100
f
POPULAR SCIENCE
RADIOSECANNUAL
PREPARED BY THE EDITORIAL STAFFOF POPULAR SCIENCE MONTHLY
POPULAR SCIENCE PUBLISHING CO., INC.353 FOURTH AVENUE, NEW YORK 10, N.Y.
COPYRIGHT, 1943BY POPULAR SCIENCE PUBLISHING CO., INC.
All Rights Reserved
Printed in United States of AmericaSecond Printing-May, 1944
TABLE OF CONTENTSNOTE-The first page of this book is numbered 101 for theconvenience of those who file their Annuals from year to year.
Midget AC -DC Receiver 101
Book -End Radio for Your Den 104
One -Tube All -Electric Set 107
Superhet for Beginners 108
Pocket -Size Radio Tester 111
"Wireless" Radio Phonograph 112
Low -Cost Home Recorder 116
Tom Thumb Radio 119
Suitcase Phonograph 120
Two -Tube Portable 123
Library -Table Radio 126
All -Purpose Portable 130
One -Tube Receiver 133
High -Fidelity Amplifier 134
Small Radio with 400 -Mile Range 137
Dressing -Table Radio 138
Unit Kills Fading 142
All -Wave Amateurs' Receiver 144
t4 Camper's Radio 146
Television Antenna 148
Universal Power Supply 150
Tiny Radio Uses Two Detectors 152
Portable AC -DC Signal Tester 154
Book -Light Radio 157
Around -the -World Receiver 158
Two -Tube Radio Phonograph 162
Cabinet Ideas 165
Cane -and -Seat Radio 166
Vacation Portable 168
Bed Radio 172
Suppressor Reduces Static 175
Emergency Receiver 176
Light -Beam Transmitter 179
Blackout Receiver 182
Vest -Pocket Receiver 185
Football -Fan's Radio 186
Pocket -Notebook Radio 189
Novel Lamp Radio .. 190
SERVICING YOUR RADIO
Fixed Condensers, Reception, Fade Out, Humming 115
How to Refinish Your Radio Cabinet 129
Faulty Heater, Extending Reception, Noisy Condenser,Midget Circuit 141
Neon Condenser Tester, Defective Resistors, Pilot Light,Fading Reception 156
Battery Testing, Loop Antenna, Substitute Batteries,Loose Batteries 171
How to Correct Dial Troubles 184
,
Measuring just over5" square and lessthan 3- thick, thisreceiver and its 2"speaker cone equalthe performance ofmuch larger sets.The tiny cabinet is
made of wood boundin genuine cowhide
Midget AC -DC ReceiverPULLS IN LOCAL STATIONS AT FULL STRENGTH
Set Has A 50 -Mile RadiusALTHOUGH small enough to slip easily
into a woman's handbag, this midgetthree -tube radio receiver is neverthelesscapable of tuning in all local stations withina 50 -mile radius at full loudspeaker strengthon its tiny 2" speaker, which handles up to%-watt power without distortion.
Front view of panel on which receiver is mounted
Making use of the new tiny electric tubesand iron -core coils, the set pictured on thesepages was mounted panel (nochassis was used) of unusual design andmeasuring only 4114" square. In the upperleft-hand corner a circle 2s," in diameterwas punched out for the speaker. Beside it,on the upper right-hand side, and mounteddirectly on the front panel, is a .00036-mfd,
Close-up of the wiring. All parts are accessible
TWO -GANGTUNING
CONDENSER
.0002. 1
LINECORD
ANT. ANT.COIL
VOL.CONT.30,000w
SW.
200 auLINE -CORD RESISTOR
r zoo'MFD.
2." SPEAKER Y
OUTPUTTRANSFORM
70L7GT
Le,
aLL
250,000W
2000W
1500
.01 MFD.600V.
200 ca)
o rinFD. 25 V.
In this pictorial diagram, the compact placing of the parts of the midget receiver is shown in detail
two -gang tuning condenser. Clamped tothe framework of the tuning condenser bymeans of two 6-32 machine screws, 14" long,is a small bracket on which were mountedthe midget seven -prong wafer sockets forthe tiny RF pentode amplifier ( 9003) andthe high -mu detector triode (9002). An-other bracket was used for mounting thestandard -size eight -prong socket of the
70L7GT tube, which is a combination powerpentode and half -wave rectifier. The midgetoutput transformer, coupling the pentodeportion of the 70L7GT tube to the 2"speaker, was mounted directly under thespeaker.
The iron -core antenna coil was mountedon the two -gang tuning condenser just nextto the speaker. The RF coil was mounted
next to the bracket hold-ing the 70L7GT tube. A50,000 -ohm variable re-sistor acts as a volumecX)03
ANT.COILar GRID
s, IIIAMT. III
II/
ill
CH.
TWO -GANG TUNING COND.
war.FIR 6,
9003
9002
7
25V.
O
MOT usai6
9002
.005w
11C1
70L7GT
20 NW° 6,ur SW.
AC -DC
Complete wiring diagram, including base layouts (at bottom) for tubes
control and was put inplace on the front paneldirectly under the two -gang condenser.
The four -stage TRFcircuit which is used inthis midget receiver con-sists of a tuned RF stagecoupled to a biased de-tector stage, which inturn is resistance coupledto the beam power pen-tode. A half -wave recti-fier furnishes plate volt-age to the tubes, and thisplate supply is amply fil-tered by the 2,000- and1,500 -ohm, 2 -watt resist-
1112
ors, the 20-mfd. and 16-mfd., 150 -volt elec-trolytic condensers, and the .01-mfd. papertubular condenser (400 volts) in the platecircuit of the rectifier.
The new midget tubes used employ mountstructures similar to those of the olderAcorn tubes, but the new ones have glassbutton bases which provide short leads andlow -lead induction. Each tube employs twocathode leads which cause a reduction in in-put loading and provide an increased gain.The single -ended design of the new tubeshas the added advantage of requiring aminimum mounting space. Although thesetubes were designed primarily for use byengineers, experimenters, and amateursworking in the ultra -high frequencies, theycan also be used for operation in a receiver
.designed for service in the regular broad-cast band.
When completed, the receiver and itsfront -panel mounting were placed in a smallwooden cabinet covered with cowhideleather stitched at the corners. The outsidedimensions of the cabinet used were 5 3/16"by 5 3/16" by 21K". A heavy piece of card-board, also covered with leather, was usedfor the back cover. This cover was held inplace by four small brass wood screws. Anopening 214" square was cut in the backcover to make allowance for the necessaryventilation of the tubes.
As this is an AC -DC receiver and there-fore already grounded through the electricoutlet, no further ground connection wasneeded. If a ground should be used it mustbe connected to the receiver through a .1mfd., 400 -volt tubular paper condenser,otherwise a short will occur, blowing out thetubes. Only a short antenna (about 15') isemployed and may consist of strandedS.C.C. wire strung along the floor.
Back view of receiver, wiredbut with the tubes removed.The RF coil mounted on themetal panel by means of a'A" angle bracket, is shownjust next to the 50,000 -ohmvariable resistor. The 9002midget high -mu triode tubeand 9003 pentode are at left
Leather -covered cabinet fromthe rear. The opening at thelower left corner ventilatesthe three tubes and is 2'"4'"
square. The back cover is a
piece of heavy cardboard andleather held by four screws
LIST OF PAR TSCabinet, 53/16" by 53/16" by 2%".Two -gang tuning condenser, .00036 mfd.Iron -core antenna and RF coils.Midget permanent -magnet 2" speaker.Midget output transformer.Midget tubes (two): Super -control pen-
tode, 9003; high -mu triode, 9002.Pentode -rectifier tube, 70L7GT.Line cord resistor, 220 ohms.Variable resistor, 50,000 ohms.Plate cover switch, S. P. S. T.Electrolytic condensers (four): 20 mfd.,
150 volts; 16 mfd., 150 volts; 5 mfd.,50 volts; 10 mfd., 25 volts.
Mica condensers (three): .0001 mfd.;.0002 mfd.; .01 mfd.
Tubular condensers (two): .006 mfd.,400 volts; .01 mfd., 600 volts.
Carbon resistors (seven): 1/2 watt, 20,-000 ohms; 1/2 watt, 250,000 ohms; 1/2watt, 200 ohms; 1/2 watt, 1 megohm;1 watt, 200 ohms; 2 watts, 2,000 ohms;2 watts, 1,500 ohms.
Midget wafer sockets: Seven prong(two), eight prong (one).
103
Decorative and doubly useful, the efficient radio shown above is a worth -while addition to one's desk top
COMBINING usefulness withnovelty, this folding book-end radio provides some-
design of radio cabinets. It is a setthat should appeal to every one who Book -End Radiobuilds radio receivers as a hobby.
The circuit chosen uses four of thelatest all -metal electric tubes andwill operate equally well on eitheralternating or direct current. Al-though not a tuned -radio -frequencyreceiver, this set is just as sensitiveand selective, and is cheaper to buildbecause only one coil and one tuning condens- ance-coupled to one of the new triodes, whicher are needed. A regenerative detector built is in turn resistance -coupled to a 25A6 outputaround one of the new radio -frequency pen- pentode. The fourth tube, the 25Z6, rectifiestodes is used in the first stage, and is resist- the current for the receiver, changing the al -
BUILD THIS
Make the wooden cabinet to dimensions in thedrawing. Note how ends fold in for carrying
RECESSED
FOR BACK
HINGES r
104
An A.C.-D.C. Four -Tube Set
ternating house current to direct However,when plugged into a D.C. outlet, this tubemerely acts as a resistance.
Regeneration is controlled by a 20,000 -ohmvariable resistor, which is connected acrossthe tickler coil. The slider, or arm, of thisvariable resistor is grounded to the chassisthrough a .0005-mfd. mica condenser. A6-mh. choke is inserted in the plate lead ofthe detector tube in order to keep radio -frequency currents out of the audio stages,and also to add to the smoothness of the re-generation control. The grid -lead detection,chosen because of its greater sensitivity overthe usual bias method,consists of a one-meg.fixed resistor in parallelwith a .00025-mfd. micacondenser.
A plug-in coil must beused with this circuitsince a standard A.C.-D.C. antenna coil has notickler winding. This coilis tuned by a .00036-mfd.single -gang variable con-denser. Inserted in theground lead of the pri-mary winding on the coilis a new type of tuningcontrol, which may bebest described as a selec-tivity control. It consistsof one of the new ultra -flat tuning condensers
using molded -plastic insulation, and is mount-ed on the front panel behind the plug-in coil,between the tuning condenser and the 20,000 -ohm variable resistor.
Decreasing the capacity of this condenserincreases the selectivity of the receiver. Atits minimum setting, it will cut down the vol-ume by decreasing the coupling of the pri-mary and secondary coils. This is especiallyuseful when tuning to a strong signal.
A .0005-mfd. and a .002-mfd. mica condens-er are used in the plate leads of the first andsecond audio stages, and help to cut downfeed -back and unwanted oscillation, whichwould cause distortion in the loudspeaker.
To avoid any excessive hum that might becaused by the compactness of the circuit, a
The chassis end of the unit and,at right, the loudspeaker end.Diagram below shows the wiring
rte-
.00042
TMFD.
1/. .00025MFD.
.00036MED.
.0005MFD.
lI16'
MFD.
ON -OFFSWITCH
R.F.C.6 Mh.
000
65J7
0.1MFD
2 MEG.
24 MFR.
500001- MS
.0005 .002MFD. MFD.
2.05MED.
SPEAKE1
0
25iE
2 7
10MFD.
4
25Z6
sY.o...."_
700 16 ma '`,. 4000OHMS MFD OHMS
6;J7 i6F5 25A6 2526A.C.-
. 0 IMP: A F D. mi.
200 OHMS
sh K2BOTTOM VIEW
OF SOCKETS
105
total of 76 mfd. should be used for filteringthe rectified direct current. The filter circuitis made up of four electrolytic condensers,one 20 mfd., two 16 mfd., and one 24 mfd.
The cabinet is easy to build, and may bemade at home from any kind of wood, suchas pine, walnut, or mahogany. After care-ful sandpapering - with the grain, notagainst it-the finished cabinet can be givenone or two coats of stain, depending upon howdark a finish is desired. Once these coatshave thoroughly dried, a thin layer of shellaccan be applied and rubbed down to kill someof the gloss. Rubber feet will raise the bookends and provide room for the speaker cordthat runs from the compartment box at oneend which houses the chassis to the compart-ment at the other end which houses the 5"permanent -magnet speaker. A built-in aerialis used so that no antenna or ground connec-tions are needed for good reception.
LIST OF PARTSFOR BOOK -END RADIO
One 6SJ7 tube.One 6SF5 tube.One 25A6 tube.One 25Z6 tube.Antenna condenser, .00042 mfd.Tuning condenser, .00036 mfd.Electrolytic condenser, 24 mfd., 150 v.Electrolytic condenser, 20 mfd., 150 v.Two electrolytic condensers, 16 mfd.,
150 v.Electrolytic condenser, 10 mfd., 25 v.Electrolytic condenser, 5 mfd., 25 v.Mica condenser, .0005 mfd. (two)Mica condenser, .00025 mfd.Tubular condenser, .1 mfd.Tubular condenser, .01 mfd.Tubular condenser, .005 mfd.Tubular condenser, .05 mfd.Tubular condenser, .002 mfd.Line -cord resistor, 200 ohm.Resistor, 1 meg., 1/4 watt.Resistor, 2 meg., 1/4 watt.Resistor, 1/2 meg., 1/2 watt.Resistor, 750,000 ohm, % watt.Resistor, 200,000 ohm, 1/4 watt.Resistor, 100,000 ohm, 1/4 watt.Resistor, 5,000 ohm, 1/4 watt.Resistor, 4,000 ohm, 1 watt.Resistor, 700 ohm, 1 watt.Switch and volume control, 20,000 ohm.Radio -frequency choke, 6 mh.Six-prong,.plug-in coil.Miscellaneous:-Speaker, four midgetoctal sockets, aluminum chassis, cab-inet, wire, etc.
106
Beginner's
This sturdy setis compact andhas a range upto 1,000 miles
WITH a broadcast -band rangeup to 1,000 miles, the one -tube, all -electric, A.C.-D.C. re-ceiver illustrated is designed
especially for beginners. It requires aminimum of parts, costs little to build,and is easy to assemble and wire.
The set is built around one of the newbantam -type, midget, octal -base tubes.This particular tube, the 25A7GT, is re-ally a pentode tube and a half -waverectifier in one. The pentode section isused as a regenerative detector.
In spite of the compactness of the steelcabinet (5" by 4" by 3"), it is an easymatter to mount and wire the parts, asthe top and bottom panels can be un-screwed and taken off. The six -prongplug-in coil, the insulated earphone ter-minals, and the tube are all mounted onthe top panel. The .00014-mfd. tuningcondenser and the 15,000 -ohm wire -wound combined regenerative control andswitch are mounted on the front panel.
One -Tuber ISELECTRIC
25A7GSCREEN
PLATE
HEATE
SEC.
CATHODE(RECT.
15,000 -*-OHMS
.00014m F D.
25A7G
br-7F SWITCH
INDICATES GROUND TO CHASSIS
GRID
PLATEPLATE (REcr)
A.C.- D.C.
All connections are clearly shown in the diagram.Note particularly the negative phone connectionto pentode -tube screen, and absence of a ground
Bottom view of the chassis, below,indicates how parts are arranged.Both top and bottom panels can beremoved to facilitate the wiring
A very simple filtering sys-tem, consisting of only onehigh -capacity electrolytic con-denser, is used. The earphonesare connected in series withthe cathode of the rectifier,and the reader should notethat the screen of the pentodetube is connected to the neg-ative terminal of the phonesinstead of the positive. Noground should be used withthis receiver, as circuits ofthis type are groundedthrough the house wiring. Arubber grommet should beused to protect the line cordwhere it passes through thesteel cabinet, otherwise thesharp metal sides of the holemay cut the insulation andcause a short circuit. Shouldthe set hum, reverse thephone leads.
LIST OF PARTSOne 25A7GTtube.
Tuning condenser.00014 mfd.
TbElectrolytic
condenser,40 mfd.uular condenser,
.02 mfd....Two mica condensers,.0003 mfd.Switch
10,000 ohmand variable
resistor,Resistor,
2 meg,% watt.Line cord, 360 ohm.
Radio-frequencychoke,
2.5 inh.Set of six -prongcoils.
(steel),octal -tube
socket,
solprong-coil
sockettwo insulated
bananader, etc.
jacks and plugs, earphones,wire,
I, IT
Superhet for BeginnersUSES ONLY TWO TUBES
SEVERAL years ago, a superhetero-dyne receiver had to consist of at
least seven tubes-sometimes nine, forin those days the multiple tubes weuse so frequently in our present setshad not been perfected. A two -tubesuper is possible these days, and oneof them is shown on these pages.
A 1A7GT pentagrid tube is used asthe first detector and oscillator. Atwo -gang .00036 condenser tunes thesetwo stages. To insure proper trackingof the oscillator tuning condenser afixed mica condenser with a capacityof .0004 mfd. is wired in series withthe oscillator coil. This condenser mustbe accurate within +3 percent.
Across the other tuning condenser( in parallel with it) is a low -capacity( .00005 mfd.) midget variable trim-mer condenser. This is adjusted to com-pensate for any irregularities between thetwo tuning condensers. It is not necessaryto adjust it for each station.
The output of the 1A7GT is coupled tothe detector stage through a 456-kc. I. F.transformer ( this may be either an iron -
This two -tube superhet gives loudspeaker reception oflocal stations with a 20 -foot antenna laid on the floor
core or air -core type). Both primary andsecondary windings of the I. F. transformerare tuned by tiny built-in trimmer condens-ers which have to be adjusted before thereceiver will operate. This may easily bedone by tuning in the strongest local signaland turning the two screws on top of the
Topside of chassis, showing the two multiple tubes This is the underside of the chassis. The on -and -that do the work once performed by seven or more off switch is operated by the volume -control knob
108
AUDIOTRAN3R
15+
TO CAP C,IIDEIGT4
N
GREEN --I. F.
TRANSF.
RED
YELLOW
YELL0z1.4
REDFLACK
.1 MFD
10 CAP ONIA7GT
.00036
.00036
SW. ANDVOLUMECONTROL500,900,LI
ANT E NW.CONDENSER
OSCILLATORCONDENSER
010.
if
% SHIELDED.!.1. WIRE
I'GREEN I
As shown in thispictorial diagram,the IA7GT penta-grid tube is usedas the first detec-tor and oscillator,with the two -gangtuning condenser
The detector (tri_ode portion of theID8GT) is trans -Former -coupled tothe power -ampli-fier pentode of thesome tube, asshown at the right
r
ANTENNA,COIL
ANT./. 00036 MFD.00005MrO
I A7GT
3., 'TEL C,w rN
.0001 MFD
54000 - -
7 2
3 f-------,,,,,,ca - .....00036 ,, .
MFD -- .° 1 Tit,F0nt
O GNI)
1
AL-_-____, ,..\-c.T .-=- , 0004 1,4 P C k-OSCiLLAFOR - E+
± 3% COIL NC NC W - /DIODEBOTTOM VIEWS (NC- N000NA..7.CIIONS)
TRIMMER- BALANCING
CONDENSER.00005100
-
"..0J04
.1M1 fM141
MI, MO =I11=1 OM al
54000,o
002MFG '.. I
MFD
*---I F. GREEN
IRANSE , 2ME64, II. .41
-I\C> C>
,C> IO tC> 18
ANTCOIL
CSC. COIL
s 3 zLo< 0 w
12:1-I ItUJ ,,,)-
. )..
IN -..`P- -
OUTPUTTRANSF
4"SPEAII,EP
ti0 0in4, 02
1§71C sao
GRID(0uotie)
9
IR.ODE1D8CIT
7
0-
0
0+
109
I. F. transformer can with a screw driver,until the signal is at its loudest. Then tuneto a weaker station (near 1,500 kc.) andgive it a final adjustment.
The detector (triode portion of the1D8GT) is transformer -coupled to the power -amplifier pentode of the 1D8GT. Unrecti-fled RF currents in the plate circuit of thedetector stage are by-passed to the chassisthrough a .002-mfd., 400 -volt tubular con-denser. Volume is controlled by means ofthe 500,000 -ohm variable resistor across thesecondary of the audio transformer. TheS.P.S.T. on -and -off switch is also controlledby the shaft of the resistor.
Output of the two -tube =per ,s fed intoa 4' or 5" permanent-magnc weaker in-stalled in the lid of the gray -wrinkle steelcabinet. A 33k" diameter opening must bedrilled for the 4" speaker, and anotheropening about 4" in diameter in the frontpanel for the dial.
Loudspeaker reception of all local stationsis possible, using an antenna of only about20 feet, stretched across the room on therug. No ground is necessary, although itmay improve the volume on weak signals.A small trimmer condenser should be in-serted in series with the antenna whenevera long antenna is used with the set.
Although 90 volts is shown in the wiringdiagram as the plate or "B" battery voltagethis value may be safely increased to 110 or120 volts with a correspondingly greatersignal strength. In this case the "C" batteryvoltage should be advanced to 9 volts.
LIST OF PARTS
Steel chassis, 7" by 7" by 2".Shielded iron -core antenna coil.Shielded air -core oscillator coil.Two -gang tuning condenser, .00036
mfd.Four -inch round dial.I. F. transformer, 456 kc.Audio transformer, 3:1 ratio, un-
shielded.Output transformer, universal type.PM speaker, 4" or 5".Steel cabinet, 8- by 12" by 8".Volume control, 500,000 ohms.Switch cover plate, S. P. S. T.Variable trimmer condenser, .00005
mfd.Pentagrid converter tube, 1A7GT.Triode -pentode amplifier tube, 1D8GT.Octal (eight -prong) wafer sockets
(two).Mica condensers: .0001 mfd., .00025
mfd., .004 mfd. (padder).Tubular condensers: 1 mfd., 400 volts ;
.05 mfd., 400 volts; .005 mfd., 400 volts;
.002 mfd., 400 volts.Carbon resistors: 1/4 watt, 50,000
ohms; % watt, 70,000 ohms; 1/4 watt, 2megohms.
Knobs, cable, binding posts, etc.
110
Front of the gray -wrinkle steel cabinet, showingthe three control knobs and four -inch round dial
1
Both primary and secondary windings of the I.F.are tuned by turning the two screws on top of thetransformer can until selected signals are loudest
Output is fed into cl permanent -magnet loudspeaker(either four or five -inch) set in the cabinet lid
The author using the test circuit shownat the left to measure a small resistor
PROVIDING a quick and easymeans for measuring small resist-ances and testing grounds, the unit il-
lustrated forms a handy piece of equipmentfor the radio experimenter and service man.
The circuit consists of a buzzer and bat-tery, three fixed resistors, a rheostat, an in-duction coil, two switches, and an earphone.As shown in the diagram, three binding -postterminals ( A, B, and X) are provided. Totest a resistance up to 50 ohms, terminals Aand B are connected together with a shortpiece of wire, the toggle switch is flipped tothe "50" position, and the unknown resistoris connected across terminals X and A. Thebuzzer is then turned on and the rheostatadjusted until the buzz cannot be heard inthe earphone. The position of the pointer onthe rheostat scale indicates the value of theresistor. For resistors from 50 to 500 ohms,the toggle switch is moved to the "500" posi-tion and the same procedure followed, butthe scale reading must be multiplied by ten.
Pocket -Size
Radio TesterIS EASY TO BUILDFROM ODDS AND ENDS
In testing grounds, simply connect theground in question to terminal X, attachterminal A to some other ground, and ter-minal B to a third ground, which can benothing more than a screw driver pushedinto the earth. When the rheostat has beenadjusted to eliminate the buzz, the resistanceof the ground can be read directly from therheostat scale.
LIST OF PARTSFixed resistor, 500 ohms.Fixed resistors, two 5,000 ohms.Rheostat, 0-50 ohms.Single -pole, single -throw switch.Single -pole, double -throw switch.High -frequency buzzer.Induction coil, 1-100 ratio.Battery, 41/4 volts.Binding -post terminals, three.
Mounted in a smallwooden cabinet, theunit can be carriedin your coat pocket
111
"Wireless" Radio PhonographREQUIRES NO CONNECTION TO SET
=rIEQuENCv-:ONTROt.:ONDENSER
ruinSOCKETS
Gomm2.7.5. VIEW 6A7
.35m F0
ELEZ'rvrkA,r,t5t.
290 DH MS
Sw 7CH
igCAP) ANT..00001 .002 AMFD. MF 3. ,
1, 1tiir350 11-1E--MMFD 0
.00304 0MIF 3. 0
E.EN30 etauras
0.5 MEG.VOMMECON1110L
P.+040
25Z5 0,7 mIcrac.o..0e.4rnA.rgsFonm
Study the wiring diagram carefully The set is actually a two -tube transmitter. Ports are best arranged as in the photo above
INEXPENSIVE and easy to build,this "wireless" record playermay be used with any alternat-
ing -current radio receiver with-out making any actual connec-tions to the receiver circuit. Inaddition, when you plug in a mi-crophone and flip a switch, therecord player becomes a public-address system that will allowyou to do your own program an-nouncing through the radio'sloudspeaker. In use, the device issimply plugged into a convenientelectric outlet.
Essentially, the "wireless" fea-ture of the pick-up consists ofa tiny radio transmitter ( oscil-lator) tuned to a frequencyaround 550 kilocycles, or to anyunused band at the upper end ofyour radio's tuning dial. The pow-er of the little transmitter is, ofcourse, very low, and the distanceit may be placed from your re-ceiver depends upon the radio'ssensitivity. With a good super-heterodyne, the phonograph can beused in an adjacent room. Withless powerful receivers, it may benecessary to wrap the insulatedantenna of the phonograph -trans-mitter loosely around the re-ceiver's antenna lead-in, withoutmaking an electrical contact.
If the reader already has asynchronous electric turntable,and either a magnetic or crystalpick-up, he may use these withthe wireless oscillator and avoidthe expense of buying new ones.
The oscillator circuit is builton a compact aluminum chassismeasuring 1!4" by 21/2" by 61/2".A black crackle panel serves asthe mounting for the trimmercondenser that adjusts the tiny550 - kilocycle radio - frequencytransformer, the microphone jack,and the switch for changing frompick-up to microphone.
The two tubes used are a 25Z5,as a rectifier, and a 6A7, as acombined modulator and oscil-lator. Two 6,000 -ohm, one -watt
112
Plugged into an outlet any-where in the rcom with yourradio, the unit "broadcasts"records through the loudspeaker
Volume is controlled by a knobmounted near the pick-up arm.The antenna, seen lead'ng outof the cabinet is st..etc hedon the floor du-ing operation
11`
dy flipping a switch and plugging in themicrophone, you've got your own public-address system for entertaining friends.How the synchronous motor for the turn-table :s installed may be seen at the left
resistors, and two 16-mfd. electrolytic con-densers are used for filtering the rectifiedcurrent from the 25Z5.
To obtain the best results with the carbon -
LIST OF PARTS NEEDEDRectifier tube, 25Z5.Oscillator -modulator tube, 6A7.Synchronous motor (2- deep).Magnetic or crystal pick-up.Radio -frequency transformer, 560 Kc.
(special).Microphone transformer (midget).Hand microphone (carbon -type).Toggle switch.Midget wafer socket, six -prong.Midget wafer socket, seven -prong.Two electrolytic condensers, 16 mfd.,
175 volt.Two tubular condensers, .05 mfd.Tubular condenser, .1 mfd.Trimmer condenser, 350 mmfd.Mica condenser, .00001 mfd.Mica condenser, .00004 mfd.Mica condenser, .002 mfd.Two resistors, 6,000 ohm, 1 watt.Resistor, 1,500 ohm, 1 watt.Resistor, 10,000 ohm, 14 watt.Line -cord resistor, 290 ohm.Volume control and switch, 500,000
Ohm.Miscellaneous: -Aluminum chassis,
crackle panel, microphone jack andplug, A.C.-D.C. antenna, cabinet, etc.
114
type microphone recommended, a suit-able transformer with a 200 -ohm pri-mary is used in conjunction with a 41/2 -volt "C" battery. As shown, the batteryfits snugly inside the cabinet.
Care must be taken to follow the exactvalues of the fixed condensers and resistorsspecified in the wiring diagram, and under nocircumstances ground any of the wiring tothe chassis or panel. This is to prevent anypossibility of shock should the user acciden-tally touch the chassis of the receiver whileholding the pickup. Standard A.C.-D.C. an-tenna wire can be used for the aerial of thetransmitter, which may be approximatelytwenty-five feet long. When making thewooden cabinet shown below, be sure and cuta ventilator opening in the back.
Details of the inexpensive wooden cabinet. Nobottom is needed, but rubber feet should be used
Servicing Your Itaidio
516i.
1110
FIXED CONDENSERS. Nine types commonly used, left to right: .5-mfd. paper condenser; 8-mfd. midgetelectrolytic condenser; two (.05 and .02-mfd.) small paper condensers; three mica condensers (.002, .00075and .0001-mfd.); a 50 -volt, 25-mfd. electrolytic condenser; and a 1-mfd. paper condenser in a steel shell
IF RECEPTION STOPS suddenly and resumes when the cabinet is knocked, the cause may be a looseconnection or a bum tube. Or it may be a bad audio coupling condenser. This looks like the .05 or .02paper tubular shown above. Diagram shows location. Before replacing, connect an .05 across it for a test
HUMMING h usually ale to a faulty electrolytic condenser like the 50 -volt, 25-mfd, shown above, acrossthe power -tube bias resistor, or one like the 8-mfd., in the filter circuit. Connect an 8-mfd., 450 -volt con-denser across each electrolytic condenser in the set until the faulty one is found, and replace it
6J7
oa
2 POWER0 al TO
TUBE
FADE-OUT experienced on many old sets is due to a fixed carbon resistor that has become crystallized.It usually can be recogeized by a coating on its surface. If not, try connecting a 50,000 -ohm, 2 -watt re-sistor across each carbon resistor in the B+ circuit. If playing resumes, replace with one of correct value
115
Low -CostHome Recorder
EXTREMELY versatile, this home recorderwill enable anyone to record radio pro-
grams, to record home programs, to boostthe volume on weak stations, and to playregular phonograph records.
The recording unit on top of the cabinetis very reasonably priced. It consists of arecord -cutting mechanism with a concealedfeed screw situated underneath the baseplate, a high -impedance crystal play -backpick-up, a powerful induction -type self-starting motor, a weighted ten -inch turntable, a cutter -armrest, a pick-up rest, and a baseplate. The unit will cut recordsup to 10" in diameter and willplay records up to 12" in diameter.
A compact three -tube ampli-fier is used with the home re-cording unit, and is installedjust inside the back of thecabinet on a steel, cadmium -plated chassis measuring 2" by7" by 7". Output from theamplifier is fed into a good -quality 5" permanent -magnetspeaker mounted in the frontof the cabinet. The rear of thecabinet is left open to ventilatethe tubes and motor.
The amplifier proper consistsof a high -mu triode, resistance -coupled to a beam power pen-tode. These two tubes provideall the power needed for eitherrecording or playing records.
116
Making a record withthe home recorder. Itis just as easy to re-cord radio programs,amplify weak sta-tions, or play regularphonograph recordsby the simple flick ofone of its switches
A 250,000 -ohm variable resistor controls thevolume for pick-up or microphone. Thevoice current for recording is tapped offthe plate circuit of the audio output tubethrough a .1-mfd. tubular paper condenser.
mounted on top ofthe chassis regulate the various operationsof the instrument. Although an A.C.-D.C.amplifier is used, the motor will operateonly on alternating current, so that the unitmust not be used on direct current. For
Bottom view of the finished unit, showing arrangement of parts
The three tubes, transformer, and control switches All wire leads ci,e kept a:. :bort as po:.sible under -as they are mounted conveniently on tie top of the reath the chassi... Be sure c II :onnections are fghtchassis before mounting in a corner o: the cabinet and arrange the parts appro::i-notely a: shown here
VOLUMECONTROL
0 0.25 MEG.
PICK -U..
S.P.-D.T."IV11/.E SW,TCN
OUTPUT TRANSF,OF SET
B+
mAorricNG,,fiTPAN SF.
125F5
CUTTINGHEAD
50 .-6GT3
.005mFO.
0.25MEG MEG
.005MFO.
SPEAKEROF SET
500OHMS
R. I.% ILL VARYOEPENOING ON TCNEDESIRED BY OPE RATOP
L.
OUTPUT'TRAP. 5F.
0.1 POIMFO. C,
1
UTTINGHEAD
3 5 IAGT 5101.6GT
N.C:"N.C. < N.C.
SOCKETS (BOTTOM V EN) N.C. NO CONNECTION
25 /GT
5" P -6ASPEAF EP
D.T.SWITCH
50 Oi-imE3JILT iNT3_WE CORD
0115,4 AC.
PkONOMO-OR
o.P.-SWITCH
This circuit diagram will ci'.e you all iFe needed info -motion for hooking up tFe Fame recoraing unit
LIST OF PARTS
Home reccrder unit.Cadmium -plated steel chassis.Triode amplifier tube 12SF5.Beam power tube 50:AGT.Half -wave rectifier tribe 35Z4GT.Octal wafer sockets .31.Volume control 250,000 ohm.D.P.S,T. toggle switch.S.P.D.T. toggle switches (2).Filter choke, 11 henrys.Line cord with 150 -ohm resistar.Single -circuit jacks (2).
Permanent -magnet speaker (5').Universal output transformer.Electrolytic condensers (2), 8 mid. 150 v.E:eetrolytic condenser, 25 mfd.. 25 v.Tabular paper condensers (2) 005 mfd., 600 v.Tubular paper condenser, .1 sold., 400 v.Carbon resistors: 50C ohm, 1 watt; 250,000 otim,
watt; 1 meg., 1,2, watt.Crystal microphone.Atosce. [anemia : Cutting and play -back needles.
Wire, so:der, etc.Blank records.
rh
117
CABINET -CONSTRUCTION DETAILS
Note that the back of the cabinet is leftopen to supply ventilation for the tubesand the phonograph -turntable motor. The re-corder unit comes with its own base plateready to be set on the top of the cabinet
OPEN SPACE
.41,4" DIA.SPEAKEROPENING
1 3/.4
EIACKOPEN
use in recording radio programs, themicrophone is switched on and placedbefore the speaker of the radio re-ceiver. In this manner, a permanentrecording, right in one's own home,of a favorite dance tune or a historicspeech can be made for preservation.
If better quality is desired in recordingoff the air one should connect the cuttingheard directly to the output of the radioreceiver through a special matching trans-former.
For best results the radio must not be
How the mike is used for recording or boosting programs
played too loud, as distortion will occur onthe record. The correct volume can best befound by trial and error. When this hasbeen obtained it is a good idea to "monitor"future recordings in order to keep thevolume constant. To monitor a recording,
headphones are plugged in across thecutting -head circuit so that the qual-ity and volume may be checked.
In recording a voice or home pro-grams directly the microphone shouldbe placed close to the source of thesound. It is advisable first to makea test through the speaker. Whenthe results are satisfactory, switch inthe cutting head and record on theblank disk.
A good crystal microphone may beobtained for about six dollars, com-plete with a desk stand, a 7' cable, anda spring protector for the cable at themike.
The unit may also be used, as al-ready stated, to boost the volume onweak stations by placing the micro-phone as close to the receiver's speak-er as possible and using the amplifierin the home recorder to increase thevolume further.
11S
Tom Thumb Radi
These three views of the tinyset will help you arrange theparts. If a cabinet is used, besure to allow for ventilation
I I SING a new cone -typeu permanent -magnet speak-er only 2" in diameter, thisradio midget among midgetsis inexpensive, easy to build,sensitive, and powerful de-spite its size. The broadcast -band set usesa four -prong coil consisting of a grid wind-ing and a tickler winding (L, and 1.2) Re-generation is obtained by a variable resistoracross the tickler winding and grounded tothe chassis through the .0005-mfd. fixedmica condenser. The 25-mh. choke in theplate lead of the detector ( 6F5) smooths re -
Note dimensions below for thealuminum chassis and panel.The wiring diagram gives allresistance and other values
VA. PLYWOODWOOD PANEL
ALUMINUM PANELAND CHASSIS OLE FOP
LINE CORD
0FOR A
MIDGET SPEAKER
generation and keeps stray radio -frequencycurrents from the resistance -coupled audiostage. Another new item in the set is thecompact 24-16-10 mfd. tubular electrolyticcondenser. A 200 -ohm line -cord resistor re-duces the house current for the three metaltubes. A %." wooden panel in front of thealuminum panel acts as a speaker baffle.
.s7
M.I
inmoo
0005
1.3 v.-- (OPTIONAL- TO BE CONNECTEDMFD. ONLY IF GROUND IS USED)
AKER
CAP
I I9
SuitcasePhonograph
FOR
PORTABLE RECEIVERS
120
You can "broadcast" a record directly from thesuitcase phonograph to any portable radio
THISSUMMER there will be thou-
sands of battery -operated portableradios in use on beaches, in parks, andon picnics and excursions everywhere.They will range from the camera -stylemidget or "personal" radios to the"twenty -pounders," capable of bringingin Europe on the short-wave band.
This battery -operated "wireless"phonograph was designed foruse with these portable sets.As the name implies, the unitwill transmit recorded soundsto the portable, through themedium of radio waves. Theradio in turn reproduces themthrough the loud -speaker. Noconnections between the phono-graph and the portable arenecessary.
This combination of radio andphonograph will enable you tofill the interludes between radioprograms that appeal to youwith recorded dance music,symphonies, or whatever youwish. The volume control will
Underside of the phonograph's Ma-sonite baseboard, showing transmit-ter, batteries, and turntable motor
7 FOOTANTENNA
140 MFD
VOLUMECONTROL ,
ANDSWITCH
(500,000u,)
PNONO GRAPHPICA -UP
Above, sketch of the set-up of the suitcase"wireless" phonograph, as shown in photo onprevious page. At right is wiring diagram
make it possible to lower the soundof your music so that it will not dis-turb your neighbors, or turn it upto fill a room with music for dancing.
Being light and compact ( 12 by8% by 4% inches) the phonographcan be carried along as easily as asmall suitcase. It is entirely self-contained and self -powered. Twosmall batteries ( similar to thoseused in the personal -type radios-a1 ih-volt flash -light cell and a 671/2 -volt "B" battery) supply all thecurrent necessary to operate thetransmitter. The power of thistransmitter is ,so small that it will not radi-ate signals beyond a few feet. It is the onlytype of transmitter that can be operatedwithout a license under Federal Communica-tions Commission regulations.
The transmitter uses a 1A7GT convertertube which acts as a combined modulationand radio -frequency amplifier. The electricimpulses from the phonograph pick-up aremodulated by the first and second grids ofthe tube. In a 1A7GT, or similar tube, thesecond grid acts as a plate. The modulatedsignal is then superimposed on the carrierwave, generated by the second portion of the
*A.SATTSKYItVOLTS
CONTACT-- TOSHELL
ES' BAT TERN, 67 AvOLTS
PNO NODSAN+TURNTABLE
MOTOR
IA7GTTUBE SOCKET
i4oMMFD
tube, and transmitted to the receiver, whereit is amplified and transformed back intosound.
The transmitter is tuned by a .00014-mfd.( or 140-mmfd.) tuning condenser and astandard four -prong broadcast coil. To cutdown costs, the four -prong socket for thecoil and the eight -prong ( or octal) socketfor the tube are both wafer types, invertedso that they can be mounted on the Masonitebaseboard and used instead of the more ex-pensive molded socket.
The suitcase into which the phonograph isfitted can be purchased almost anywhere for
121
MASONITE PANEL
2NUT5(WS2")ON BOTH SIDES
OF BRACKETKNURLEDNUT /32"
BRACKET
44eiltool of.41ototting
Wattel1.f
was1111.
Y 4-MI g11
7t_:LOCKWASHER
Completed phonograph seen from above, with eight-inch turntable. The seven -foot antenna is coiled
less than $1. A spring motor can be pickedup secondhand for about $2.50, or it may beordered direct from a manufacturer. Aneight -inch or smaller turntable must be used,otherwise a larger suitcase than is shown inthe accompanying illustrations will have tobe purchased. A Masonite panel 111.i " by
" is used for mounting the motor, pick-up, and transmitter parts. In the set shownhere, the panel was left in its natural brownfinish, since this blended with the brown -
122
Left, detail oF the suitcase, and above, a stageOf mounting, before installation of the turntable
striped covering of the suitcase. The twobatteries are strapped to the undersides ofthe panel with a flexible brass band %-inchwide. This will keep them from knockingabout in the bottom of the case, and theymay easily be replaced whenever necessary.
To operate the phonograph, place thes'ven-foot antenna near the back of the por-table receiver ( where manufacturers usuallyplace the loop antenna). Then turn thetransmitter on by means of the switch onthe 500,000 -ohm volume control, and tunethe unit to some free spot on the receiver'sdial by rotating the 140 mmfd tuning con-denser. Records can then be played on thephonograph, and will be heard over the set'sloudspeaker.
LIST OF PARTSConverter tube, 1A7GT.Volume control and S.P.S.T. switch,
% megohm.Crystal pick-up.Four -prong broadcast coil, 350 to
565 meters.Tuning condenser, 140 mmfd.Octal and four -prong wafer sockets.Carbon resistor, 10,000 ohms, 14 watt.Mica condenser, 50 mmfd.Seven -foot antenna.Small, brown -striped suitcase.Masonite panel, 11%" by V.i".Midget "B" battery, 67% volts.Stlndard flashlight cell, 1% volts.Spring -wound motor with 8" turn-
table.
TWO -TUBE
PORTABLE
A COMPANION FOR A
SUITCASE PHONOGRAPH
DESIGNED as a com-panion piece to thesuitcase phonograph
shown on page 120, this battery -operated portable will provide many hoursof radio entertainment in your home, atparties, or on summer trips and picnics.Though it has only two tubes, it has suffi-cient power to bring in all local stations.When used with the phonograph, which itmatches in size and appearance, it will giveyou your choice of recorded music or broad-cast programs anywhere and at any time.
The suitcase in which the set is built canbe purchased at almost any five -and -ten -
As a receiver, the sei brings in local stations with good volume
cent store for 50 or 60 cents, and shouldmeasure 12" by 9" by 41". If you made thesuitcase phonograph, you will probably wishto get a second suitcase with a similar ex terior finish.
The two -tube chassis of the set, the four -inch speaker, and the batteries are fitted inthe lower half of the suitcase, while the
loop antenna is placed inside the lidand fastened with two 1,i,." -long 6/32" ma-chine screws.
The upper picture atthe left shows top viewof the chassis withtubes and controls inplace. Separate tuningcondensers were foundto be better than aganged condenser.Lower photo shows thearrangement of portsunderneath the chassis
123
Connections to theloop are made withtwo Fahnestock clipsmounted on the lowerinside edge of the lid.To hide the loop an-tenna, the author cov-ered it with a 10" by8" sheet of stiff paper.
The metal chassisand speaker aremounted on a woodenpanel measuring 14"by 81,L," by 1114". Thepanel has a 33.i" di-ameter hole drillednear the top for thespeaker, which is dec-orated with a bronzeescutcheon plate.
The set uses a three -tube tuned -radio -fre-quency circuit, builtaround the two tubes-a 3A8GT ( diode -triode - pentode) anda 1Q5GT (beam -pow-er output tube). The3A8GT is used for thetuned radio -fre-quency stage and theregenerative detector.The diode portion ofthe tube is not usedand is wired directlyto the chassis.
Separate tuningcondensers were foundto be better than aganged condenser, as
>CC >-< crO 4 b
w o.s__
z II0
cvE E dcc ).e11u s ft.:
II
II
II
11
11I
III
L --GROUNDEDTO CHASSIS
.0005MFD
.00025 MFD
MFD.05
r=,.."11, jw iDOGMFD 20 MFD
NOTE: DOTTED LINESINDICATE SHIELDEDCONNECTION
500,000.
- r >V. "" 90 V.+
0
O 0Uri0
ce <FO G.4 0
000M111
How the six -prong plug-incoil is wound. Below aretube -socket connections
3A8GT CND GRID Sc I QSGTGRID
124
F 44.
BOT TOM;D Nc FIc
Complete wiring details for the circuit are given in this picture diagram
... and the schematic drawing below. Bios for the receiver is automatic
3ABGT
5-7AATENNAOPTIONAL) 3
5141EL050WIRE
L .0004.1 ...MFD 1--:: 2MEG
CAP'!
MED SW
S7 .000'2 j
MN) g
500,000.0 .00011 d C .00025
REGENERATIONCONTROL
456T
MFD
r<
ows 4NFL) 3
II
VOLUMECONTROL
MFD
PLUG-INCOIL
MN)
20MFD
OUTPUTTRANSFORMED
0+90 V
0 -1.5V
SW
5
Metal chassis and speaker, mounted on a woodenthe lower half of the suitcase along with the
they enable the receiver to operate at its best.Single -pole, single -throw switches are
mounted on the back of both the volume andregeneration controls. The one on the re-generation control turns the receiver onand off, while that on the 1-megohm volumecontrol disconnects the grounded side.
Bias for the receiver is automatic and issupplied through the 600 -ohm, 1, -watt re-sistor and the 20-mfd. electrolytic by-passcondenser in the 'B-" and "A-" circuits.
If more volume .s desired from the set,the "B" power supply can be increased from
-A4
The 9'/2" by 71/2" loop antenna goes inside the lid ofthe case and can be concealed by paper if desired
panel, fit intoset's batteries
90 to 135 volts.wire ( about 10grid side of the
Panel and chassis details. Panelhas a 33/4" hole for the speaker
Or a short length of antennafeet can be connected to theloop antenna.
LIST OF PARTSLoop antenna, 9112" by 7'_". Suitcase.Tuning condensers, .00041 mfd.Three-inch tuning dials (two).Four -inch PM speaker.Universal output transformer(Six -prong plug-in coil.Six -prong wafer socket.Volume control, 1 megohm.Regeneration control, 15,000 ohms.S. P. S. T. attachable switches (two).Diode -triode -pentode tube, 3A8GT.Beam -power output tube, 1Q5GT.Octal wafer sockets (two).Carbon resistors (two), 2 megohm,
watt.Carbon resistor, 1,4 megohm, watt.Carbon resistor, 200,000 ohms, % watt.Carbon resistor, 4,000 ohms, 14 watt.Carbon resistor, 600 ohms, 1/2 watt.Electrolytic condenser, 20 mfd., 150
volts.Paper tubular condenser, .05 mfd., 400
volts.Paper tubular condenser, .006 mfd., 400
volts.Paper tubular condenser, .003 mfd., 400
volts.Mica ccndenser, .0005 mfd.Mica condensers, .00025 mfd., .0002 mfd.Midget 1.5 volt "A" battery.Midget 45 -volt "B" batteries (two).
125
Library -Table Radio
'Agri 1414toww.,,jaii.
Putting thethe finishingtouches on the chassisof the extremely corn-pacrscra pbook" radio
Notice the shield sep-arating the two tubes,with its soldered con-nection to the metalframe of the perma-nent -magnet speaker
126
AKING an attrac-tive decoration
for the library ta-ble, this book radio willappeal to all builderswho prefer housing theirreceivers in some un-usual type of cabinet. Itcan be easily assembledand built at home by anybeginner, and the entireset, including the cabi-net, should cost less thanten dollars.
The cabinet for the re-ceiver was made fromthe wooden covers re-moved from a large scrap-book which cost less thana dollar at a local de-partment store. The cov-ers serve as the bottomand hinged top of thecabinet, while the sideswere built up, as shownin the drawings, frommatching wood. A jig-sawed decoration on thecover, left intact, addsto the attractiveness ofthe cabinet.
Resembles a BookThe circuit, which appears to
use only two tubes, has in realityplenty of punch behind it, sincethe two tubes do the work of four.The first tube ( 12B8GT) containsa high -frequency pentode anda high -mu triode. The high -fre-quency pentode is used as aregenerative detector which isresistance -coupled to the triode.The triode is in turn resistance -coupled to a pentode element inthe second tube. This second tube( 25A7GT) contains, besides theoutput pentode, a half -wave rec-tifier. A 260 -ohm line cord reducesthe house current to the voltagerequired by the heaters of thetwo tubes.
For the sake of compactness, asmall A.C.-D.C. antenna coil isused. As the coil has no ticklerwinding, the reader will have to add one him-self. This can be done easily by winding ap-proximately thirty-five turns of double -silk -covered wire around the bottom portion of thegrid coil. If tests show too little volume, re -
The booklike radio in use, showing its recessed control knobs
verse the connections to this tickler windingA five -inch permanent -magnet speaker is
used, with its cone facing upwards. Choosea speaker whose depth does not exceed 2%"otherwise the cabinet will have to be madedeeper.
The reader will notice that a shield Is usedbetween the two tubes. This is necessary asthe feed -back coupling between these .ubeswould cause a terrific hum. Arrange the 3nieldso that its top portion almost touches theframe of the speaker and make a so,dered
When the cover is shut,the set makes an orna-mental piece to put onthe library -table top
1'27
LIST OF PARTSMidget antenna coil.Radio -frequency choke, 16 mh.Potentiometer (75,000 ohm) and switch.Line cord, 260 ohm.Triode -pentode tube (12B8GT).Rectifier -pentode tube (25A7GT).Permanent -magnet speaker
(5" diameter).Tubular paper condenser, .1 mfd.Tubular paper condenser, .05 mfd.Tubular paper condensers (two) .02 mfd.Tubular paper condenser, .01 mfd.Tubular paper condenser, .006 mfd.Mica condenser, .0005 mfd.Mica condenser, .00015 mfd.Mica condenser, .0001 mfd.Midget tuning condenser.Electrolytic condenser, 5 mfd., 25 volt.Electrolytic condenser, 16 mfd., 150 volt.Electrolytic condenser, 24 mfd., 150 volt.Electrolytic condenser, 30 mfd., 150 volt.Resistor, 500,000 ohm, 1/4 watt.Resistor, 250,000 ohm, % watt.Resistor, 150,000 ohm, % watt.Resistor, 75,000 ohm, % watt.Resistor, 1 meg., 1/4 watt.Resistor, 3 meg., % watt.Resistor, 2,500 ohm, 1 watt.Resistor, 250 ohm, 1 watt.
Miscellaneous: Two octal sockets;aluminum chassis, 1,14" by 3- by 7%";special cabinet; knobs; dials, etc.
connection between the shield and the frame.Regeneration is controlled by varying the
screen voltage on the detector with the75,000 -ohm potentiometer, one side of whichis connected to the plus "B" voltage througha 250,000 -ohm, half -watt resistor, the otherside grounded to the chassis.
Sc.(PENT)
P (PENT.)
P (T.4.1=DOE)
---!,(FENT.) Sc.LPENT.
P (PENT.)
-H SOCKETS SHOWNBOTTOM VIEW
K (PENT.) G (FR OCE) K(RECT.)--FC 16mh (12613GT)'ocKP
CAP
3 3 MEG
..L. 00015ca,6 MFD._ .0005
_ /AFC).
1.46C
2
006(25A7GT)
.
3 5 .0001 Vr0 MF4
oarA -AIL
0.1 1 AECMFD.
75,000 OHMSVOL. CONT. 0.25MM
m ..02F0..1:1
N1S
a
0 r1 20 I 0CiM00
LK1- 0.5
MEG
2
5 MFD.(2514)
-P(RECT)
(PENT.)
[1.
SPEAKER
64(
24MFD.
250 OHMS
77 -SHIELD (GROUNDED)
2500OHMS
30 16MFD MFO.
.05 KARD.--"F260 OHMS
0ON -OFFSWITCH
110-V. A.C.-D.C.
Top, the cut-out cover decoration. Diag-ams-cabinet, panel, and wiring-give all construction details
128
You can get a repair kit for fixing up your old cabinet
Deep scratches and dents are filled with stick shellac
. . . while slight ones are removed by a special polish
111111110...,_French polishing can be done with new synthetic materials
... and rubbing with fine steel wool gives a satiny finish
Servicing Your RadioMANY radio cabinets today are in
sad need of repair. The old finishhas peeled off in places, nicks have beenmade in the wood, and maybe a fewscratches have appeared.' Various kits are now on the marketwhich will enable even an inexperi-enced person to patch up anything froma small scratch to a bad dent. Firsttake the spatula which comes with thekit and heat it over the alcohol lamp.With the heated spatula melt a shellacstick cement of the proper shade andcolor into the hole, scratch, or dent.Once the imperfection is filled, it issmoothed off as well as possible withthe spatula. The high spots are scrapedoff with a razor blade or sandpaper,and then rubbed down with fine steelwool and polished. Care should betaken not to injure the surroundingfinish. Always make sure the spatulais clean and never use matches or acandle to melt the shellac stick.
For slight faults and scratches onthe cabinet, you can use a specialscratch remover and polishing liquid.This usually is made up into a con-venient applicator with a felt brush at
brush and the mark disappears.Synthetic materials now on the mar-
ket make French polishing a simplematter. A small additional amount ofthe liquid is placed on a pad alreadysaturated with the French polish andrubbed over the surface of the cabinetwith a circular motion until a highgloss is obtained. Fine steel wool, light-ly used, will tone down the gloss.
FRENCH POLISH PADSOFT MUSLIN COVER
`,.\ TIED AT TOP
COTTON STUFFING
-3* DIA. PAD
4. CABINET SHOULD BEPOLISHED WITH A
CIRCULAR MOTION
129
All -PurposeEXTREMELY compact and
light, this battery portablewill work anywhere-ontrains, in the home, on a
boat, in the city or country.Having a loop inside the case,the set needs neither antenna norground connections. It has anighttime range of 500 miles anda daytime range of more than100 miles. If greater distancesare required, an antenna may beattached to the loop by an extraconnection provided for this pur-pose. It consists of two extraturns which are loosely coupledto the other turns on the loop.One side of this extra "coil" isgrounded to the chassis-the an-tenna being attached to the oth-er side.
The circuit consists of a four -tube superheterodyne using the
Notice the coiled loop antennainside the hinged back of thecabinet. The chassis rests on ashelf cut away for the speakerframe. Under the shelf go thebatteries for the four -tube set
PortableEASILY BUILT RECEIVER FOR USE
AT HOME OR ON YOUR VACATION
new small -size 1.4 -volt tubes. The first tube, may be purchased already punched, readythe 1A7GT, acts as a combined first detec- for the parts to be mounted on it. The vol-tor and oscillator, while the third tube, the ume control, with its on -off switch, and the1H5GT, fills the role of second detector (a tone control are mounted on L -shape metaldiode in this case) and first audio stage. The1H5GT also controls the automatic -volume -control current which keeps distant stationsfrom fading. The other two tubes ( 1N5GTand 1C5GT) are used as the inter-mediate -frequency ( I.F.) amplifierand power pentode, respectively.
Iron -core input and output trans-formers are used, as their high im-pedance matches these new batterytubes better than th air -core type,resulting in a higher gain andgreater selectivity. Ontop of each of the I.F.transformer cans aretwo screws which areconnected to the rotorsof the primary and sec-ondary trimmer con-densers. These must beadjusted carefully witha small insulated screwdriver while the set is inoperation in order totune the I.F. transform-ers to their resonancepeak of 455 kilocycles.
Care must be takenin punching out the 112 "by 4 %" by 5 % " chassis.If desired, of course, it
Careful placingof parts on thechassis is neces-sary to get themall in the limitedspace. Diagramsare given below
AOOPNT.(1A7GT)3
Ci!R
NL .05poFD.
250,000OHMS
.00005MCC,.
U
.000313NIFD.
.0004UFO
3
2 NIEL..0001mFO.
VOLUMECONTROL
RI.
F4
A7GT)
0145G -
P.
.003 mFD.01 mFO.
.01
(1C35611
P.M.SPEAKER
CONTROL J
D.P-S.T. SWITCH ONVOLUME CONTROL
-90V.+.C. 0100E Sc.
F + F4 F4N.C. Slif NC.. Sh: N.C- 1.1.c. N.C.(11,50T) (1NSGT) (IC 5GT)SOT Tem VIEW OF SOCKETS SHOWN
r-
brackets attached to either side of the chassis in front.A piece should also be cut out in front of the chassisto clear the speaker frame, which otherwise is toolarge to fit in the bottom compartment. The speakerframe must be grounded to the chassis.
Mahogany, %" thick, was used to make the cabinet.A shelf is placed inside the case to support the smallsteel chassis, which is held in place by two 1h" right-angle brackets.
The tiny loop antenna (1/16" by 5" by 8"), whichcan be purchased ready-made, comes with a paperbacking so that it may be easily glued in place on anysurface. The best place for it is on the inside of theback cover. Any ordinary glue may be used, but notpaste. The back cover which is only %" thick ishinged to the front portion by two 1 %," brass hingesand is fastened by two small brass latches.
This good - lookingporta ble is light.Follow the sketchbelow for cabinet
LIST OF PARTSP -M speaker, 5".Tuning condenser, 2 gang,
.00036 mfd.Oscillator coil.Iron -core I.F. transform-
ers, 455 kc.Volume control with
D.P.S.T. switch, 1 meg.Tone control, 100,000 ohm.Tubes: 1A7GT, IN5GT,
IH5GT, 1C5GT.Portable 45 -volt "B" bat-
teries (two).Portable 1.5 -volt "A" bat-
teryCarbon resistors:
% watt, 2 meg. (three).% watt, 500,000 ohm.% watt, 250,000 ohm.',4 watt, 75,000 ohm.% watt, 800 ohm.
Tubular condensers:.05 mfd. (three)..01 mfd. (three)..003 mfd.
Mica condensers:.0001 mfd. (two)..0004 mfd. (padding)..00005 mfd.
Miscellaneous: Chassis, cab-inet, 4 octal wafer sock-ets, dial, wire, batterycable, loop antenna, etc.
SHELF----- TO HOLD
CHASSISO
o Cabinetand Shele
132
One -TubeReceiverUSES RECTIFIERAS DETECTOR
BASED on a novel and simple one -tube circuit, operating on the
house current, this new type of radioreceiver brings in stations as clearlyas a crystal set, with absolutely nohum, and with fine selectivity andsensitivity.
Using a rectifier as a detector tube,no filtering system is needed be-cause no plate voltage is used! TheA.C. voltage is needed only to heatthe filaments of the 25Z5, with noconnections between the A.C. lineand the receiver proper. The 25Z5acts as a diode and has its two platesand cathodes tied together. Theplates are connected to the fixedplates of the .00015-mfd. tuning con-denser, to the four -prong coil, and tothe antenna. The cathodes are con-nected to the moving plates of thecondenser and to the other end ofthe grid winding on the coil. Thephones are placed in the cathode cir-cuit of the tube.
The rectifier heater voltage mustbe adjusted critically to from five tosix volts, instead of the rated 25, forsatisfactory operation. As there isno line cord on the market rated at600 ohms, use an 80 -watt fixed re-sistor with an adjustable tap. Other-wise connect two 300 -ohm line cordsin series.
LIST OF PARTSTuning condenser, .00015
mfd.Set of two 4 -prong broad-
cast coils.Rectifier tube, 25Z5.Six -prong wafer socket.Four -prong wafer socket.Wieb - wound resistor
( tapped), 80 watt, 750 ohm.Headphones, 2,000 ohm.Line cord.Black Bakelite panel for
baseboard, 4" by 7".Miscellaneous: Wire, solder,
20' indoor antenna, etc.
The complete receiver with its tube removed from its socket
This close-up view shows how the four main parts are placed
Bottom view, revealing the extremely simple wiring of the set
Only the heater uses power, so a ground may be safely used
133
High -Fidelity AmplifierFOR YOUR P A SYSTEM, PHONOGRAPH, OR F M RECEIVER
EXTREMELY versatile, this 10 -watt audio amplifier may beused with either a crystal or
magnetic pick-up for playing re-corded music, with a crystal mi-crophone for public-address work,as an amplifier for a high-fidelitytuned radio frequency or super -het tuner, or as an amplifier foruse with an FM receiver, such asthe one described in book one( page 44) of this two -book series.
The amplifier employs a uniqueoutput, two different types oftubes being used in a push-pullcircuit. A 6AD7G is used on oneside of the circuit while a 6F6G isused on the other. The pentodesection of the 6AD7G, however,has the same characteristics asthe 6F6G, and the triode portionof the 6AD7G is used as a phase -inverter tube. No push-pull audiotransformers are used. Insteadthe first audio stage ( 6SJ7) isresistance -coupled to the push-pull output stage.
The input is fed through a .005-mfd. mica condenser and 500,000 -ohm volume control to the grid ofthe 6SJ7. This tube is one of thenewest types, using the single -endconstruction where the grid caphas been eliminated from the topof the tube-the grid being con-nected to one of the prongs on thebase of the tube. This, of course,helps to enhance the general ap-pearance of the amplifier, sinceno wires are visible above thechassis.
In the plate circuit of the 6SJ7there is a tone control consistingof a .006-mfd., paper tubular con-denser in series with a 250,000 -ohm variable resistor-one sideof the variable resistor beinggrounded to the chassis. This tonecontrol really acts as a treble con-trol. In other words it cuts outonly the high notes without af-fecting the low notes. No controlfor the low notes was considerednecessary, as these are reproducedby the amplifier to their fullvalue.
The new amplifier (right, above) in use with a FM receiver
Close-up top view of amplifier with cover removed. This andthe bottom view of chassis below will help you locate parts
134
The plate and screen -grid circuits of the63J7 are elaborately decoupled to avoidfeedback. This helps to reduce hum and in-stability in the amplifier. An adequate fil-tering system is also used in the rectifiercircuit-the 350 -volt output from the 5Y3Gfull -wave rectifier being filtered by the
8-mfd. and 16-mfd., 450 -volt electrolytic con-densers and the heavy-duty, 25 -henry filterchoke. This choke is rated to pass 140 mil-liamperes. The power transformer, ope-rating on 110 to 115 -volt A.C. only, has threesecondary windings : a 6.3 -volt winding ratedat 312 amperes for the tube heaters, a 5 -volt
--,
500,0001 /*". 44, 1.50,To.,000,(10ccwVOLUME CONTROL(
- - --- ...SEM123
7".11111
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40 -SHED DUALE L EC TROLTDC
FILTERCHORE OUTPUT
TRANSFORMERCON D...
Elry SHIELDED
INPUT .-, WIRE .1., SPEAKER- irL
115 VOLTS AC
The plan diagram above (compare with undeTchassis view at bottom of opposite page) shows not onlywhere each port is placed but the general path of the connecting wires. Below is shown the schematicwiring diagram. Bottom view of the tubes (below, left) identifies terminals (prongs) of tube elements
TONE .00CONTROL S0,00 0 UP (M D
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SW 11SA.C.
POWERTRANSFORMER
BEA ° 2c1. 501°)x ERP.
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1 ;5
VOLUMECONTROL SiSWITCH
TONE CONTROL
DL/.L ELECTROLYTICCONDENSER
FILTERCHORE
Dimensions of the chassis and additional data onpositioning of parts are given in the drawing above
winding rated at 3 amperes for the rectifier'sfilament, and a high -voltage winding ratedat 375 volts, 90 milliamperes. The powertransformer is fully shielded to protect thewindings and to reduce hum pick-up to aminimum.
In connecting the high-fidelity audio am-plifier with the FM receiver described in arecent issue of POPULAR SCIENCE, the powertube ( 6F6) in the receiver will have to beremoved from its socket and a short piece ofshielded hook-up wire connected to the con-trol -grid pin ( No. 5) on that socket. Theother end of the wire is connected to termi-nal "a" on the amplifier, while the braidedshielding on the wire is connected to termi-nal "b." The shielding on the wire will alsohave to be grounded to some point on thechassis of the FM receiver.
The speaker should be at least an eight -inch model-preferably 10 or even 12 inches.It should be a permanent -magnet type witha voice coil of 6-8 ohms. It should bemounted on a good -size baffle ( 40" square )or a properly designed cabinet. There areseveral special console -type speaker cabinetsavailable which are especially useful withhigh-fidelity speakers. In selecting one ofthese for use with the amplifier, the setbuilder may be governed by his individualtaste and requirements. -
Back of amplifier with the cover removed is shownat the left. Note the jocks for the microphone,phono, or radio connections in the center. At farleft are two banana -plug jacks for speaker ,unnec-Hons. In the front view below, the cover is being re-moved. Note the attractive ventilators, bandies, andmoldings. Chassis finish, marine gray ripple vname!
---;1151111110V
LIST OF PARTSShielded power transformer, 115 volt, A. C.Universal output transformer.Filter choke, 25 -Henry, 160 ohm.Amplifier cabinet, 8" by 9" by 12..Control knobs, red (two).Dial plates, chrome, (two).Volume control and switch, 500,000 ohm.Tone control. 250.000 ohm.Octal wafer sockets (four).Tubes: 6SJ7, 6AD7G. 6F6G, and 5T3G.Electrolytic condensers:
Dual. 8-16 mfd., 450 volt.Tubular, 8 mfd., 950 volt.Tubular, 10 mfd., 25 volt.Tubular, 25 mfd., 50 volt.
Mica condenser. .005 mfd., 600 volt.Paper tubular condensers:
.01 mfd., 400 volt (two).
.005 mfd.. 400 volt (two).
.006 mid., 400 volt.
.5 mfd.. 400 volt.Carbon resistors:
400.000 ohm, 1 watt.300.000 ohm. 1 watt.200.000 ohm. 1 watt.150,000 ohm. 1 watt.100.000 ohm, 1 watt (two).50.000 ohm. 1 watt.40.000 ohm, 1 watt.1,000 ohm. 2 watt.
Wire -wound resistor. 600 ohm, 10 watt.Insulated phone jacks, red (two).Insulated banana jacks, red (two).Line cord and rubber plug.
136
Front, rear, and bottom views of a compact one -tube receiver That you can take along almost anywhere.Power to operate the tiny set is supplied by a 1.5 -volt flash -light cell and a midget 45 -volt battery
MIDGET RADIOHAS
400 -MILE RANGESMALL enough to be carried in a
knapsack on hiking trips, this midgetone -tube broadcast set will receive radiosignals over distances up to 400 miles atnight. It uses one of the new small -sizeRF pentode tubes as a regenerativedetector, and all parts are standard.
To conserve space, an unshielded an-tenna coil is used instead of plug-in coils.Twenty-five turns of No. 34 d.s.c. wireare wound around the lower portion ofthe grid winding of the coil. This servesas the tickler.
A midget seven -prong wafer socket forthe tube is mounted on the 2%" by 2%"by 1%" chassis in the conventionalmanner. A S.P.S.T. switch placed inthe "A" -plus lead turns the set off andon and is mounted on the panel besidethe variable resistor.
The antenna plug is mounted at theupper right-hand corner of the panel.The ground plug is mounted on thechassis beside the tuning condenser, whilethe insulated phone -tip jacks are mountedat the back of the chassis.
A 1.5 -volt flash -light cell and a midget45 -volt battery supply power for the set.Outdoors, a long nail driven into theearth can be used as the ground, while a40 -foot wire strung to the branch of atree serves as an antenna.
TICKLERwinINNG
SW
RFC
r.IONFS
45 VOLTS
15 VOLTS
A small -size RF pentode tube, mounted in a seven -prong wafer socket, serves as a regenerative detector
This give,space, an
on idea of the size of the set. To saveunshielded antenna coil replaces plug -ins
137
MARIE ANTOINETTE DOLL CONCEALS
Dressing -Table RadioWHEN a friend asked me to design
an all -electric dressing -table re-ceiver that would provide goodbroadcast reception, yet would not
look like a radio, I hit on the idea of housingthe circuit in one of the old-fashioned dollspopular some years back for concealing bed-room telephones. The result is the MarieAntoinette radio shown in the photographs.It provides good loudspeaker volume on allmajor stations, yet it matches the frilly fem-ininity of the average dressing table.
Compactness being an important factor,the A.C.-D.C. circuit was designed aroundtwo brand-new dual-purpose tubes, makingit possible to obtain five -tubepower and sensitivity with littlemore than half the number. Thetwo new tubes are designated asthe 6CSG and the 25A7G. The6CSG, which consists of twotriodes, is used in the set as thecombined detector and first audio -amplifier stage. The detector, us-ing the bias method of detection,is resistance -coupled to the firstaudio stage. Bias for the detectortube is obtained by means of a100,000 -ohm, 1/2 -watt resistor by-passed by a 5-mfd. electrolyticcondenser.
The second new tube used, the25A7G, serves as a combinednigh -power pentode output tubeand a half -wave rectifier. The25A7G, although similar to theolder 12A7, has a much higheroutput rating. Consequently, ex-xeme care must be exercised toobtain adequate filtering to elimi-nate any possibility of troubleiue to excessive hum. In thecircuit shown, additional filteringwas incorporated in the design inthe form of a 1,000 -ohm fixedresisror and a 24-mfd. electrolyticcondenser. These parts are in-dicated in the wiring diagram ofthe circuit. Resistance couplingwas used to couple the first andsecond audio stages, and a 5 -inchpermanent -magnet dynamicspeaker was chosen to handle theoutput.
All parts, with the exception of
the speaker, can be mounted directly on the2 by 51/2 by 8 -in. aluminum chassis. Forconvenience in wiring, the antenna coil canbe mounted directly on the ganged tuningcondenser, while the radio -frequency coil, thefilter choke, and the electrolytic condenserscan be mounted under the chassis. The gridlead to the 6C8G should be shielded to insurestability of operation and to avoid any possi-bility of troublesome coupling with otherparts of the set. Both the 6K7G ( radio -fre-quency tube) and the 6C8G can be shieldedby means of the conventional ventilatedaluminum cans.
When connecting up the rectifier and power
Wire and glue can be used to mount the porcelain figurine on thealuminum brace at the top of the cabinet. Suitable dolls, oncepopular as telephone covers, can be obtained for a dollar or less
138
circuit, care should be taken inwiring the power cord and itsbuilt-in filament resistor. Theresistance wire, generally dis-tinguishable by its asbestos cov-ering, must be connected to thefilaments.
To form the cabinet and providea mounting for the doll figure,two rounded wooden panels, 81in. high, 8 in. wide at the base,and 5/16 in. thick are attached tothe chassis. The panels shouldbe reenforced at the top with analuminum strip 51,i in. long and11,:; in. wide. This also serves asthe support for the doll. Strongglue and wire can be used tohold the porcelain body of the
The completed Marie Antoinette coil forms anattractive and useful decoration for a dress-ing table. The cabinet is shown at the left
.004MPD.
5K7G4N
300OH M5
150,000OHMS
.0002WFD.
0.1MEG.
11 PC(23 ,..1n.)
.)2MFD.
M F D. 0.. M
`20,000 OHMS 24N 6K7G iC8G 2.5A -G I. kiFO.(vpiLumE CONTROL)
5PEN10E.7-
.01
a-
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1
MEG
0.5MEG.
1000 C116,1-5
1_ DICATES CONNECT ONTO CHASSIS
-2P0 OHMStJ 510-V. A.C.- LC. 0--
6K7G
G1
4 5 3K102 .6
P2
2.HH.
NC. K2 T
1 8
6C8G
25A7G
When wiring thetubes followthese socket dia-grams carefully
139
doll securely in place.In the original, thedoll's dress with its fullskirt was made of taffe-ta and bordered aroundthe bottom with a lacefrill 2 in. wide. Anymaterials available,however, can be used.This completely hidesthe receiver. The dollwas purchased at asmall antique shop fora dollar. With a littleshopping, however, less -expensive figures canbe obtained.
To provide adequateventilation for thetubes, a large holeshould be cut in therear panel of the cabi-net. A sheet of card-board; or better still,asbestos board; shouldbe placed at the top ofthe cabinet to protectthe skirt material from the heat of the tubes.The two tuning controls, one for the gangedtuning condenser and the other for the combi-nation volume control and switch, can eitherbe hidden under the doll's skirt or mountedon the outside.
No outside ground connection is needed with the re-ceiver. Only the short flexible antenna wire, atthe left, is necessary to get excellent reception
Condensers and other small parts are mounted under the compact chassis
In use, the receiver requires only an an-tenna, twenty or thirty feet long, strungaround the baseboard or ceiling molding. Noground should be used, as the A.C.-D.C. hook-up is grounded through the house wiring andadditional grounding would blow the tubes.
LIST OF PARTSTwo -gang tuning condenser, .00036
mfd.Fixed condensers, two, .1 mfd.Fixed condensers, two, .02 mfd.Fixed condenser, .05 mfd.Fixed condenser, .01 mfd.Fixed condenser, mica, .004 mfd.Fixed condenser, mica, .0002 mfd.Electrolytic condenser, dual, 8-8 mfd.,
250 volts.Electrolytic condenser, dual, 5-5 mfd.,
25 volts.Electrolytic condenser, 10 mfd., 50
volts.Electrolytic condenser, 24 mfd., 150
volts.Fixed resistor, 100,000 ohms, 1,4 watt.Fixed resistor, 150,000 ohms, 1,4 watt.Fixed resistor, % meg., % watt.Fixed resistor, 1 meg., % watt.Fixed resistor, 300 ohms, % watt.Fixed resistor, 700 ohms, 1 watt.Fixed resistor, 3,000 ohms, 1 watt.Fixed resistor, 1,000 ohms, 1 watt.Plate choke, 2.5 mh.Filter choke, 12 h.Midget volume control with switch.
20,000 ohms.Miscellaneous.-Power cord with 280 -ohm resistor, matched antenna coiland radio -frequency coil, speaker,chassis, tubes, sockets, shields, doll,dress materials, wire, solder, etc.
140
Servicing Your RadioOFTEN minor adjustments that can be
made at home will correct radio -re-ceiver difficulties or give additional recep-tion to an old set. Below are methods for
65 70 90 0 13060; 1150,55 mi..41 :1601
REPLACE THE POWER PENTODE TUBE if the pilotlight goes on and then immediately goesoff again each time the receiver is turnedon. The trouble is a faulty heater in the50L6GT tube. It makes proper connectioninside the tube when cold, but when itwarms up the contact is broken.
NOISY TUNING IN THE LATEST RADIOS may becaused by filings between plates of the oscil-lator tuning condenser. These may beburned out by connecting the secondary of a700 -volt power transformer across the con-denser, as shown above and in the diagram.Be sure to disconnect the house current andthe grid connection to the oscillator unit.
detecting a faulty heater in the power pen-tode tube, extending reception at the upperend of the dial, eliminating noise in a newcondenser, altering a midget filter circuit.
1,600 -KC. STATIONS CAN BE BROUGHT IN on an oldAC -DC midget set by readjusting the trim-mer on the oscillator tuning condenser. Thecapacity is reduced a sixteenth of a turn ata time by loosening the nut shown at leftabove. Then loosen the nut on the other con-denser to increase volume.
THIS SIMPLIFIED FILTER CIRCUIT for a midget AC -DC receiver uses only one electrolytic con-denser, and may prove worth substitutingwhen servicing a filter circuit that containstwo or more electrolytic condensers if re-placements for these condensers are hard toobtain. The diagram below shows how thesubstitute wiring is done.
PD+
.E--WYWIYMTO
CIRCUIT
2ECEIVEP.
1000 u.n'ATT '1000
WATT
2..C., TO -tanIrn.(ELEcrgoLs. TIC
CONDENSER''=-
TOHEC`'FIER
141
Easily Assembled UnitWITH the easily assembled unit illus-
trated, you can convert your regularsuperheterodyne set into a "diversity
receiver" that will provide signals of constantstrength free from the bothersome fading socommon to short-wave reception and oftenexperienced on the broadcast band. Called a"diversity coupler," the unit operates on thesame principle as the "diversity receivers"with this one main difference-what the re-ceivers do electrically, the coupler does me-chanically.
The secret of diversity reception is basedon the fact that the degree of fading of a
given signal varies with different antennas.The coupler, acting as a link between two an-tennas-one vertical and the other horizontal-and the receiver, automatically couples thereceiver by means of a motor -driven variablecondenser to the antenna providing thestronger signal.
The coupling condenser, sold complete withits motor, consists of three plates, two sta-tionary and one movable. One stationaryplate is connected to the vertical antenna,the other to the horizontal antenna, while themovable plate is wired to the receiver's an-tenna terminal. As the signal in one antenna
or the other fades, the movableplate is turned automatically toa new position. Thus, the an-tenna receiving the strongersignal becomes "capacity cou-pled" to the receiver in di-rect proportion to the signalstrength.
Three tubes are used in thecoupler circuit, a 6B8G ampli-fier, a 2A4G control tube, and a25Z6 rectifier. The 2A4G, theheart of the circuit, acts as asensitive relay to control thetiny magnetic motor that op-erates the dual coupling con-denser. When the signal fades,this tube allows current to flowto the motor until the condenserhas been adjusted to the new
All connections must be soldered carefully, and a meta! shieldas at the right, must be used to eliminate radio interference
142
Kills Radio Fadinsetting for maximum signalstrength. The action is en-tirely automatic, and soswift that audible signalstrength is constant.
In wiring the circuit, fol-low the diagram carefully.Make sure that all connec-tions are well soldered,since any poor joints willcause losses that will com-pletely upset the balance ofthe hook-up. When connect-ing the coupler to your re-ceiver, use shielded cableand ground the outersheathing as indicated.Shielded cable also shouldbe used for the antennalead-ins. A wafer adaptercan be used to make thenecessary connection to theplate prong of the receiv-er's last intermediate -fre-quency tube, as shown inthe diagram.
To adjust the coupler,tune in a short-wave signalwhich is fading, as indi-cated by the rise and fall involume, or by the fluctua-
HORIZONTALANTENNA
VERTICALANTENNA
SHIELDED CABLE(GROUNDED TOCOUPLER CHASSIS)
TO CHASSIS
SHIELDED CABLE(GROUNDED TO RECEIVERCOUPLER CHASSIS)
COUPLER
SHIELDEDCABLE
TO PLATEOF FINALI F TUBE
How the various part, are arrarged above and below the metalchassis The iny motor, which turns the dual condenser to tune in theantenna giv ng the strong signcl, is on the top face of the chassis
D2
P
\21.8
7
0N.C.
6066N
C -I ;624303
I MEC,
SHIELDEDVi CABLE
IC PLATE OFLAST I.F. TUBEOF RicE vv.;
110-V. A.C.(50-60'.0N00
I F. T. (2A40)L
SW.ON P,
LutaMFD.
r-VERTICAL ANT.
TO ANTCN SET
NORIZONTALANT.
r
.411
'44
RELAY
ANTENNASELECTING
NDENSERr 5 (2526)
These drawings show how the coupler is connected lo receiver and antennas and the complete wiring diagram
143
A close-up of the motor that turns the dual condenser
tions of the visible tuning meter if the set isequipped with one. Then advance the sensi-tivity control or 500,000 -ohm potentiometeruntil the signal remains constant. Do notturn the control too far or the motor willtend to operate continuously. It is best toadjust it to a point just below that of maxi-mum response. The best point can be per-manently marked on the dial with a pencil.
Some experimenting may be necessary inplacing the two antennas for best results.
LIST OF PARTSFOR RADIO -FADING ELIMINATOR
Antenna - selecting condenser, andmotor.
Plate transformer.Filament transformer.Intermediate - frequency transformer,
455 Kc.Trimmer condenser (C-1).Condenser, 10 mfd., 300 volt.Fixed condenser, 20 mmfd.Tubular condenser, .5 mfd., 200 volt.Tubular condenser, .1 mfd., 200 volt.Tubular condenser, .1 mfd., 400 volt.Mica condenser, .0005 mfd.Two resistors, 150 ohm, 1/4 watt.Resistor, 1 meg., 1/2 watt.Resistor, 100,000 ohm, MI watt.Volume control (500,000 ohm) with
switch.6B8G tube.2A4G tube.25Z6 tube.Three octal sockets.Toggle on -off switch.Special chassis with cover and bottom
plate.Miscellaneous:-Nuts, clips, washers,
lugs, wire, etc.
ALL -WAVEHERE is an inexpensive alternating or
direct -current receiver that covers allthe wave bands from 430 to 10,000
kilocycles. It is of the regenerative type, butthe functions of regeneration and detectionare separate. Regeneration is furnished bya 6C5 tube and is controlled independentlyof the 6J7 detector tube, while the outputtube, a 25L6, provides ample power for a6 -inch dynamic speaker, as well as earphones.
Contributing to the low cost of the set arethe four tuning coils (I., L, La, and L4),which you can wind yourself with No. 26,enameled -copper wire, on tubing 11.'2 inchesin diameter. Three of these coils (L2, La, andL.) are wound on one form as shown in thediagram below. The only coil you need buyis a universal -wound inductance coil (L.)used to tune the circuit to the very longwave bands. It may have any inductancevalue from 2.5 to 60 millihenries. A tapswitch instantly selects the coil for the wavelength desired. It should be noted that thelower end of the coil (La) goes to the cathodeof the 6C5 tube instead of being grounded tothe chassis. Plate voltage, and consequentlythe regeneration, of the 6C5 tube, is con-trolled by a .25-megohm potentiometer.
As a safeguard, a "polarized" plug is usedon the power cord. One blade of the plug isslightly wider than the other for easy identi-fication. When first plugging it into the out-let to be used with the set, take a 10 -wattlamp and test to see if there is current flow-ing between the set chassis and a grounded
-TO TAP SWITCH
FL1-190 TURNSNo.26 ENAMEL
WIRE
-'----SPADE LUG
LOWER END OF COILS SOLDERED TOGROUNDED LUG
TO SWITCH
L4-7 TURNS
TO SWITCH
L3 -I9 TURNS
TO SWITCH
1.1-56 TURNSNo 26 ENAMELCOPPER WIRE
Follow these specifications in winding the coils
144
RECEIVER for Radio Amateurswire. If there is, reverse the leads of thepolarized plug. Thereafter, when the plug isinserted in the outlet in the same position, asolid ground connection to the chassis willentail no danger of a shock or a short.
The -inch plywood panel measures 7 by141/2 inches, and the chassis and sidesare cut from 18 -gauge galvanized iron
or cold -rolled sheet stil, and bolted together.A 50 to 150 -foot antenna, with the antennacoupling condenser set near maximum ca-pacity, will work best for the longer wavebands. On the short waves, an aerial wirefrom 5 to 25 feet long can be used.
This view of the setshows the arrangementof --e various parts
Yeu sar switch frorr earphones toloArpe3ger of ar-, wave band
.0001 mFD..0001
m F o. 5.0002 m PO.
.000365mFO.
L1 1-2 1.3 1.4
POLARi2EDPLUG
110-14A D.0
220 OHMS
0.25 MFD.
0.25 MEG.
k,IFD. ROO
NA4.4>CHOKE (30
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CLOSED-CIRCUITJACK FOR
PHONES
FIELD COIL(3000 OHMS)
Two novel features of the hook-up are the polarized power plug, and the use of a separate tube for regeneration
145
CAMPER'S RADIOHow the resistors andcondensers are wired.The fishpole antennaplugs into a jack onthe top control panel
SMALL enough tobe slung overyour shoulder
like a camera, thislightweight portablehas its own built-inbattery supply anduses an ordinary steelfishing rod as an an-tenna. Housed in anattractive yet sturdycloth -covered cabinet,the midget earphoneset forms an idealcompanion for use oncamping and fishingtrips, picnics, and long automobile tours.
Three of the latest -type midget tubes formthe basis for the compact circuit. Operatingon a filament voltage of 1 L z volts and a platesupply of 45 volts, the tiny tubes not onlymake it possible to pack a three -tube hook-upinto a small amount of space, but they cutdown the number of necessary batteries to aminimum-a 45 -volt "B" battery and a small11i -volt dry cell. These tubes, of Englishmanufacture, can be obtained from almost
The chassis and panel assemblyat the right fits snugly intothe top of the compact cabinet
any large radio -parts supply house and mustbe used with the special midget sockets de-signed for them. In wiring the tubes, bear inmind that the metal connection at the top ofeach XSG tube is not the grid, as on mostAmerican tubes, but the plate.
To obtain the highest possible volume, iron -core coils are used. However, to save spacethey should be removed from their aluminumshielding cans. To obtain the necessary feed-back in the detector or second coil, fifteen
146
Uses Fishpole AntennaCamera -Size Set
turns of No. 28 double -cot-ton -covered wire should bemounted close to the gridwinding. This is indicated inthe wiring diagram.
Composition -insulated tun-ing condensers are used in-stead of the usual air -spacedvariety. They cost no more,and do conserve space. How-ever, since these condenserscannot be ganged or mountedon a single shaft, each stagemust be tuned separately.
A 500,000 -ohm variable re-sistor connected across the"B" battery serves to controlthe volume. To avoid any un-necessary drain on the bat-teries while the set is not inuse, a switch should be placedin the ground lead of the vol-ume control. This switch canbe ganged with a secondswitch placed in the minuslead to the "A" battery, sothat both then can be con-trolled by a single knob onthe panel.
The cabinet used for theoriginal set illustrated wasmade of wood and coveredwith airplane cloth. If youdesire, it can be covered withimitation leather. To simplifythe problem of changing thebattery supply, two hingedcovers are used-one at thetop to cover the control paneland the other at the bottomto close the battery compart-ment. A leather strap runthrough leather loops mount-ed on the sides of the cabinet makes it easy to carrythe receiver.
No ground is needed in operating the set, only anantenna, which need be nothing more than the businessend of an inexpensive telescoping steel fishing rod. Forthe original, the author sawed about %" from the bot-tom end of the main ferrule that fits into the socket inthe fishing -rod handle and soldered a short banana -typeplug in place. This plug fits in a small jack mounted inthe lower left-hand corner of the control panel as shown
If you wire the receiver carefully according to the diagram belowyou wi l be able to get good radio reception miles from home
AT (1.5 V.)
03GRID
5CnEEN PLATE
0z 40
10 05
Follow these socket diagrams when makingthe various connections to the three tubes
147
Two small batteries housed in the bottom of the cabi-net form the tiny receiver's complete battery supply
in one of the photographs. If desired, a wirefrom the antenna plug can simply be clippedto the end of the fishing -pole handle. As amatter of fact, the receiver can be used whilefishing-the steel rod serving not only tocatch fish, but to snare the radio waves. Inthis case, however, the fishing rod must beone fitted with a cork or other nonconductinghandle to insulate the antenna and eliminatebody capacity.
In laying out the parts and wiring, followthe photographs and diagram closely. Thetubes and coils have been arranged with aneye to compactness and ease in wiring. Beparticularly careful in making the connec-tions to the three tube sockets, and followthe socket diagrams to the letter.
LIST OF PARTSFOR CAMPER'S RADIO
Volume control, 500,000 ohms.Iron -core antenna coil.Iron -core radio -frequency coil.Variable condensers, two, .00042 mfd.Fixed resistor, 2 megohm, lh watt.Fixed resistor, 1 megohm, 34 watt.Fixed resistor, 300,000 ohr..d, lh watt.Tubular condensers, two, .01 mfd.Tubular condenser, .02 mfd.Mica condenser, .002 mfdMica condenser, .0002 mfd.Mica condenser, .0005 mfd.
Miscellaneous: Tubes (two XSG, oneXY), one 45 -volt "B" battery, one 114 -volt midget dry cell, chassis, cabinet,special tube sockets, switches, four in-sulated plugs and jacks, cabinet, fish -pole, knobs, dials, wire, solder. etc.
HOW /frajaiiTO a'
HE problem of setting up an effi-cient television antenna for your-self or one of your friends willbe greatly simplified if you will
follow a few simple rules. First of all,although television antennas may varyin appearance and size, they are basicallyall alike, consisting of a quarter -wavedoublet antenna of predetermined length.A "reflector" antenna, similar in designbut from eight to ten inches longer, ismounted beside the doublet when neces-sary to eliminate "ghosts," or unwanted,out -of -register duplications of the tele-vision image on the fluorescent screen.Such ghosts usually are caused by a re-flection of the signals from near -by build-ings, just as visible light waves-whichtelevision waves resemble in many char-acteristics-reflect from a wall or othersurface.
It is advisable to have a helper whensetting up the antenna, so that one per-son can adjust its position while the othertests the operation of the receiver. Locatethe antenna in a clear space on the high -
148
a Tele io 74PlieKita
0.14.
a) WITHDOUBLET
1/4a1 WITH REFLECTOR
MAST TO SET )
.FREQUENCY
(a) 44-56 MC
(b) 66 -72 MC
(c) 78 -90 MC(d) 96-109 MC
Li
122 INCHES
83 INCHES
70 INCHES
58 INCHES
LI
DgWutkiLE
Li134 INCHES
93 INCHES
80 INCHES
66 INCHES
.NOTE: REFLECTOR IS ALWAYS 8 TO 12 INCHES
LONGER THAN RECEIVING ANTENNA
est possible part of a building, and on the sidetoward the transmitter. If the antenna and re-ceiving set are not within easy shouting distancemuch time can be saved with a two-way, portabletelephone system strung up temporarily betweenthe two points.
Television antennas on awooden or an iron mast of substantial design, se-curely anchored to the building. They are Usuallyfurnished properly matched for the 45 to 55-millicycle "vision"transmitters. If yourlocal television sta-tion operates on adifferent frequency,the metal rods mustbe cut to the overalllengths shown in theaccompanying table.Regardless of its de-sign, the antennawill have to be ro-tated until the maxi-mum signal strengthis obtained, which isusually when thequarter -wave rodsare at right anglesto a line between theantenna and thetransmitter. Severalcommercial anten-nas are fitted withflexible couplingswhich simplify thisadjustment.
Several types of antennas.A two-way telephone be-tween fhe set and antennahelps to make adjustments
149
Lin is/ors:a IPower Supplyfor Battery iteceivers,
EASILY BUILT UNIT FOR FARMS ANDVACATION CAMPS USES STORAGEBATTERY OR 110 -VOLT HOUSE LINE
The vibrator is mounted on top of the chassis withthe transformer, rectifier Tube, and 110 -volt socket
One six -volt battery is oll that is needed. It cawbe either a storage battery or a dry -cell unit
Operating a battery radio with the power supply
NO MORE worrying about "B" batter-ies for that battery -operated radio atthe summer camp or on the farm if
this universal power supply is hooked to theset. One 6 -volt battery is all that is needed.And, by simply snapping a switch, the powersupply can be converted to operate a batteryset on 110 -volt A. C.
Easy to build, this vibrator unit steps upthe 6 -volt power to from 100 to 300 voltsfor the tube -plate current. It will operateany receiver or amplifier circuit using six -volt tubes where the total B drain is notmore than 100 milliamperes.
Four -tube battery receivers with 1.5 -volttubes also may be operated with this unit ifthe tube filaments are connected in series( 4 x 1.5 volts=6 volts). However, this maybe done only when the vibrator is operatedoff a 6 -volt battery, as the heater voltage isA. C. when the unit is connected to a 110 -volt line, and the unit must NOT be used on110 -volt D. C. current.
The pack is built on a steel chassis meas-uring 2" by 7" by 9". On top are mountedthe power transformer, four -prong vibrator,84 rectifier tube and 110 -volt socket. Be-neath the chassis are the chokes, resistances,and condensers.
150
SOLDEREDTO CHASSIS-
TO 6 VOLTSTORA-:E BATT.
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I6MFD450 V.
16MFD450V.
ALAOL,--J.
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SOLDEREDTO CHASSIS
POWEATRANSFORMERON UPPER SIDEOF CHASSIS
AC -DC SWITCH RF.A'Cifone
Schematic drawing of power -supply circuit. It will
The 30 -henry filter choke should pass atleast 100 milliamperes to avoid overheatingand damage to the windings. The 8 -milli-henry radio -frequency choke and 1-mfd. by-pass paper condenser in the "B +" lead, andthe "A" choke and 25-mfd. condenser in the"A -" lead provide the necessary RF filter-ing to prevent interference with the receiver.The "A" choke can be easily made at homeby winding closely 20 turns of No. 14 enam-
operate a 20-,Ao+, CW or I2 -watt phone transmitter
eled wire xi a bakelite tubing with an out-side diameter of 1/2".
A little vibrator static may be experiencedwhen the unit is connected to a 1.5 -volt bat-tery receiver, but with 6 -volt tubes no humor vibrator static is noticeable. In each casethe power -supply chassis should be con-nected to a good external ground and the re-ceiver's ground connection disconnected.
The grounded side of the tube heaters
The special power transformer makes it possible to Bottom view of chassis, showing the positions ofoperate a battery set on a 110 -volt A.G. supply the various chokes, resistances, and condensers
151
V P.D.T.ROTARY'.
SWITCH
84(RECTIFIER)
ooH o.
BOTTOM VIEW
RrA'04015E (SEE TEXT)
I=36V.r--' HEATERS
0
IS AMP 6VFUSE BAT T.
110 V. AC
V.HEATERSI
20,000.,SO WATTS
Wiring diagram for power pack. It's easy to build
must be connected to the grounded side ofthe 6 -volt heater connections on the powerunit. Do not use a "B -" lead to the unit.The "B -" connection is automatically madethrough the grounded 6 -volt heater lead.
With an inexpensive vibrator, it is advis-able to use the 300 -volt tap when running thepower supply off a 6 -volt battery, because aninexpensive vibrator will have a low output.
PARTS FOR POWER SUPPLYSpecial 6 -115 -volt power transformer.Full -wave rectifier tube 84.Four -prong vibrator.Steel chassis, 2" by 7" by 9".Rotary D. P. D. T. switch.Fuse and holder, 15 ampere.Filter choke, 30 henry, 200 ohm.Semivariable resistor, 20,000 ohm, 50
watt.R. F. choke, 8 millihenry.Carbon resistors (two), 50 ohm, 1 watt.Electrolytic condenser, 20 mfd., 150 volt.Electrolytic condenser, 4 mfd., 450 volt.Electrolytic condensers (two) 16 mfd.,
450 volt.Electrolytic condenser, 8 mfd., 450 volt.Paper tubular condenser, 1 mfd., 400
volt.Oil -impregnated condensers (two) .01
mfd., 1,600 volt.Five -prong wafer socket.Four -prong wafer socket.Wire, tubing, 110 -volt socket, etc.
MIDGET RADIOUSES TWODETECTORS
GREATER EFFICIENCY from thedetector stage is achieved in this
simple detector set by using separatetubes for detection and regeneration. Itis so efficient that in some localities itwill operate a small magnetic speaker.
When using a screen grid or even apentode in the detector stage, it is neces-sary to apply a low "B" voltage on thescreen, sometimes as low as 22 volts, sothe tube cannot be operated at maximumefficiency. However, with this arrange-ment of a separate triode tube for regen-eration, 671/2 or even 90 volts can beapplied to the screen.
The one -tube detector stage may alsobe used as a medium -power audio ampli-fier for a crystal pick-up. The pick-upmust have an output of at least 1% volts,preferably 3 volts. The clip on the gridcap of the 1N5G is removed and one lead( unshielded) of the pick-up is connectedto it, while the other lead ( shielded) isconnected to the chassis. Phones, a smallmagnetic speaker, or a PM speaker ( 5 -inch) may be used to listen to phono-graph recordings.
A 100,000 -ohm potentiometer is usedto control regeneration. One side of thecontrol is grounded to the chassis. Asthis puts a resistance across the "B" bat-tery, even when the receiver is not in use,and causes the "B" battery to run downsooner, it is advisable to include a switch,as shown in the diagram ( SW2). Insteadof buying two S.P.S.T. switches, a D.P.S.T. may be used.
Any good antenna and ground installa-tion may be used with this set. However,if the amateur has not, as yet, an antennarigged up he can test the set with a 20 -foot piece of wire lying on the floor.Short waves may be received on thisradio with suitable plug -in -coils. In thiscase it is advisable to use a small trim-mer condenser in the antenna lead( .00005 mfd. or 50 mmfd.)
A 7" by 7" by 2" steel cadmium -platedchassis is used, and, by mounting thetuning controls as shown in the photo-graphs, the metal panel can be dispensedwith, thus cutting down the cost.
Separate tubes for detection and regeneration give this simple receiver its remarkable efficiency
TAlsomEP,CONDENSERMA USE ONSHORT WAVE)
PRIMARY
DISCONNECT WHENUSING PICK-UP
2.5Ah.PLATE CHOKE
O&TiCyLER
.0001P.IFD
DISCONNECT WHEN..'-USING TRIMMER COND.
ESECONDARY
0 .00014 m6
CRYSTALPICK -LIP
MEG
CAP 3 ",,s6
C:=3 .0001 MED 'N..
Top and bottom views of chassis. By mounting thecontrols thus, the metal panel is dispensed with
5 MEG
0PHONE OASPEAKER
1001000 GI/ t. GC BSI7-F r
45 V. q. eciiswz TO 90V. N....Lit/
-- -0-5 IG4G
0I.5 -AV.
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DISCONNECT FSWITCH IS USED
How the circuit is wired. Any good antenna andground installation may be used with this set
EV;
Portable AC -DC Signal TesterOscillator Can BeUsed as Wave Meteron Broadcast Band
INEXPENSIVE to build, this port-able AC -DC test oscillator recom-
mends itself to present conditions,for it may be used either in theshop or taken out on calls. It oper-ates on both AC and DC currentand there is no need to worry aboutwhich current a customer mighthave. The latest tubes are used andthe output is a pure, audible soundthat can be heard easily through the speakerof the set being tested.
This oscillator will align the I.F. stages ofany superheterodyne having a frequencybetween 300 kc. and 850 kc. Most I.F. stagesare peaked at about 456 kc. Additionally itwill serve as a wave meter for the broadcastband, enabling one to identify unknown,distant, or local stations. A simple two -tubecircuit is used, consisting of a half -waverectifier (25Z6-GT/G) and a parallel -feedoscillator of the electron -coupled typa,(6SK7).
Two coils are needed for this operation,
Below, front view of coils on chassis and other parts
Oscillator shown in use with test prong hooked to screen grid cap
one to cover the 300-kc. to 850-kc. band andthe other the broadcast band ( 1,750 kc. to550 kc.). Directions for making these coilsare in the accompanying sketch. Beforeusing, the oscillator must be completelyinclosed in a steel cabinet. A .05-mfd. con-denser grounds the cabinet to the chassis.Do not ground the cabinet directly to thechassis, for there is danger of a short,especially where the oscillator is being usedon a DC line. The .02-mfd. condenser mustbe mica insulated and not paper insula ted.unless it is non -inductively wound. Thesimplest way to calibrate the oscillator for
Bottom view of chassis. illustrating wiring of unit
154
FILTER2 5 76-GT/6 CHOKE 8-8MFO
9
.11111.
330 KC 6.510TO TUNING
850 HC CONDENSER
ON -OFF 00036ROTARY COIL CONTROLSWITCH SWITCH
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5 MEG. 6
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METAL CABINET 704MFD-1.1.6:A'- AN TENNA
G'.- GROUND (CHAS', I OF
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r I-I GRID CAT H. CATH:
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As shown in the wiring diagram, two taps are used on resistor. Thesecon be moved up and down resistor until correct voltages are ob-tained. Directions for making coils are given in detail at left. Coils aremounted on chassis with a 11/4" machine screw and a'" brass spacer
aligning a superheterodyne is to use a goodradio receiver having an I.F. frequency of456 kc. Then remove the grid clip from thefirst I.F. tube and connect point "A" on theoscillator to the grid cap of the I.F. tube.Rotate the dial until a signal is heard in thespeaker. This point on the dial will corre-
Cabinet with dial, brass handle and two self -tap-ping screws appears below. Note its compactness
spond to the I.F. frequency of the receiver.In order to align the I.F. stages of other sets,tuned to 456 kc., switch the oscillator on andturn the dial to the correct position for 456kc. The oscillator is now sending out a notetuned to 456 kc. Next, adjust the trimmerson the I.F. coil cans, until the note is heard.
LIST OF PARTSCabinet, 'Tu.:" by 8" by 10U". Black
wrinkle finish.Cadmium -plated chassis, 2" by 7" by 9".Tuning condenser, .00036 mfd.Coil form, rte.:" diameter by 314" long.Coil form, 11i" diameter by 3112" long.Filter choke, 15 henrys, 550 ohms.Dual electrolytic condenser, 8I8 mfd.,
450 volts.Octal tube sockets (two).Rotary switch, S.P.S.T.Variable resistor, 75,000 ohms.Wire -wound resistor, 10,000 ohms, 25
watts.Porcelain binding posts (two).Tubes: 25Z6-GT/G and 6SK7.
155
Servicing Your RadioI" DIA.WOODHEAD
6' WOODLEGS
PICTUREWIRE(BRASS)
6-32 MACHINESCREWS
SOME SERVICEMEN HAVE TROUBLE with the AC -DC receivers that use the 35Z5-GT rectifiertube. This is shown by the pilot bulb blow-ing out when a new tube is inserted and theset is switched on. When these blowoutsoccur, the section of the tube's heater-justacross the pilot bulb-is defective. As theset is switched on, the pilot bulb carries theload and, of course, blows out. The remedyis to replace the 35Z5-GT.
WORK CAN BE SPEEDED UP inservicing old radios with thisgadget made from woodenphoto tongs, such as is usedon wet prints. Switch on thereceiver and begin short-ing the different fixed resist-ors. When the defective oneis shorted, reception improves.
WPB HAS REDUCED TUBE TYPES bysome 349 varieties, because theymade up only about Ira of thetotal yearly sales. Smaller GTtubes will replace the larger Gtypes, without loss in output.Sets formerly using a 25Z6 -G, ora 25Z6-GT, will now obtain iden-tical results with the 25Z6-GT/Gtube.
INTERMITTENT AND FADING RECEPTION on someAC -DC receivers and phone combinationsis caused by faulty construction of newsingle -ended tubes, without grid caps. Totest for this trouble, tap all tubes with awooden screwdriver handle, as shown be-low, and the defective one will induce static.It should be replaced.
HANDY DEVICE TO HAVE on the workbenchis this homemade neon condenser tester. Itwill work with either paper tubular or micacondensers of any capacity. If the condenserbeing tested is good, the neon will flash once.But if the condenser is leaky or shorted theneon will glow and sputter, showing it shouldbe replaced. Tester works on DC or 90-v.battery.
156
Book -LightRadio
MOUNTED on the bracket of abook light, this tiny broadcast
receiver will be found extremely use-ful while reading after you've gone tobed. Earphones, two small batteries,aerial, and ground complete the set.Instead of the usual radio -frequencychoke in the plate lead, a 10,000 -ohm,half -watt resistor is used, while a pad-ding condenser with a maximum ca-pacity of .0004 mfd. forms the tuningcondenser. A similar padding conden-ser is used as a regeneration control.The tuning coil (L.) is a midget -typeantenna coil such as used in A.C.-D.C.receivers, wound with thirty turns ofdouble -silk -covered wire around thelower end. This unit comprises the ticker coil (1..).
The cabinet is %" thick walnut, 1 %" by 3 %" by3%", glued together and shellacked. Fiber boardmay be nailed on the back, and the unit fastenedto the stem of the book lamp with a single bracket.
Podding condensers replace the usual -type tun-ing condensers. Note cabinet details at right
Controls are handy a you read with this radio turned onWOUND ON'COIL
Details of the cir-cuit are shown indiagram. Its tinyI V2 -volt tube uses a45 -volt "B.' batteryfor plate current
10,000 OHMS
1 5BOTTOM VIEW
OF TUBE
157
Six tubes give thiscircuit enough pow-er to bring in sta-tions from all overthe globe. Note con-trol panel above
K N'.:.71IL$
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158
Receiver COSTS $20 TO BUILD
TWENTY DOLLARS will build this pow-erful six -tube communications receiver,which includes all the more important
features necessary for good all-around re-ception. Although only six tubes are used,two of these are dual-purpose tubes (the6A8 which acts as acombined first detec-tor and oscillator, andthe 6Q7 which servesas the second detectorand first audio am-plifier) resulting ineight - tube perform-ance. This set hasplenty of power tobring in short - wavestations from anypart of the globe.
The circuit coversthe entire range fromtwelve meters up to550 meters. This range is divided into fourbands (550 to 200 meters, 200 to 75 meters,75 to 35 meters, 35 to 12 meters) by meansof a selector -type switch which is noiselessin operation. Among its other features area sensitive, self-contained dynamic speaker,a stand-by switch, a beat -frequency oscil-
The set includes many of the outstanding features of the latest models
lator with pitch control and on -off switch,tone
control, and provisions for the use of eithera doublet or L -type antenna.
No band spread has been used in the orig-inal design as satisfactory results were ob-tained by the small friction -drive vernier
O.Sly K
(607)
Stfi
p.
3A
5
.4,2 Co) 7
(6r6G)
(61(7)
(6.17)
('W)
COMPLETE WIRING DATAfor the circuit and the tubes aregiven in the diagrams at the leftand above. A supersensitive bondspread ;s shown in dotted lines
159
liSSMSAND SW
BROADCASTANTENNA
COIL
QSCILLATOR. TONE CONTROL VOLUME,SWiTC4 .ANO SWITCH, 'CONTROL.
STAND-BYSW ITCH dial shown next to the
outer rim of the largetuning dial. Mostreaders will find thissystem sufficient fortheir needs, especiallyas this vernier controlcosts only ten cents!However, those whowish to add an elec-trical band -spreadsystem may do so. Itinvolves only a slightadditional cost offrom one to two dol-lars, and consistsmerely of wiring atwo -gang tuning con-denser having a verylow maximum capaci-ty (approximately fif-teen micromicrofar-ads) across the maintwo -gang tuning con-denser as shown bydotted lines in thediagram. It can bemounted between thespeaker and maintuning control. Thissystem is used onlyfor the short-wavebands and becomesinoperative on thebroadcast band.
All -metal tubes areused in all stages butthe last. In this stage(the output or secondaudio amplifier) an
PARTS FOR THE COMMUNICATIONS RECEIVERAll -wave coil kit.Two -gang tuning condenser, .00036 mfd.
(two).Two -gang tuning condenser, 15 mmfd.
(two) (see text).Beat -frequency coil.One 4 -prong and five octal wafer sockets.Tone control, 50,000 ohm; and switch.Volume control, 500,000 ohm.S.P S.T. rotary stand-by switch.Five -inch dynamic speaker, 2,750 ohm.Power transformer.Two radio -frequency chokes, 2.5 mh.Padder condenser, 3-30 mmfd.Two carbon resistors, 1/2 meg., 1/ watt.Carbon resistor, 1 meg., 1/2 watt.Carbon resistor, 250,000 ohm, 1h watt.Carbon resistor, 200,000 ohm, 1/2 watt.Carbon resistor, 100,000 ohm, 1/2 watt.Carbon resistor, 50,000 ohm, 1/2 watt.Carbon resistor, 25,000 ohm, 1 watt.Carbon resistor, 10,000 ohm, 1/2 watt.Carbon resistor, 15,000 ohm, 1 watt.Carbon resistor, 3,000 ohm, 1/2 watt.
Carbon resistor, 500 ohm, 1/2 watt.Carbon resistor, 400 ohm, 1 watt.Carbon resistor, 300 ohm, 1/2 watt.Carbon resistor, 200 ohm, 1/2 watt.Five tubular condensers, 1 mfd., 400 volts.Five tubular condensers, .05 mfd., 400 volts.Tubular condenser, .01 mfd., 400 volts.Three mica condensers, .0005 mfd.Three mica condensers, .0001 mfd.Two electrolytic condensers, 12 mfd., 450
volts.Electrolytic condenser, 10 mfd., 25 volts.Mica condensers, .003 mfd., and .0016 mfd.
(included in coil kit).Converter tube 6A8.Amplifier tube 6K7.Amplifier tube, 6J7.Detector -amplifier tube, 6Q7.Power -amplifier tube, 6F6G.Rectifier tube, 80.Dial, 4 -inch, with vernier attachment.Vernier dial, 3 -inch (see text).Miscellaneous: Five dial knobs; aluminum
panel and chassis, phone jack; wire.
130
octal glass tube is used. The same thingapplies to the 80, or rectifier, tube.
The purpose of the beat -frequency oscil-lator is to introduce oscillation into theintermediate -frequency stage so that whentuning to weak or distant stations they willnot be missed. With this oscillator in opera-tion, a slight whistle is heard each time astation is passed, much the same as thatheard when tuning a one or two -tube re-ceiver with the regeneration control full on.
Another helpful feature is the stand-by
Cabinet, panel,and chassis di-mensions and ar-rangement ofcontrols should befollowed closely
The back of the cabinet isleft open for ventilation.The chassis is slipped inand held in place by screws
switch. This control cuts out the plate andscreen voltages and silences the set butdoes not disconnect the heaters of the tubesso that the set can be switched back intoimmediate operation without any waitingfor the tubes to heat up.
The intermediate -frequency transformersare of the tuned -grid, tuned -plate type, andare adjusted to the proper frequency beforeleaving the factory. This adjustment allowsfor normal stray capacities found in theaverage receiver, and, unless excessive capac-
ities are introduced such as grid -wireshielding, only a slight touch of thetrimmers is necessary to align the in-termediate -frequency amplifier. How-ever, an oscillator should be used toalign the set on the short waves. Thiswill be done for a nominal sum byyour local radio service man.
The cabinet for this set should notcost more than fifty cents and is easilybuilt at home. It is made of whitepine three quarters of an inch thickand consists of only four sides, with nofront or back. As the aluminum paneloverlaps the chassis in length, it willprevent the chassis from sliding in toofar, and at the same time provides ameans of anchoring the whole setfirmly in the cabinet by means ofscrews at each end of the aluminumpanel. The back of the cabinet is leftopen to provide adequate ventilationfor the tubes.
To give a dressier appearance to thecontrol panel, apply a coat of brown,crackle -finish lacquer.
161
The finished set, ready for use as a radio or phonograph
USING only two tubes, this radio -phono-graph combination provides output and
quality equal to sets using five tubes ormore. It boasts all the features found incombinations using eight tubes, for thetwo -tube chassis incorporates a variabletone control working on both radio and
Two -Tube RadioPhonograph
records, a volume control (also workingon either radio or records), a phono-radio switch, and a sensitivity control.The latter is really a regeneration con-trol but differs slightly from the conven-tional type in that it can be adjustedto below the oscillation point and leftin that position while tuning throughthe entire broadcast band. Usually aregeneration control must be reset aseach station is tuned in. Another ad-vantage is that it does not detune the set.
The reader should be able to dupli-cate this phonograph combination, in-cluding the commercial walnut cabinet,for $27.50. This price includes tubes,pick-up, and phonograph motor.
Fundamentally, the circuit is a tunedradio -frequency receiver using a triodein the detector stage, and a 3:1 shieldedtransformer in the audio stage. Plug-incoils have been used in both the anten-na and radio -frequency circuits. Theyare tuned by two .000365-mfd., ganged
variable condensers. To balance the circuitssatisfactorily, the original trimmers on thecondensers should be replaced by two othersof a slightly higher capacity (3 to 30 mmfd).The coils chosen must have a range- of 185to 360 meters (with a .00014-mfd. con-denser). With the .000365-mfd. tuning con -
Extreme simplicity marks the layout of the parts on This view of the underside of the chassis will es-top of the chassis. Coils and tubes are accessible sist the builder in making the various connections
162
I f4
e .30 )34,
_L
REGENEI A" tor co.i- ;0_50,300 °WA!.
A
.00045
- .000365
3-30,rT1 f ci
TO -b"
if.4
.000 lb
0P1050 PIC* UP
45C V.
OUT PUTTR ANSE
111111
MM.+
-.40
,C> 14-EFEA
9:70, ZOO ;
OHMS
6000W...5,C43H
400C OHMS A
I25V25sAFD
10000MEM
EIMFO .119m150V. 4.50V. -
SW
Complete wiring details are shown in this diagram. Be sure a l connections are carefully soldered
densers, they will cover the entire broadcastband up to 550 meters. A four -prong coilis used in the antenna stage, and a six -prongcoil in the radio -frequency stage.
In wiring the six -prong coil, be sure thatthe winding over the grid winding is usedas the primary. The third winding is, ofcourse, the tickler. If the set does notoscillate, the connections to the ticklerwinding should be reversed.
The .00065-mfd. capacity placed betweenthe plate of the triode and chassis consists
of two fixed mica condensers wired inparallel-one having a capacity of .0005mfd. and the other .00015 mfd. However,any combination of capacities v. hich totals.00065 mfd. may be used.
Regeneration is controlled by a 50,000 -ohm variable resistor in series with a.00015-mfd. fixed mica condenser. Tone iscontrolled by means of a 100,000 -ohm vari-able resistor and a .1-mfd. tubular con-denser in the plate lead of the output pentode.
The phonograph pick-up is connected into
Another view of the completed chassis. Actually, ii All of tie controls are neatly grouped :cr corven-hangs perpendicularly when installed in the cabirei iert mcnipul3tion whet the ca Dine lid is raised
163
LIST OF PARTS
Six-inch permanent -mag-net speaker.
Output transformer.Radio -phonograph cabinet.Radio -frequency and de-
tector tube, 12B8GT.Output and rectifier tube,
32L7GT.Line cord and resistor, 220
ohm.Four -prong plug-in coil.Six -prong plug-in coil.Two -gang tuning con-
denser.Eight -inch slide -rule dial.Aluminum chassis, 2" by
7" by 9".Audio transformer, 3:1
ratio.Variable resistor, 50,000
ohm.Variable resistor, 100,000
ohm.Variable resistor, 500,000
ohm.S.P., D.T. rotary switch.Filter choke, 13 henry.Radio -frequency choke, 2.5
mh.Crystal pick-up.Phonograph motor.Trimmer condensers, two,
3-30 mmfd.Electrolytic condensers,
three, 8 rr.fd.Tubular paper condensers,
two, .1 mfd.Electrolytic condenser, 25
mfd., 25 volt.Tubular paper condenser,
.05 mfd.Mica condensers, three,
.00015 mfd.Carbon resistor, 1:2 watt, 2
meg.Carbon resistor, 1 watt,
600 ohm.Carbon resistor, 2 watt,
1,000 ohm.Mica condenser, .0005 mfd.Green pilot bulbs, two, 110
volt.
Top view showing phonograph turntable, radio dial, and controls
View from behind the set, showing how the parts o -0 mounted
the grid of the output pentode. A single -pole double -throw rotary switch cuts out thesecondary of the audio transformer whenthe pick-up is used. It is important to usea crystal pick-up with this circuit, as amagnetic type will not give sufficientvolume.
164
Three 8-mfd. electrolytic condensers, a13 -henry choke, and a 2 -watt. 1,000 -ohmresistor, make up the filter network. In thewiring diagram, note that the "S" positiveside of the output transformer is connectedto a point between the filter choke and the1,000 -ohm resistor.
Built to match the walk, one cabinet holds the phonogreph and radio, the cther the loud
Cabinet Ideas for Radio BuildersWITH unmounted radio chassis,
home owners handy withtools can build cabinets to harmon-ize with the decorative scheme ofa particular room. Four sugges-tions are shown in the photographson this page. At the top, twin cabi-nets were built of knotty pine, onehousing the radio and phonographand the other the loudspeaker. Oth-er schemes pictured are a unit builtinto the end of a divan, a modernis-tic cabinet separating a living roomand foyer, and a cabinet foran entrance hall.
Here are three other methodsof adapting the radio to thearchitectural and decorativescheme of on individual room
I
165
SPORTS RADIO Is
RECEIVIR
Cabinets mcunted othe handler of thecane -seat lose theset and batted.;
fi
CONSISTING of acompact yet power-ful battery receiver
mounted on a conventionalcane -seat which can bepurchased for a dollar ortwo, the radio illustratedforms a handy set forhikers, sports spectators,and campers. The circuit,designed around three ofthe new American -mademidget tubes, consists ofa pentode regenerative de-tector, resistance coupledto a pentode amplifierwhich in turn is resistancecoupled to a second audio -amplifier stage. Regener-ation is controlled by a25,000 -ohm potentiometer.Since the commercial typeof antenna coil shown inthe diagram has no tickler
The receiver cabinet with its slidingcover removed to show the three smalltubes, tuning condenser, and chassis
Combination Cane and Seatwinding it will be necessary toprovide one by winding approxi-mately thirty-five turns of No. 38double -silk -covered wire aroundthe lower end of the long, fiatgrid coil.
With the maximum of 45 voltsof "B" voltage used, 1 2 watt, oreven 14 watt, resistors can beused, while a maximum rating of200 volts is sufficient for the .01mfd. by-pass and coupling con-densers. These condensers can beof the paper or tubular type. Allother condensers, however, shouldbe of the mica variety.
The receiver and its batterysupply are housed in two cabinetseach 1 " by 414." by 512" andmounted on the handles of thecane. Sliding panels serve ascovers and make it an easy matter 'fro knobs, one forto change batteries or make re- fining, the otherpairs. The cabinets are fastened fog regeneration,to the aluminum handles by means control the setof long bolts and metal bushings.
Two 11, -volt cells connected in parallelserve as the "A" battery supply, and maybe obtained as a single unit or by breakingopen a four -cell "A" battery. The midget
r.TZ7--', . 4 +Ketir-.,
45 -volt "B" battery fits snugly"A" cells in the battery cabinet.
For an antenna, a steel fence,cane, or a 35' piece of wire will serve.
25.000OHMS
N
.00025 mFD
3
2 MEG.
.00036
Ny115.10'125-G
-DOTTOm viEw", SOCKETS
0005r t".)
Whiny 0.09 rant
0002MFD.
(.4v125).01MFD.
2 MEG.0.3 MEG.
PHONES0 0
.003MFO.
-013+ 3- A+ -
45V. -5v 1,2v.
beside the
the
5e6-.1
Reeeivei Section
metal
vi
n r '
-.1 -.W la 4 5 V.
"ErU U
C 1EBATTERY
. .
IBattery Section.
i1*.
How the circuit should be wired and cabinets and chassis constructed eicalVA.
167
MidgetPortableFOR YOUR VACATION
Careful arrangingof the parts andbatteries makes itan extremely com-pact portable set
The completed radio, ready to take to vacationland
FOR SET builders who want a reallycompact vacation receiver here is theanswer. Built around five of the new-
est midget tubes, the set illustrated re-quires only two small batteries and weighsonly 7% pounds complete with loudspeakerand built-in antenna. The superheterodynecircuit is particularly powerful and givesexceptional tone and volume.
The 4%" by 57/8" by 8%" cabinet wasbuilt up of 3/16" pressed composition wood,although plywood could be used as well.When the top, bottom, and two end pieceshave been cut to size and carefully trued up,each of the four joints should be fastened
168
with cellulose cement and two" round -head wood screws.
These screws serve mainly tohold the pieces together whilethe cement dries thoroughly. .Allholes should be counter-sunk so the screw headscan be covered with woodputty. As a finish the casecan be given two coats ofclear lacquer with a sand-ing before and after thefirst, and a final rubbingdown with pumice and rot-tenstone after the second,followed with wax.
The receiver itself isbuilt on a 1/16 -inch alu-minum chassis. The vari-ous parts should first bespotted in place and themounting and connectionholes drilled.
A homemade vernierdial is used to drive the
Top and bottom viewsof the chassis, aebw,will aid you ie build-ing the tiny receiver
variable condenser. It consistsof a celluloid disk fastened tothe shaft, and turned by thedrive shaft taken from an oldfriction dial. A regulation dialcan be used, of course, if de-sired.
A fiber form should be usedin winding the built-in loopantenna. The tabs should becut 1" deep all around with theslots %" wide. In making theform first mark the 51i.>" cen-ter line. Then with points A
and B as centers, drawsemicircles with a 1%"radius joining them topand bottom. Place the %"
_OOP ANT.SHORT POREHERE FOREXTRA PICKUP
osC
C7 T
I FTI
to
(Bono, .z- s.zicz7 .erg
("9 IFS(1 2
SbP
N.0F -
F F-s" r4)
T4)
4&2% 'A"
BAT -E41PLO,
.5)IFT3
SG. S.G
.1 T. Cl
3 F-P 2 6*
F+ F- FC12 -"13-6tr
(154)
(154
P
(155)
PHONE SPEAKJACX
C,0 1P5A .,14c 134
+G135 tT4 f 45 v.'15'BATTERY PLUG
G
P'0
C,4
4
Follow this wiring diagram careful y. The symbols refer 'o the text and the parts list on the next page
169
svecuAssuslieALUMINUM.1-CCEP socxv MOLLS yid DIA.
diameter circle at the center of the oval.The figures on each tab show the widths atthe inner ends. Starting at tab C, mark thesides by lines such as those shown dotted.Proceed all around the oval in this man-ner, then cut out the openings. The loopcontains 34 turns of No. 24 D.S.C. wirewound on opposite sides of adjacent slots.
A 30-mmfd. trimmer condenser (C:) shouldbe fastened to the loop form so that it canbe reached through a hole drilled in the backof the case. A terminal for the connectionof a short wire for extra pick-up in poorlocalities also should be provided. The loopshould be fastened to the rear of the chassiswith two screws.
An on -off switch is provided at the top ofthe case under the handle. For reception ofweak signals, or in locations where quietmust prevail, headphones are required.A jack on the front panel is included.
When the set is in use, the loop antennawill be found highly directional, minimumsignal pick-up occurring when the plane ofthe loop is at right angles to the station be-ing received. This minimum, or "null point,"can be used for approximate direction find-ing in the woods or on a boat.
170
You should have no trouble making thisdirectional loop antenna. Note the ex-tra connection below for attaching asupplementary antenna when desirable
LIST OF PARTSCondensers: C,.-Two-gang variable,
350 mmfd. each section.Cr-Trimmer, 3-30 mmfd.C,.-(On C,).C..-Padder, 500 mmfd.C C,,.-Mica midget, 50 mmfd.
Cf, Cs, C,, C,,.-Paper, .01 mfd., 400 volt.C,r-Paper, domino type, 25mfd., 200volt.C,,, C,-Midget electrolytic, 10 mfd.,
50rvolt.C,,.-Mica midget, 100 mmfd.C,..-Mica midget, .004 mfd.C,,.-Paper, .05 mfd.Resistors: R,.-100,000 ohm, % watt.R,.-.25 meg., % watt.R,.-20,000 ohm, 1/4 watt.R..-2 meg., 1,4 watt.R,.-Variable, .5 meg.R,.-10 meg., % watt.R,.-3 meg., 1/4 watt.R.-1 meg., 1/4 watt.R,.-.5 meg., % watt.R0.-200 ohm, % watt.Transformers: T,. - Midget, output,
8,000 to 3 ohm.IFT,.-Iron-core, input, intermediate
frequency.IFT,.-Iron-core, interstage, interme-
diate frequency.IFT,.-Iron-core, output, intermediate
frequency.Miscellaneous: Batteries. 3 -inch permanent -
magnet speaker, five tubes sockets, 14 poundNo. 24 D.S.C. wire for loop, case, etc.
Servicingyam'
rit
it 60'7;
Your itiatlia
STRIPS CONNECTING THE LOOP ANTENNA in-side the cover of some midget portablereceivers are likely to wear and breakoff near the hinge as a result of frequentopening and closing of the lid. Whensuch a break occurs, reception stops.These metal strips can be repaired,however, with short lengths of twistedmetal cord like that used in repairingradio dials. Remove thebroken part of the originalmetal strips at their connec-tion with the two wires in-side the set; then solder oneend of the cord to each ofthese wires, and solder theother end to that portion ofthe strips connected to theantenna.
TESTS ON BATTERIES from a portable receivershould always be made with a voltmeter-itis the only testing instrument that will showaccurately the effective voltage of a battery.It is risky business to use any other device,including a flashlight bulb or an ammeter,since many of them are likely to affect thelife of a battery materially. An ammeter-especially bad to use for this purpose-has a low resistance and will short a batterywhen placed across it. Invariably it will alsogive a high reading even when a battery isbadly run down.
TWISTED METALDIAL CORD
SOLDER
BROKENMETAL STRIP
CONNECTIONINSIDESET
BATTERIES THAT DO NOT FIT TIGHTLY in theircompartment will not knock about ifwrapped in corrugated cardboard. A roundhole cut in one piece of the material will per-mit insertion of the battery plug. To con-serve your battery, turn off the receiverwhen it is not in actual use. If you have athree-way set, use electric current whenpossible. Batteries kept outside the case at60 -deg. temperature may last 18 months.
Where a loop -antenna connection is likelyto break. Repair it as shown at the left
THREE SEPARATE PLUGS can be substituted for asingle battery -pack plug, as shown in thedrawing below, to permit use of separate"A" and "B" batteries with a portable if abattery pack cannot be obtained. In remov-ing any plug from a battery, always graspthe plug itself. Never tug on the wire, forthis may pull the insulation back, and ashort caused by wires touching can rendera battery useless in 15 minutes.
1.3 VOLTS
OLD PLUG.TO Si REMOVED
BAT- ERYCABLE
I 5 v DLTS
BATTERY- CABLE
90 VOLTS
itill141111ri-PT \\ A,
-A- BATTERY/ ..... PLUG INTO
PLUG INTO45 -VOLTBATTERIES
90 VOLTS
COP. NECTING SEPARATE PLUGS TO BATTERY CABLE
171
How the electric speaker set fits conveniently on the head
NZ L. ID RADIO
CONVENIENTLY hung over the back ofa bed, this small radio will bring pro-
grams within easy control of late nightreaders or convalescents. The cabinet andchassis layout have been carefully plannedin order to compress the complete set in assmall a space as possible without loss ofefficiency.
Measuring 71/2" square, and with a depthof only 2 %,", the cabinet houses apowerful tuned -radio -frequency re-ceiver with a radio -frequency de-tector, and audio and rectifierstages. Yet only two tubes areused-an important factor in re-ducing the overall size. One tube,the 12B8GT, contains an RF pen-tode and high -mu triode, *hich areused for the RF and detectorstages, respectively. The other tube,the 25A7GT, contains the pentodeoutput amplifier and the half -waverectifier.
Instead of these tube types, thereader may use the newer low -drain models, the 25B8GT and the70L7GT. If these tubes are used,the line -cord resistor will have tobe changed to one having a built-inresistance of 135 ohms instead of220 ohms. Also, the connections tothe 70L7GT tube differ slightly.Connections to the 25B8GT aresimilar to those of the 12B8GT.
172
No other changes are neces-sary in the circuit.
A compact two -gang vari-able condenser tunes the an-tenna and the RF coils. Theantenna coil is unshielded,and is mounted right nextto the 12B8GT tube. TheRF coil, though shielded, ismounted in an unconven-tional way - upside down.However, it is still thorough-ly shielded and in this newposition makes the wiringof the set a lot easier. Thesmall screw on top of thecan, which holds the coil inplace, is unscrewed andpassed through a hole in thechassis to anchor the shield-ing can and coil securely inplace.
Volume is controlled inthe conventional manner, byvarying the grid bias on theRF tube, using a 50,000 -ohm potentiometer betweenthe antenna and cathode ofthe RF pentode. A 300 -ohm,1/2 -watt fixed resistor in
series with the potentiometer keeps thetube always slightly biased. Ganged withthe 50,000 -ohm volume control is the S. -P.,S. -T. on -and -off switch. To provide greaterstability in the RF stage, the screen of thepentode (12B8GT) is decoupled by meansof the 5,000 -ohm, 1/2 -watt resistor and the.05-mfd. tubular by-pass condenser.
The cabinet is constructed of pine, with
of a bed
The wired chassis looks like this before being placed inthe cabinet. Careful designing makes it extremely compact
AN T.m/
--OUT PU1 MANSE
tiO -VOLTAL - 01.;
INC CORO'
5,000 o440,1
.00036
D5MFD 750,000 ORM aoo aim :t imp
.05 MFD
ANT NNA (OIL
MFD
.00036
.02MFD
1213861
35V. 10IAFD --ISO OWN
3 MEG4.000I5MKA
16 MUD150 V
TD MFDIsoV
OUTPUT TRMISF.
25A7GT §
SPEAKER
ZOMFD.6 ION "6 .,pow
The pictorial diagram above makes it easy to follow the wiring connections
View below shows the cabinet and speaker, with chassis ready tobe installed. Right, angle view of the finished set and brackets
PLATE y
-CRID
RATEECT.
HAtcYPeN Y.
ZSA7GT
173
the sides %" thick and the frontabout 5/16". A round hole of3%" diameter is cut in front forthe speaker and decorated with around escutcheon from a tuningdial. The escutcheon may be pur-chased separately at any largeradio store. Aluminum 1/16" thickis used for the back. To it, twobrass strips 8%" wide, previouslybent in a vise, are attached-eachwith two 6/32 machine screwsand hex nuts. The angles shownin the drawing will do for mostlow -backed beds.
The large ventilator hole on thebottom side of the cabinet pre-vents the heat generated by thetubes from remaining inside thecabinet and melting the wax in-sulation on the tubular condens-ers, or causing damage to thefinish on the bed ( if wood) byheating up the aluminum back.
Rear panel before mounting. Chassis and cabinet details below
LIST OF PARTSTwo -gang tuning condenser,
.00036 mfd.Antenna coil, unshielded.RF coil, shielded.Filter choke, 10 henry.Line cord, 220 ohm (see text).12B8GT tube (see text).25A7GT tube (see text).Permanent -magnet speaker, 4'.Output transformer.Potentiometer, carbon, 50,000
ohm.S. P. S. T. switch.Octal wafer sockets (two).Carbon resistor, 150 ohm, 1 watt.Carbon resistor, 300 ohm, 112
watt.Carbon resistor, 5,000 ohm, 1,12
watt.Carbon resistor, 150,000 ohm, %
watt.Carbon resistor, 800,000 ohm, %
watt.Carbon resistor, 3 meg., % watt.Electrolytic condenser, tubular,
10 mfd., 25 volt.Electrolytic condenser, tubular,
16 mfd., 150 volt.Electrolytic condenser, tubular,
20 mfd., 150 volt.Tubular condenser, .1 mfd., 400
volt.Tubular
400 volt.condenser, .05 mfd.,
Tubular400 volt.
condenser, .02 mfd.,
Tubular condenser, .01 mfd.,400 volt.
Mica condenser, .00015 mfd.Mica condenser, .0002 m:d.
VENTILATINGHOLE
(72etaitof CA assi s
De tadof BrassSapierrts
I.1
-R1c
SECTIONMOUNTED ON
ALUMINUMBAN
Oetagof Cabinet
octi ETHOLES 111- DIA.
174
Suppressor Built from Junked Parts Reduces Man -Made Static
OWNERS of sets ranging all the way fromheadphone "one-lungers" to 12 -tube con-
sole models may find good use for this static sup-pressor. Easily attached across the output cir-cuit, it will in most cases reduce severe man-made static by at least 50 percent. There is aslight loss of volume, but this can be overcomeby turning the volume control.
The unit comprises a full -wave rectifier tube( such as the 6X5-GT/G or 50Z7 -G), a push-pulloutput transformer, a 220 -ohm line -cord resistor,a S.P.S.T. toggle or rotary switch, and a 412 -volt"C" battery. The secondary of the output trans-former must have a high resistance. One of theold output transformers used with magneticspeakers will do admirably, and may be retrievedfrom the junk box. A class "B" interstage trans-former might be tried, but make sure that theprimary winding passes enough current. It isuseless to attempt to use a transformer withan 8 -ohm secondary.
Any kind of chassis, wood or metal, may beemployed. The transformer, eight -prong tubesocket, and switch are mounted on the top of thechassis. Drill " holes in the back for the linecord and the two leads to the "C" battery. Thesecondary leads of the output transformer ( pri-mary side if it is a class "B" interstage trans-former) are connected to two plastic bindingposts, which provide connections to the set.
Remove the radio chassis from the cabinet.Connections are then made to the plate of thelast tube and the B+ lead. The leads from theunit to the radio chassis should not exceed 3' or 4'.
LIST OF PARTSChassis, metal or wood Small "C" battery, 4 Si voltsFull -wave rectifier tube Line -cord resistor. 220 ohms6X5-GT/G or 5027-GEight -prong octal water Binding posts (2)
socket Rotary or toggle switch.Output transformer (see text) S.P.S.T.
Left the unit connectedto a small AC -DC receiv-er. It may be used withalmost any type of radio
A bp view of tie unitis ihcwn in the firstphoto below. Few con-nections are necessary,m con be seen in secondphoto. Drawing shows howthe static suppressor is
hocked up to a receiver
PLATT)F LAS--UBESECEIV
1+OUTPUT -
TRANSFORMER2:1RATIO
OR 1: 1 RATIO
6X5 -GT/GOR
50Z7 -G
4C5. -VOLT-IS -VOLT
C.BATTER.'
SWITCH -220 0) 0--0LINE -CORD 110 VOLTSRESISTOR AC. -DC
175
BATTERIES POWER COMPACT
EMERGENCYRECEIVER
This portable set, designed For defense organizations,picks up near -by broadcasts without the use of a ground
ESIGNED to meet the require-ments of defense organizations,this is a portable emergency
short-wave receiver that eliminatesthe use of bulky "B" batteries withoutsacrificing power, quality, selectivity,or sensitivity. Instead of the usual 90 -to 135 -volt "B" batteries, the metalcarrying case comfortably encloses, inaddition to the usual "A" battery, twosmall "C" batteries which reduce theplate voltage to eight or nine and cutthe filament voltage in half. Since the"emission life" of a tube is greatlylengthened when operating at a re-duced voltage, the life of the tubes inthis novel receiver may be at leastdoubled.
The new single -ended, all -metal 6.3 -volt AC -DC tubes ( 6SJ7) or the sin-gle -ended glass type 11-volt batterytubes ( 1SA6GT) can be interchangedin the set without any rewiring. Thechoice of either tube depends onwhether you want a husky tube thatwill take more abuse in the field or atube that will conserve the batteriesto their limit. The metal tubes con-sume more electricity, but they aremore robust for portable use and areslightly more sensitive. The glasstubes conserve electricity, but theyare not so serviceable in the field.
Filament voltages are extremelycritical in both types of tubes and anyvariation-even as little as 1/10 of avolt-will reduce the power of the re-ceiver. In fact, if the tube& are
Tubes shown here are the new all -metal, 6.3 -volt View of the receiver chassis from the underside,6517 type, but battery ISA6GT tubes will serve showing the wiring and connection to phone clips
176
operated at their normalvoltages, the set will stopfunctioning. For this rea-son a voltmeter is placedon the front panel for avisual indication of the"A" batteries' current. Theknob directly under thevoltmeter operates a 10 -to 15 -ohm rheostat whichcontrols the filament sup-ply.
But for special tube con-nections, the set is builtwith a standard detectorplus two audio -frequencyamplifying stages. Inter -stage coupling is achievedby means of two un-shielded 1:3 audio trans-formers, one mountedabove the chassis and onebelow. The output of thereceiver is fed directly in-to a pair of magnetic 2,000 -to 4,000 -ohm headphones.These phones can beplugged into the front ofthe panel.
Plug-in coils enable anoperator to tune in on va-rious wave bands rangingfrom 16 to 1,000 meters.This includes ship -to -shore, broadcast, aviation,police, foreign, and do-mestic transmission. If
Two midget "C" batteries withlow voltage replace the bulky"B" battery, lengthening the"emission life" of the tubes
-A(4.5 MILS
VOLTS)
ANT.
.00015M FD.
2 MEG.
.00014
R F CHOKE2.5r.sh
SIX -PRONGPLUG-IN COIL
VOLTMETER-SV.
VO TMETERSWITCH
POWERSWITCH
RHEOSTATIS C21
PHONEJACK
AUDIOTRANSFORMER
RUISRER GROMMETS
CHASSIS I
ALL TwigsARE EITHER:
G.SJ 7-GT
655715A6-GT
(9 VOLTS)
+A(4.501..1.5
VOLTS)
(9 VOLTS)
Pictorial diagram of the compact, portable emergency receiver. Thisshows in detail the placing of the ports both on the chassis and onthe front panel. In making the connections, solder should be used
Below is a complete wiring diagram in simplified form to be followedin making connections. A fickler -winding coil is used in the antenna
ANT TICKLER R F. CHOKEWINDING
-1
A A TE NW AWWESNG
SIV PRONGPLuO.IN COIL
N.R.o.. 6.66E a 3-66"A" ...A ...AAC.0C 3- bw. . wag1 rid ISAA-OT 0.4 I.,-..0 A, al ,',
2 I111 AUDIOIEJA6010 TRANS F.IRAN SF. e I:300.115
3OR :5
5ND. ALL TURES USED ARE EITHERG,S.T 7-6T ; 65.17 ; SA6-GTSHIELD PRONG'.
IS GROUNDED ..S.5/50TO CHASSIS eorTom viEw VOLTMETER SW
PHONES
SW -8(9v)
4.svOR1.55
A
+13(953
177
Clips on the cabinet next to the tuning condenser hold thetelescopic, metal antenna when the radio is in operation.These clips must be fully insulated from the metal cabinet
the set should fail to oscillate on a particular band,this may be remedied by increasing the capacity ofthe variable condenser used for regeneration to.00036 or by increasing the number of turns on thetickler winding. It should be noted that the coilspecified by the manufacturer for use as the ticklerwinding is used instead as the antenna winding.
Batteries and headphones can be housed in themetal carrying cabinet. When the operator is readyto use the set, he merely removes the headphonesfrom the case and plugs them into a jack on thefront of the panel. Two inexpensive luggage straps,purchased from the five -and -ten -cent store, can beplaced around the cabinet for carrying purposes.
The antenna consists of a telescopic metal rodsimilar to those used on cars. It is clipped to thecabinet next to the tuning condenser when the setis in operation. The clips must be fully insulatedfrom the metal cabinet, which is connected to theground circuit.
If the receiver is operated within 20 or 30 milesof a transmitter, it will work efficiently without aground connection. For distant or weak stations,however, a ground must be used. A suitable groundcan be made by connecting 5' of insulated wire to ametal rod about 5" long and 1.1," in diameter. Thewire should be connected to either the front panelor the chassis and the rod inserted in the ground.
The emergency receiver shown in the illustrationwas constructed for about $15, but changing con-ditions in the radio -equipment field make it im-possible to give any definite figures on cost. Theset has been tested under adverse conditions wherereception was faint on similar radios and has givenexcellent results.
178
VOLTMETER0-5V.
RHEOSTAT
PHONE JACK
REGENERATION CONTROL
8"
ANTENNACLIPS
TUNINGCONDENSER
4Vz V 'A 4112.v.
QTUBES Gi"
(-1 14 0AUDIO 'MANSE
COIL
I I n 1
AV:C:VOLT METE R
TUNINGCONDENSER
LIST OF PARTSBlack wrinkled steel cabinet, 7"
by 7%." by 8".Cadmium -plated steel chassis,
2" by 51/2" by 7".Telescopic chrome -plated an-
tenna.Tuning dial, 21/2" diameter.Octal wafer sockets (3).Six -prong coil socket.Six -prong broadcast and short-
wave coils.Tuning condenser, .00036 mfd.
or .00014 mfd.Regeneration condenser, .00014
mfd. or .00036 mfd.Audio transformers (2), un-
shielded, 1:3 or 1:5 ratio.Ground clamp and wire.RF choke, 2.5 millihenrys.Rheostat, 10-15 ohms.Voltmeter, 0-5 volts.Toggle switches (2).Phone jack and plug.Mica condenser, .00015 mfd.Carbon resistor, 2 megohms,
watt.Midget "C" batteries (2), 4%
volts."A" battery, 1% or 4% volts.Pentode amplifier tubes (3),
6SJ7 or 1SA6GT.
-
rri
*Iire°4446e04,0 -.'064
LIGHT -BEAMTRANSMITTER
The operator taps out a code message onan ordinary telegraph key. Below he sightsalong the barrel to aim the flash light
CODE MESSAGES that canbe detected only with a spe-cially built receiver may be
sent with this light -beam transmit-ter. With a 3- or 4 -cell flash light,preferably of the type with whichthe light rays can be focused intoa spot, messages can be sent up to200 feet. By increasing the volt-age of the buzzer circuit from 71/2to 22?,Z volts and using a morepowerful light, this range can beincreased.
The transmitter works on thesame principle as a radio broadcasttransmitter. Tl.e light waves fromthe flash light act as the carrierwave, and the code signals tapped
The fop o' the transmitter. Note rubber grommets Bottom view shows handmade coupling transformer
179
The receiver, showing thephoto -electric cell shieldand the magnifying glass,connected by a black tube
out on the telegraphkey are superimposedon this light beam.Light variations form-ing the message are in-visible, and to all ap-pearances the raysfrom the flash lightform a steady beam.
With this device,messages can be sentthrough windows, foli-age, or anything thatallows at least part ofthe light beam to reachthe receiver.
A gas -filled photo-electric tube is used asa detector in the re-ceiver, and this is fol-lowed by a two -stage,resistance -coupled au-dio amplifier. The photocell transforms themodulated light beamInto sound which is am-plified by the audiostages.
As a matter of fact,any good audio ampli-fier can be used afterthe detector stage. Caremust be taken, however, to see that thephoto tube never has more that: 90 voltson its plate, to avoid damaging the tube.
To focus the rays of light from the trans-mitter, a magnifying glass is placed 4"from the curved cathode inside the photo
PHOTO-ELECMICTUBE
MAGNIFYINGLENS
GRIDPENTODE
SCREEN _GRID7>c-7; TRIODE.PLATE s
.02 Nee
TPRLARDTDE,PENTODE .°3
p.,700
NC" DIODEID8GT
florrom view)
.00'2.'AFT)
. - -
BO -90N
Schematic diagram of the receiver, with a bottom view of the I D8GT tube
tube. To enable the receiver to work in alighted room or outdoors (if the sun is notloc_ bright) a black composition tube, 1"in diameter and 21,z2" long, is placed be-tween the can and the magnifying lens.
For the amplificatical a 1D3GT tube with
The top of the receiver with the shielding token off a Eottorn viaw, showing the condensers and resistors
l
5'
PHOTO-ELEC.SHIELD TUBE
(4°Sw. A
PHONE TERM
7"
a 1.4 -volt filament is used. Tooperate the receiver, two 45 -volt "B" batteries and a 11/2 -volt "A" battery are needed.
If the photo -electric cell failsto operate during reception ofa signal, discharge the .25-mfd.condenser by shorting it.
To make the coupling trans-former used in the transmitter,use an iron core made from anold chisel or a hollow ironshaft about 1/2" in diameter.Around this wind 100 turns ofNo. 24 d.c.c. wire and thenanother 150 turns of the samewire to form the primary andsecondary windings.
RECEPTACLE
BATT.TERM.
I V2VOLT CELLS. REFLECTOR
Putting the photo -electric tube in place. Note the hole inthe shield. At the left is the detail of the receiver chassis
BACK OFFLASH LIGHT(SHOWN IN PART
ONLY) an BLUE
PART OFSOLDER
---RUFISER-INSULATED FLASFFUGHT BARRELWIRE
SOLDER
SWITCH ONOl ROOSTER l'AV. BATTERY BARREL IS NOTTOGGLE SWITCH CONNECTION (SHORTED USED (LEFT IN
4:--4:--C) WHEN BATTERY IS NOT OPEN POSITION)USED)
g
.`COUPLING TRANSFORMER
KEY
BUZZER( MOUNTED+ 71V.- ON RUBBER GROMMETS)
The transmitter circuit. The chassis detail is at the left
LIST OF PARTS FOR THE LIGHT -BEAM SET
Transmitter
Three -cell flash light (with metal barrel)Four -section tripod.Metal chassis (7" x 5" x 11/2").Household buzzer.71,2 -volt "C" battery.Telegraph key.Special coupling transformer.S. P. S. T. toggle switch.Four insulated binding posts.Spool of No. 24 d. c. c. wire.
Itimeivier
Photo -electric cell, type 923.Dual amplifier tube, type 1D8GT.Four -prong socket, wafer type.Eight -prong socket, wafer type.
Metal chassis (7- x 5" x 11/2").Coil shield.Electrolytic condenser, 8 mfd., 150 v.Paper tubular condenser, .25 mfd., 400 v.Paper tubular condenser, .002 mfd., 400 v.Mica condenser, .005 mfd.S. P. S. T. toggle switch.Insulated 'phone terminals.Carbon resistor, 500,000 ohms, 1/2 watt.Carbon resistor, 50,000 ohms, 1/2 watt.Carbon resistor, 2 megohms, % watt.Two portable "B", batteries, 45 v.Midget "little six" "A" battery, 1% v.Black tubing 21/2" long.Pair of head 'phones, 2,000 or 4,000 ohm,Approximate cost of receiver parts is $5.84.Transmitter, 3.10.Batteries for both sets, 1.86.
Prices include tubes.
1' i
BLACKOUT
RECEIVERBortery-Operated Setwith Only Two TubesCosts Little to Run
This is o back view of the cabinei shown ab)ve atright. The 2" by 7" by 7" chassis is on the left,and the "A" and "B" batteries are seen at the right
Below a top view of the chassis. Note the smallpush-pull output transformer bell rsd cial at +le left
ALTHOUGH two tubes usually are neededjust for the push-pull stage in a re
ceiver, here is one with only two tubes mul-tiplied into an RF stage, a high -mu detectorstage, and a push-pull output stage usingtwo pentodes! Powered by dry batteries, thisreceiver is especially useful in the home incase of a blackout and is extremely eco-nomical to operate.
The 3A8-GT tube is a combination RFpentode and a high -mu triode detector,while the 1E7 -G tube is a twin pentode. Thefilaments of the 3A8-GT are connected inseries (2.8 volts), and operated straight offa 3 -volt "A" supply without a filament re -
Bottom view of the receiver chassis. The wiring is
simple and should be an easy job even for a beginner
182
sistor. However, the filaments of the 1E7 -Gdraw 2 volts at .24 amperes so that it isnecessary to insert an 8 -ohm semivariableresistor in series with the filament prong(No. 7) of the tube and the chassis.
No "C" battery is required, the necessaryvoltage being obtained through a resistorplaced in the "C" return lead of the push-pull audio transformer. For best results andgreatest volume the plate voltage ("B" -battery supply) should be increased fromthe usual 90 to 135, which means the use ofthree instead of two 45 -volt "B" batteries.However, excellent results can be obtainedwith just two 45 -volt "B" batteries. Noground is necessary.
A 250,000 -ohm variable resistor 'in thescreen circuit of the 3A8-GT's pentode con-trols the volume. Tuning is accomplished bymeans of a two -gang, .00036-mfd. tuningcondenser. On the right side facing the cabi-net is a third control-a .0001-mfd. variablecondenser connected across the antennatuning condenser. As it is not always pos-
PUSH-PULLAUDIO TPANSF,
TOUTPUTRANSF.1
PUSH- PU
VOLUMECONTROL t SWITCH
R.F.COIL
ANT.COIL
i DIALTUNING \CONTROL .0001 MFD.
BALANCINGCONDENSER
The layout of the variousparts of the receiver isshown in the drawing above,actually a top view of thechassis. Follow it care-fully in building the set
sible to obtain perfect tracking across theentire broadcast band with a battery re-ceiver, this additional trimmer condenser isused to balance the RF and detector stages.It does not have to be set for each station-just portions of the broadcast band. Thesmall trimmer condenser already on the an-tenna tuning condenser is not used and isleft at minimum capacity.
The cabinet measures 81/2" by 91/2" by151/2" to accommodate the batteries and asix-inch, permanent -magnet speaker. Thechassis is 2" high and 7" square.
LIST OF PARTSCabinet, 81h" by 9%" by 15%".Black wrinkle chassis.PM speaker, 6". Slide -rule dial.Push-pull output transformer.Push-pull interstage transformer.Tuning condenser, 2 -gang, .00036 mfd.Iron -core shielded antenna coil.Iron -core shielded RF coil.Variable condenser, .0001 mfd.Tubes: 3A8-GT and 1E7 -G.Octal sockets.Variable resistor, 250,000 ohm.Coverplate switch, 3 -position.Paper tubular condenser, .05 mfd.Electrolytic condensers, 25 mfd., 50
volt, and 10 mfd., 25 volt.Semivariable resistor, 10 watt, 8 ohm.Antenna -ground binding post.Carbon resistor, 300 ohm, 1,.e.; watt.Mica condenser, .0002 mfd.Mica condenser, .001 mfd.
Left, view of chassis showing the 3A8-GT tube beingplaced in its socket. At its left and right are theantenna and RF coils. Wiring is diagrammed below
1
.00036 '05MFD.`, MFD.
1
250,900E0 dj1
NVVVvVINI-A
3, C,
OE. .-
.0001MFD.SCREENPENT.
PLATEPENT.
F+
3A8-GT NC
-P0
0
--//\_/\,01
1
NOTUSED
GRID'TRIODE
__GRIDPENT
Pt ATETRIODE
D,ODE
PUSH-PULLT,RAN',CORMER
3Ab -GT 0002
611,D GRIDPENT.(2) /PENTSI
PLATE PLATEPENTU) PENT 01
FNC SCREENS 1E 7-G
001MFD.
1E7-6
PUSH-PULLOUTPUT
TRANSP.
.,PEatsER
-3V.
0+3V.
O+135V.
-135V.0
Servicing Your RadioDIAL troubles often can be corrected easi-
ly. Many of the older AC -DC midgetshave a direct -drive dial, in which the tuningknob is mounted right on the shaft of thetuning condenser. The only thing that cango wrong with this type is a loosening of the
A squeaky slide -rule dial is remedied easily by alittle oil on the wheels over which the dial cordruns. Oil very lightly with a light lubricant, be-ing careful not to get any oil on the cord itself
If the pointer is not calibrated properly with theFrequency marks on the dial, it is an easy matterto move it to the correct position. Once it is setproperly for one station, it will be correct for all
A slipping belt, when it is not too serious, canbe remedied by applying a special wax obtainablein stick form. Rubbed lightly on the belt as shown,it usually is found to give a smoother -working dial
184
knob, which is remedied by tightening theset screw. Slightly more complicated arethe friction drives using a belt or cord. Thephotographs below show various troublesencountered on these sets, and how they canbe eliminated.
gr._
In some friction -drive dials, the cord is kept tautby a spring behind the dial plate as shown above.To tighten the cord, it is necessary only to removethe spring and make another knot in the drive cord
To get of a spring behind the dial plate as shownin the photograph above, it is necessary to takeoff the plate. To do this, loosen the two hexnuts indicated by the pencil in this illustration
If moving plates touch fixed plates in a tuningcondenser, loud static is produced whenever thedial is touched. On most condensers this can becorrected by adjusting with a screw and lock nut
Vest -PocketReceiver
BUILTinto a simulated -leather playing -
card case, this one -tube receiver ispowerful enough to get distant stations.The tube is the new all -glass 45 -volt minia-ture diode pentode (1S5) with the unuseddiode portion grounded directly to thechassis or "A" minus. A "vest pocket" 45 -volt "B" battery should last three or fourmonths and an ordinary flashlight "A" bat-tery will give several hours of use. Padder( compression) condensers less than 1"square are used, one for tuning and one forregeneration. They tune only about half thebroadcast band, so the capacity most desir-able should be decided before purchasing.An unshielded antenna coil, to which a tick-ler winding may be added by winding 15to 20 turns of No. 30 d.c.c. wire to the lower
Two padder condensers,one 155 glass pentode,an unshielded antennacoil, and two batteriesare used. Two Fahnestockclips on the card -casecover connect to phones.No ground is necessary
.7)etoue
BAKELITE,.PANEL 2.V4s-Ae
Really small enough to fit into the vest pocket
portion of the grid winding, is satisfactory.Oscillation may be reduced by decreasingthe turns or stepped up by reversing theconnections to the tickler coil. No groundis used. The aerial is 20' or 30' of loose in-sulated wire.
DIG
sc(55 TICKLER
WINDINGCSEE TFXT) rlt PHONES
.45V
-45Y. k.
-I.5V.
+1.5V}d?
PL
GFI
2 MFGF - F
v,1" 250WOO.
Above, a complete wiring diagram for the one -tubereceiver, and below, the radio in its tiny case,showing tuning and regeneration controls, phoneshooked up, and (between knobs) antenna connection
185
Football -Fan's RadioWITH THIS COMPACT RECEIVER, YOU CAN LISTEN TO
THE BIG -GAME BROADCASTS AS YOU WATCH THE PLAYS
Handy for picnics aswell as football games,this compact receivergives good earphonebroadcast reception
186
HOW would you like to hear aplay-by-play radio descriptionof the football games you see-
while you're seeing them ? All youhave to do is build the compact radioreceiver described on these pages, takeit with you to the next big game, and
tune in on the radiobroadcast of thegame while youwatch. When thefinal whistle blows,you'll know a lotmore about the playsyou saw than willyour neighbors in thefootball stands.
The cabinet of thereceiver, designed tolook like a vacuumbottle, is compactenough to be car-ried under the arm,and contains boththe receiver circuitand the batteries.Open at one end andclosed at the other,it measures 41/2," by
Several spectators of a football game can use this receiver at once. Compact and provided with a con-venient handle, the receiver has its own built-in battery supply, and operates on a short antenna
5" by 10" and is constructed of pine %"thick. Shellac can be used to seal the grain,while a few coats of enamel provide a dur-able and attractive finish. If desired, the casecan be finished in the colors of the builder'sfavorite school or college.
To complete the realistic appearance ofthe outfit, the cap from a regular vacuumbottle is fastened to the closed end of thecabinet. This can be bolted permanently inplace, or, as was done in the original shown,it can be used to house additional plastic
Batteries and circuit ore slipped into the open end ofthe cabinet. The chassis holds the batteries in place
drinking cups. If this is done, a disk of woodshaped to be a snug fit in the outer cap mustbe bolted to the cabinet. If a particularlyneat job is desired, coarse threads can becut into the wood disk to take the threads onthe inside of the cap.
The circuit chosen is a standard three -tube,tuned -radio -frequency hook-up. However, in-stead of using 2 -volt battery tubes, theauthor decided to employ the new 11/2 -volttubes, because of their extremely low fila-ment consumption. The parts are mounted
To allow -he vacuum -bottle cabinet to stand upright.the control panel should be recessed, as shown above
187
IN% (BOTTOM viEW)N.C.-NO CONNECTION
tASG(BOTrom vicw)
Tube -base dia-grams showing theconnections to bemode for the tubesused in the circuit
IN56
CA
LOO,)00
IN5C
2 MEG.
!A 5G003roc0
6+22iv.
0 5+90V.
.;,?c,-0 A+ ON.5w
0 0 -qv.C+ 41 v
B+64,,f.
Wire the circuit carefully and
on a midget aluminum chassis measuring 1'by 41/2" by 41,.".
A two -gang variable condenser is used totune the radio -frequency and detector stages,and a small radio -frequency choke in theplate circuit of the detector tube preventsany stray radio -frequency currents fromentering the audio -frequency stage and caus-ing distortion. The antenna coil, a regularcommercial unit removed from its shieldingcan, can be mounted directly onto the backof the control panel just above the two -gangtuning condenser. The radio -frequency coil,which should be shielded, is mounted betweenthe two pentode tubes. A 100,000 -ohm vari-able resistor inserted in the screen lead tothe radio -frequency tube controls the volumeof the receiver by varying the screen voltageto that particular tube.
If a wood disk is used to hold the cop in place,additional cups can be carried inside the cop
make sure that all soldered connections ore tight
The "A," "B," and "C" batteries, bound to-gether with a heavy black elastic band, canbe slipped into the cabinet after being con-nected up to the six -way battery cable of thereceiver. The set is then placed inside ofthe cabinet in front of the batteries, andanchored securely in position by four smallbrackets. Four insulated plugs mounted onthe face of the control panel provide con-nections for the antenna, ground, and phones.A twenty -five-foot antenna, of the type com-monly used with small A.C.-D.C. receivers, isrecommended.
LIST OF PARTS NEEDEDTwo 1N5G tubes.One 1A5G tube.Two -gang condenser, .00036 mfd.Two tubular condensers, .1 mfd.Tubular condenser, .05 mfd.Mica condenser, .003 mfd.Mica condenser, .0005 mfd.Mica condenser, .00025 mfd.Resistor, 2 meg., '2 watt.Resistor, 1 meg., watt.Resistor, 250,000 ohm, % watt.Radio -frequency choke.Radio -frequency coil.Antenna coil.Three octal sockets.Switch and volume control.Six -way battery cable.Miscellaneous:-Four banana plugsand jacks, dials, knobs, batteries,chassis, cabinet, panel, earphones, etc.
188
Pocket RadioBUILT INTO A
Notebook
A one -tube receiver that is small enough to becarried in your coat pocket. How the parts aremounted inside the book cover is shown at right
BUILT into a 3" by 4" loose-leaf notebook,this tiny one -tube, earphone radio canbe carried easily in your coat pocket.Its single tube, a midget triode measur-
ing less than 3", requires but 3 volts for :Isfilament and 45 volts for its plate.
Because of limited space, no socket is used,connections being soldered directly to the pinsat the base, and the tube is sewed to the frontcover to hold it in place.
A small, .00042-mfd. insulation -spacedvariable condenser tunes the set. It measures114 " square, and has a depth behind the coverof only % ".
The tuning coil is a high -impedance pri-mary coil of the type used to replace burned -out primary windings in standard A.C.-D.C.receivers. However, you must unwind ap-proximately 100 turns to make the coil suita-ble for receiving stations between 200 and550 meters.
Current is supplied by the new midget "A"and "B" batteries, also small enough to fit inyour coat pocket, and four small jacks pro-vide connections for the antenna, ground, andphones.
x DTUBE
F -
BOTTOM VIEW
.000/2MFD.
PHONES
-C)+--C) +4S V.
20 OHMS
-45V.
3V
+ 3V.
LIST OF PARTS NEEDEDMIDGET tuning condenser, .00042 mfd.,
paper -spaced instead of air -spaced. High -im-pedance primary coil. Two mica condensers,.00015 mfd. each. Resistor, 3 meg., watt.Midget detector tube ( see text). Rheostat, 20ohms. Midget 45 -volt "B" battery. Midget 3 -volt "A" battery. Battery cable, three-way,about 3' long. Four jacks and plugs. Five sol-dering lugs. Small black loose-leaf notebook.Earphones, dial, knob, etc.
189
A
190
NOVEL
Note steplike chassisthat saves space. Shel-lacked wall paper cov-ers the wooden cabinet
LampRadio
IS EASY TO BUILDOMBINING a reading light anda broadcast receiver, this Chi-nese lamp radio will form auseful as well as an attractive
addition to a living room, a bedroom,or a den. It is easy to assemble, andthe use of a modern dual-purpose tubemakes the necessary parts few in num-ber and inexpensive.
You don't even have to be an artistto apply its artistic finish. Select any
attractive wall paper ofChinese pattern and sim-ply glue it to the woodcabinet. When the gluedries, apply a coat of thin,colorless shellac. The lightfixture can be bought at ahardware store, and alamp shade to blend withthe wall paper can be al-most as inexpensive as youwish.
The parts for the com-pact, two -tube radio cir-
Chinese in design,the radio lamp willharmonize with theusual furnishingsin a living room, abedroom, or a den
cuit are mounted on"steps" on the alu-minum chassis whichshould be cut andbent to shape according to the diagram. Thecombination lamp base and radio cabinet ismade of pine and is detailed in the drawings.Round the edges of the cabinet with a fileand sandpaper before adding the wall paper,and you will avoid any homemade appearancein your finished job.
Atop the first step of the chassis mount thetube sockets and the tuning coil, and on thesecond step place the midget output trans-former and the dual electrolytic condenser.Between these two parts, the 3" perma-nent -magnet dynamic speaker can be fas-tened to the front panel. A novel tuningcondenser whose moving plates are not air -spaced, but separated by thin layers of spe-
To use a ground, make the connectionshown in dotted lines in the diagram
cially treated paper, is only '4" thick andfits behind the aluminum panel where its con-trol knob balances that of the combinationvolume control and switch. The tuning con-denser has a capacity of .0005 mfd., which ishigher than standard types. Because of this,however, only one plug-in tuning coil coversthe entire broadcast band from 200 to 550meters, even though the actual wave -lengthrange of the coil is from 135 to 270 meterswhen used with a .00014-mfd. condenser.
The two -tube design uses a high -frequency,all -metal pentode as a regenerative detector,resistance -coupled to a sensitive, glass powerpentode containing a half -wave rectifier. Ade-quate filtering is provided by the 15-h. choke
191
Bottom view of wiredset showing arrange-ment of parts. Below,screw driver points tothe two-way connec-tor that permits easyremoval of the chassis
LIST OF PARTSLine -cord resistor, 310
ohm.Midget choke, 15 h.Plug-in coil, 4 -prong(see text).Padding condenser,
.000075 mfd.Tuning condenser,
.0005 mfd.Dual electrolytic con-
denser, 8 and 8 mfd.Dry electrolytic con-
denser, 10 mfd., 25 v.Volume control and
switch, 50,000 ohm.Mica condensers, .0005,
.0003, and .00015 mfd.
Tubular condensers, .02(two), and .01 mfd.
Plate choke, 2.5 mh.Resistors, 2 meg., %
watt, and 100,000ohm, % watt.
Resistor, 200,000 ohm,% watt.
Resistor, 750,000 ohm,% watt.
Wire -wound resistor,2,000 ohm, 1 watt.
Miscellaneous. - Chas-sis, midget sockets,output transformer,indoor antenna,speaker, tubes, etc.
192
and dual, 8-mfd. electrolytic con-denser. Regeneration and volumeare regulated by a 50,000 -ohm vari-able potentiometer, coupled with theswitch.
Should the set fail to oscillate overthe entire wave band, increase thecapacity of the .0005-mfd. fixed con-denser connected between the radio -frequency choke and the chassis, bywiring another fixed condenser of.00025-mfd. capacity in parallel. Theoriginal model worked well withouta ground, but if you wish to try one,connect it through a .02-mfd. con-denser as shown by the dotted linesin the circuit diagram. The con-denser must be used to insulate theground from the chassis and protectthe tubes, which are also groundedthrough the chassis to the electric -wiring system.
A two-way plug, placed just abovethe metal pentode as shown in thephotographs, connects the wiresfrom the lamp socket with the cur-rent -supply cord leading to the ra-dio, making. it easy to disconnect thewiring for removal of the chassisfrom the set.
ANItr,
t' ,
3140
III SO% 61111%
4244
3011013S'IS31
01 NOS
