hinged and allcables, leads and diagrams of speakersockets are kept inside.

The jacks shown on diagram aremounted in the top of the box at oneend. In all cases a lead must be con-nected between jack marked Gnd. andradio chassis or case.

Servicemen must judge for them-selves whether their car -radio servicework is sufficient to warrant makingthis unit.

REPAIRING SPEAKER CONESBy John J. Nothelfer

HAVING bought several speakerswith damaged cones (at a good priceand with an idea of reselling at aprofit), I learned, much to my chagrin,

the prices for new cones! The priceswere so high that a loss instead of aprofit would result.

'A' BATTER,/ CLIPI-- SPEAKER SOCKETS (TOP VIEW) SKIS

FORO 7 ARVIN 17 HUDSON m6tC8-6 C.ADILLAC

FEMALE SOCKET i VCFOR FORD RA010 I(BOTTOM VIEW))


FIG. 3-A repair kink for that busted speaker -cone.

The speakers were damaged at thecentering device; this is the case withnearly all damaged cones. I tried re-pairs, and with good success, after sev-eral attempts.

Cutting out the damaged area, Isandpapered the edges of the cut to arough finish. Cutting a piece of paperfrom an old cone, I pasted the patchto the damaged area, with white col-lodion.

It takes a while to dry, and the patchmust be held in place all that while,probably an hour. The ordinary hotiron came to the rescue. After raisingit to a good, hot "heat," I pressed theiron over the pasted area. In a fewminutes the collodion had dried, and itheld as firm as if the patches were met-al and soldered! To prevent the collo-dion from getting to the surface of theiron, a piece of paper is laid over thearea to be heated.

A new centering device should he in-stalled and the cone replaced on thespeaker. The cone is then as good asnew, and no fear should be entertainedthat the pasted patch will come loose.(Fig. 3.)

This idea also works very well withdynamic speakers which have beenpunctured by accidental means; in thiscase, ordinary typewriter paper is used.

NEON BULB USED IN CENTER-ING SPEAKER CONEBy Samuel H. Beverage

HERE is a kink for centering dyna-mic speaker cones. It consists of a110-V. neon bulb in series with thespeaker output transformer primary andthe A.C. line (see Fig. 4). A low-pitched hum will he reproduced. Thecentering screws should be loosened and

DYNAMICSPEAKER

BINDING POSTSFOR

TEST LEADS

FIG. 4-Using a neon bulb as a help in centeringspeaker -cones.

then, with the A.C. ripple still beingreproduced, the centering screws shouldbe tightened. The voice coil will heperfectly centered. This arrangementis much more convenient than the meth-od of disconnecting filter condensersin the set in order to produce hum.

SIMPLE EXCITER FOR SPEAKERFIELD

By H. P. KellyTHIS exciter costs almost nothing

to build and is an extremely handy unitto have at hand. The output voltageunder load of about 80 ma. will bearound 145 V. Only a 40 W. lampshould be used, as this provides the cor-rect voltage drop for the filament of the25Z5 rectifier. (Fig. 5.)

FIG. S-A.C.-D.C. speaker field exciter.


REPAIRING PHONOGRAPHPICK-UP

By Ralph BillsOTHER than actual burn -out, trou-

ble in pick-ups is usually caused bydrying up of the rubber armature dam-pers. Repairs can easily be made withrubber from an inner tube which stillhas good resiliency, and bits from a(transparent) rubber nursing nipple.

RuISSE011ILD4m40.4

mtmxruatCOO.

ItularaPIVOTDAOPER

MAGNET biussDAIMPeWOLOER

004 ,011.1

RUNE

4104AnAte

14111OLI MOLDER SLOT toe Alm.-TLAI ISLOE

NOODLE

eueeen 044.0taNT MOP.. loiraft TUBE

RUOITER PIVOT DELUDERS 1::zg?CUT FSHXC I I le.40401.4 .004LE

of

THE RADIO -VIOLINBy Fess Christiani

RADIO programs may now be re-ceived through the medium of the vio-lin. Find the center of a small magne-tic speaker disc, and solder a smallwood -screw to it at the center. Then

59

FIG. 7-The home-made "mute" is screwed to mag-netic speaker diaphragm, and attached to a violin

as in Fig. 8.

cut a wooden violin mute as shown inFig. 7, and screw it to the speaker dia-phragm. The telephone unit is now as-sembled together with the mute and theentire arrangement attached to thebridge of a violin as shown in Fig. 8.

When a program comes over the ra-dio, one may have the novel experienceof walking about the room accompany-ing it. I play an obligato, and the toneis very clear. There is a great "kick"in it for both listener and player.

mmim11111111111E1===1111111111FIG. 8-The radio violin. The improvised speakerunit, on the violin bridge, facilitates radio accom-

paniments.

CHAPTER XII

TOOLS AND ACCESSORIES

A LIGHT BLOW TORCHBy J. E. Kitchin

PROBABLY some of you ServiceMen with the ever -handy power lines

have never had to solder without asoldering iron. The following, how-ever, may interest those, who, like my-self, are sometimes in a district wherepower is not available.

FIG. 6-Illustrating the method repairing apickup.

Use care, when taking the assemblyapart as the fine wires from the coilbreak off with the slightest pull. (SeeFig. 6.)

GO PRACTICAL RADIO KINKS AND SHORT CUTS

FIG. I-Blowpipes are obtainable from laboratorysupply houses; but Mr. Kitchin takes this ingeniousway to make one of light weight. (The bottle may

be retained as a handle!)

Procure an atomizer, of the type usedfor perfuming, or for spraying the in-terior of the mouth and nose. Removethe cork, to which are attached the rub-ber and metal tubes. Cut off the stem,which dips down into the bottle in reg-ular use, and plug this tight with stop-per S. Cut the bulb off the rubber tubeand, if there is a spray nozzle at theend of the metal tube, remove this.You now have a blowpipe which willdirect a flame with needle -like sharp-ness. Carry this, with a short lengthof candle, in your kit. (Fig. 1.)

AN INEXPENSIVE SMALL -PARTSSTORAGE CABINET FOR

THE AMATEURBy Lester P. Young

MANY experimenters like to "file"their radio small parts. Stationersoften throw away convenient -sizedboxes of the type illustrated; and thewriter finds them handy for this pur-

4

1

SO RDBOX

UHHEDLABEL

PULL" INPLACE!STRIPS

ROUNDfah°PAPER FASTENER.

METHODOF BENDING.-'r

CASE OF SHELVES WITH BOXES IN PLACE.

FIG. 2-The cabinet here shown provides a place foreverything; yet it is made of paper boxes, etc.,which would otherwise be thrown away. Small wood-

en strips serve instead of partitions, as shown.

pose. A couple of dozen of these card-board boxes of varying depth may befitted into shelves spaced to fit the tiers,as they are placed in a wooden pack-ing case of correct dimensions.

To prevent the boxes from binding,and insure their sliding in the properplace, dividing strips made of /-inchsquare wood are nailed to each shelf,between each pair of boxes.

Ordinary round -headed brass paperfasteners may be used to make a prac-tical and good-looking "pull" (Fig. 2.)

CORDLESS SOLDERING IRONBy H. R. Wallin

WHEN one is using an electric sol-dering iron, especially in wiring radiosets, the cord of the iron is usually inthe way; yet at times it is not longenough.

ELECTRICSOLDERING

IRON

PRONGS FITTEDTO BOTTOMOF HANDLE

SPRINGCOVER

SOCKET ANDBOX FITTEDTO UNDERSIDE

OF WORKBENCH

FIG. 3-By inserting the soldering iron into a fixedsocket to keep it hot, it may be used without a cord.

To prevent this annoyance, the con-nection shown in Fig. 3 was used; it isvery simple to rig up. A standard 110 -volt socket is set into the top of thebench, by cutting a hole to fit. A flatmetal -plate cover is set over this hole,to prevent dirt from falling in; a springshould be used on this cover, so thatit will close automatically.

Into the handle of the soldering iron.two prongs are fitted and connected tothe terminals of the wires.


For heating, the soldering iron is setupright into the socket; and whenneeded for use, it is pulled out andbrought to the work. Where continu-ous work is necessary two irons maybe used. The heat will be retained inthe iron for some time.

SOLDERING WITH ELECTRICARC

By Oscar 0. BoucherTHE 5 V. -secondary of a power

transformer from an old radio set isused to supply the energy for the "iron"(which, unlike the usual "copper," ismade of "carbon"!) illustrated in Fig.4. One heavy, flexible lead terminatesin a heavy alligator clip which is usedas the ground connection. The otherlead, which terminates in a holder,fastens to an electrode-a piece ofcarbon rod about 2 ins. long and 1/4 -in.in diameter. (The type of rod usedin arc lamps will do, or it may be takenfrom a flashlight cell.)

FIG. 4-Set-up of the electric -arc apparatus forsoldering.

The clip is fastened to the materialto be soldered and the rod placed onthe part where soldering is to be done,When the part heats up, solder is ap-plied. The iron is particularly handy,since there is no wait for heating upand it is always ready for use.

KEEPING THE IRON CLEANBy Luther C. Welden

KEEPING the soldering iron cleanis half the job of doing a solder job;so it is a good idea to keep two handyaccessories on the work bench for thispurpose.

First, a ,small sheet of rather fine -grade sandpaper; on which flow a smallbit of resin or solder paste and a smallamount of solder (Fig. 5A). Next,

SOLDER1

SANDPAPER,

PASTE

A

CANVAS ,

9 CAN1

61

FIG. 5-The two accessories shown make It moreconvenient to do a good job of soldering.

procure a small round tin can about21/4 inches in diameter and 2 inchesdeep. Take a strip of canvas 21/4 incheswide, or 1/4 -inch wider than the can isdeep. Roll the canvas in a tight rolluntil it will fit snugly into the can, withabout 1/4 -inch extending above theedge; "fuzz" this outer edge. (Fig.5B.)

When the iron is hot, clean the tipby rubbing on the sandpaper; it willbe well "tinned" at the same time, be-cause of the paste and solder on thepaper. Then clean off the surplus pasteby passing it over the canvas pad.

An iron holder made of a strip of tinmay be fastened to the can.

A SOLDERING IRON HOLDERBy Louis Rick

THE idea is merely to connect a 60 -watt lamp in series with the solder-ing iron. When the iron is removedfrom its holder, the contact K closes,short-circuiting the lamp; the full line

-A-

110.V

-B-

SOCKET

TOGGLESwITCH 110V

HOOK FOR SOLDERING IRON

LAMP SPRING BAKELITESPRANG1,1

/11"7771-=

PLUGOUTLET

FIG. 6-A nove soldering Iron holder. A. Ifs elec.trice' circu ; I, mechanical arrangement.


voltage is then applied to the iron.When not in use the iron is placed onits holder, which opens the contact andconnects the lamp in series with theiron; reducing the voltage applied tothe iron. With the usual amount ofuse, the iron is thus kept at a constanttemperature.

At A is shown a schematic diagramof the circuit. The arrangement maybe housed in a box as illustrated in B,(Fig. 6), and then mounted in any con-venient location.

A HOME-MADE ELECTRIC GLUEPOT

By C. M. ParksSOME time ago a glue pot was need-

ed in a hurry. There was none around,so a serviceable one was made asillustrated.

Resistor R may be any unit havingthe required value, which must be de-termined by experiment. If the wateris too conductive, too much current mayflow for the resistor to carry safelyand it will burn out. It is thereforesuggested that a lamp bank be used.

The water serves a dual purpose, asthe resistor required to develop the re-quisite heat, and to isolate from directcontact with a localized heat (knownas the "water -bath" method) which isa requirement for properly heating glue,sealing wax, paraffin and similar plastics.

The rubber washer is required to insulate the inner metal container fromthe outer metal can. (Fig. 7)

FIG. 7-An easily -constructed glue pot, recommendad as an emergency construction design. It may be

used regularly for light work.

The outer container is an ordinarymetal can of convenient dimensions;the inner one is of proportionate sizeand the top is slit to form two earswhich are bent over to rest on therubber washer.

PUNCH AND JIG FOR METALWORK

By Eugene DouglassEVERY set -builder and experimenter

knows of the difficulty encountered indrilling or cutting holes of large diam-eter in the metal sheet that he uses fora chassis base. Obtaining neat holeswas a problem to me until I had madeup the outfit illustrated. The methodis as follows:

Two pieces of flat iron (about 30inches long, Yrinch thick, and two orthree inches wide) are obtained andclamped together, one above the other;so that, after drilling, the holes inboth, pieces will correspond to size andlocation. Now, drill a A -inch hole,about 1 inches from each end of theiron strips; and also, near the centerof the strips, drill holes of the sizes thatwill later be required in the chassis base.Next, remove the clamp that holdsthem together, and bolt them together.Use two A -inch bolts for this, andplace a 3/32 -inch washer at each end,between the iron strips. (See Fig. 8.)

F G. 11-With a set of jigs and punches of this kind,large, clean holes may be punched in fairly stout

sheet metal.

This completes the construction ofthe jig, but you will need punches, tofit the various holes in it; they aremade of round steel and should fitneatly into the holes.


FIG. 9-A, simple eyelet punch used to form ferminals on a coil: B, a cable with neat eyeletted ter

minals.

A CONVENIENT TERMINALBy Russell L. Woolley

MORE and more, pressed eyeletsare being used in the manufacturing ofradio parts. Radio -frequency transformers, R.F. choke coils, connectorstrips, and the ends of the wires of acable are all parts of a radio receiverwhere eyelets may be used to advan-tage. Perhaps the most notable exam-ple of the use of punched eyelets is theA. K. terminal strip.

Now then, the point is that the setbuilder may also use this comparative-ly simple manufacturing process, withpractically no expense; and at the sametime, make a really neat job of his ex-perimental home-made apparatus.

The use of the punch and eyelet kitis illustrated in the accompanying draw-ing. Here the punch pliers is shown,inserted into a hole drilled in the sol-enoid coil form. To use, without re-moving punch from hole, put on theeyelet over the punch-small end down-and close tool. (Fig. 9A.)

A second illustration, suggesting theuse of punched eyelets, shows a dynam-ic speaker's four -wire cable with eye-let terminals. It is apparent that, whenthese terminals are put on a 6/32 or8/32 machine -screw binding post, theycannot come loose. (Fig. 9B.)

RULING PEN HANDY TOOLBy Arthur Bernd

IN putting nuts on bolts in tight cor-ners and down deep in the set, I use a

draughtsman's steel ruling pen, which(as you know) is shaped like a pairof tweezers, with a stud runningthrough to tighten it.

By placing the nut in the ends of thepen and screwing down on the stud, itis easy to set the nut on the bolt; aftera few turns the pen is withdrawn andthe job finished with a "spin-tite" orwrench.

REPAIRING SCREEN -GRID TUBES

By Frank C. Atkinson

DON'T throw away a screen -gridtube if the control -grid tip should pulloff the top, leaving only the lead stick-ing up. Clean out the cap, and aroundthe top of the bulb; clean the end ofthe control -grid wire, and solder to ita short length of fine wire.

Then, procure from a paint store asmall amount of litharge (yellow oxideof lead) and a small quantity of gly-

CLEANTHOROUGHLY

SCREEN -GRIDTUBE.

ADD SHORTLEAD

CLEAN OUTCAP ANDFILL WITHLITHARGE

FIG. 10-The loosened cap of a screengrid tube maybe cemented back into place, quite satisfactorily,

in the manner shown.

cerine. Mix a small quantity of thelitharge into the glycerine, until a stiffpaste is formed; pack the grid cap withthis, and run the control -grid lead ofthe tube through the paste and outfrom the small hole in the cap. Pressthe cap down upon the glass, cleanaway the excess paste; and allow thiscement to set for twenty-four hours.Then clean the cap, and solder the endof the wire to it; and the job is fin-ished. You will find the tube as goodand as strong as slew; I have used thismethod for some time and it has neverfailed me. (See Fig. 7.)


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HUGO GERNSBACKEditor -in -Chief

R. D. WASHBURNEManaging EditorN. H. LESSEM

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Associate Editor

Contributors toRADIO -CRAFT

M. M. BrisbinKendall CloughA. A. Ghirardi

Glenn H. BrowningCharles Sicurenza

Samuel C. MilbourneF. L. SprayberryArthur H. Lynch

A. C. ShaneyBertram M. Freed

J. B. CarterClifford E. Ccnton

Milton ReinerFrank J. LesterJ. T. Bernsley

RADIO -CRAFT MAGAZINE99 -PR HUDSON STREET NEW YORK, N. Y.

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