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Sencore Tech Tips

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1 Analyzing The HV Circuits With The VA62A 2 Analyzing The Horizontal Output Pulse With The Waveform Analyzer 3 The TVA92s Horizontal Output Tests 4 Understanding the LC103s In-Circuit Capacitor Test 5 Understanding Horizontal Output Stages of Computer Monitors CONTENTS
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Page 1: Sencore Tech Tips

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Of all the TV waveforms you have toanalyze, the horizontal output transistorcollector pulse is the most importantbecause this output pulse is used toperform many other functions than to justsweep the CRT beam horizontally. It caneasily be said that the horizontal outputtransistor is the heart of a TV. Let's take acareful look at nine of the key functionsthis all-important horizontal outputwaveform is responsible for, how to fullyanalyze it, and some possible problems.

The horizontal output stage is practicallyresponsible for the complete and efficientoperation of the entire TV. The waveformat the collector of the horizontal output

transistor is the most important waveformyou should check on every TV before youbegin changing parts, and after every TVis repaired .

Looking At The Horizontal Output Pulse With In Three Simple Steps

Sencore's Waveform Analyzers use ahighly accurate low capacity probenetwork that lets you safely measure thehorizontal output transistor and otherpulse waveforms to 2,000 volts (DC +peak AC). Even if you should happen toaccidentally leave your WaveformAnalyzer input attenuator in its most

sensitive position and hook up to thehorizontal output transistor collector,there is no need to panic as no damagewill result.

1) Connect the TV AC line to an isolationtransformer, such as the Sencore PR57POWERlTE®. The isolation transformerprotects you and your equipment fromelectrical shock and damage by isolatingthe HOT chassis from you and yourWaveform Analyzer .

2) Hook up the probe ground to the TVchassis ground, then connect the probeto the collector of the horizontal outputtransistor.

Analyzing The Horizontal Output Pulse With The Waveform Analyzer

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Fig. 1: The horizontal output pulse is responsible for the efficient operation of the TV.

Page 5: Sencore Tech Tips

3) Adjust the vertical volts per divisioncontrol to the 200 volts position. Switchyour time base control all the way downto the video preset position. Push in thehorizontal preset button and you will seewaveforms that include two lines of videoinformation.

Fig. 2: A typical horizontal output pulse.

What To Look For In This Waveform

Before you do any measurements, take asecond or two to look over the waveformitself. It should look just like the horizontalwaveform in Figure 2. The waveformshould be symmetrical in shape duringpulse retrace time. If it is not, look for achange in the value of the horizontaloutput transistor stage timing capacitors,or an excessive load on a B+ supply. Thewaveform should be symmetrical beforeyou proceed with testing .

The trace or scanning “on time'' of thetransistor should also be relatively clean.Any excessive ringing is a clear indicatorof deflection system problems such as acracked integrated high voltagetransformer (IHVT) core or open IHVTwindings. The trace must be clean beforeyou analyze the pulse any further. If it is not, look for other noise pulsesriding along during the trace time. Theycould be causing faint noises or drive

lines in the video picture. Look for ashorted secondary power diode, shortedIHVT diodes or shorted windings. Thetrace time should be clean of any noisebefore performing the measurements tofollow .

Measuring The WaveformParameters (Automatically)

You'll need to make four measurements(along with the waveshape coveredearlier) of the horizontal output waveformto be sure that it is operating safely orwhen you are troubleshooting thehorizontal section of the TV:

FIRST: Push the DCV button for fullyautoranged DV voltage measurements.

SECOND: Push the VPP button forautomatic peak-to-peak measurements.

THIRD: Push the FREQ button forautomatic and noise free frequencymeasurements.

These three measurements are the firstones to be made. They tell you thecondition of the regulated B+ supply, andthat the TV is not in the shut-down mode.The next measurement is used to helpprevent future component failures.

How Important Is The DurationMeasurement Of The HorizontalOutput Transistor Waveform?

Of all the horizontal output transistorwaveform parameters, the“duty-cycle”measurement tells you the most. Becauseof the many jobs that this critical circuitperforms, TV manufacturers carefullyspecify the horizontal output transistor“duty-cycle” or time duration in exactmicroseconds as follows:

Retrace time: 11.5 - 16 microseconds

Trace time: 47.5 - 52 microseconds

They make these specifications for a verygood reason. If the time duration (duty-cycle) is too short during retrace, speedand excessive voltage will be developed;therefore, excessive power will bedissipated. This generates heat which willcause TV parts damage in time.

IMPORTANT: Always refer to themanufacturers' schematic or literature forthe particular chassis timing.

The Waveform Analyzer is especiallyequipped to measure portions of awaveform with the DELTA TIME feature.

Fig. 3: Two common faulty waveforms which cause problems with the operation of a TV.

Page 6: Sencore Tech Tips

To make this important measurement:

1. Align the pulse by using theVOLTS/DIVISION and the CAL. knobs sothe top of the retrace pulse is on the100% graticule marking.

2. Select the dual channel mode bypushing the A & B button.

3. Switch the CHANNEL B INPUTCOUPLING switch to ground, and alignthe trace with the VERTICAL POSITIONcontrol so it lies on the 10% graticulemarking.

4. Press the DELTA TIME button,

5. Adjust the DELTA BEGIN knob so theleft-side of the intensified trace aligns withthe left-side intersection of the CHANNELA and CHANNEL B traces (Figure 4).

6. Adjust the DELTA END knob to alignthe right-side of the intensified trace withthe right-side intersection of the twotraces.

7. Read the digital display directly inmicroseconds to see that you are within11.5 to 16 microseconds .

Fig. 5: The digital display shows the timing.

What If The Horizontal OutputWas Only 5 Microseconds Off?

Suppose you measure 9 microsecondsinstead of 14 microseconds for theretrace pulse. On a TV with this type ofproblem, the peak-to-peak value could begood, the DC reading could be close, andthe waveform would look close enough.Even the frequency could be right on15,734 Hz. This TV will work for a while.

Shifting the retrace duty cycle 5microseconds does not look like much, oreven sound like much. But, to thehorizontal output system, it sees a 35.7%reduction in retrace time meaning thatretrace is faster and this generates highervoltage that means the horizontal outputtransistor is “on” just a little longer at fullscan conduction. Increased conductiontime means increased heat.

Increased scan time means increasedscan derived power supply levels. Thepower supply capacitors have a longertime to charge and reach higher voltages.All the circuits are now stressed and mustwork at this higher voltage.

Isolate Start-up and Shut-DownProblems With The HorizontalOutput Pulse

The CRT can be used to watch for aninstantaneous start-up pulse. Simplyconnect the Waveform Analyzer andpreset the CRT controls as describedearlier. Then, watch the CRT as you applypower to the TV's circuitry. If you see apulse appear then disappear your start-upcircuitry is operating correctly and the setis in the shut-down mode.

If this happens, you have to service thechassis in a “powered down” condition.,at either half the normal B+ level, suppliedsepartately, or reduce the AC input powerto half power (60 VAC) and monitor thecollector of the horizontal outputtransistor with your scope.

NOTE: If the chassis uses a switch modepower supply (SMPS) as the B+ source,you need to determine if the SMPS isdefective, or if the porblem is on thehorizontal output stage. Refer to Tech Tip#205 "Identify SMPS Problems" forinformation on how to do this.

For More Information,Call Toll Free 1-800-SENCORE

(1-800-736-2673)

3200 Sencore Drive, Sioux Falls, SD 57107

Fig. 4: The time duration measurement of the retrace puse should bemade from the 10% to 10% levels.

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Form 4968Printed In U.S.A.

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This Tech Tip presents a logical, step-by-step procedure for using the TVA92’sHorizontal Output Tests. While thesetests can be used individually, this testsequence gives you the best opportunityto safely identify different types of hori-zontal circuit defects. If you need addi-tional information on how the horizontaloutput stage operates or would like an in-depth explanation of each test, pleaserefer to the following Tech Tips:

#207 “Understanding the TVHorizontal Output Stage”

#208 “Making Horizontal OutputDynamic Measurements”

#209 “Understanding the TVA92’sHorizontal Output Load Test”

#210 “Understanding the TVA92’sHorizontal Output Device Sub &Drive.”

Recommended TVA92 HorizontalOutput Tests Procedure

The following outline presents anoverview of the recommended procedure

for using the TVA92’s Horizontal OutputTests.

I. HORIZ OUTPUT LOAD TESTS –AC power removed from thechassis (TV Off)A. Check current draw - mAB. Check output pulse time - uS

II.HORIZ OUTPUT DYNAMIC TESTS – AC power applied to the chassis(TV On)A. H.O.T. removed, HORIZ DEVICE

SUB & DRIVE off1.Check unloaded B+ power

supply - DCV2.Check for horizontal drive to

the H.O.T. - INPUT DRIVEB. H.O.T. removed, HORIZ DEVICE

SUB & DRIVE on1.Check current draw for

excessive load - DEVICE SUBCURRENT

2.Check loaded B+ powersupply - DCV

3.Check flyback pulse amplitude -PULSE PPV

4.Check flyback pulse time -PULSE TIME uS

C. H.O.T. installed, HORIZ DEVICESUB & DRIVE off1.Monitor B+ power supply -

DCV2.Monitor flyback pulse

amplitude - PULSE PPV3.Monitor flyback pulse time -

PULSE TIME uS

Performing the TVA92 HorizontalOutput Tests

The following procedure is a detailedlook at the above outline. It describeshow to do the tests, what readings to

expect, and what to do when a bad read-ing is indicated. This procedure coversthe most common results and defects.

I.Horizontal Output Load TestsThe first TVA92 Horizontal OutputTests to perform are the Load Tests.These two tests give an indication ofany major defects in the horizontalstage. During the Load Tests theTVA92 supplies a B+ voltage atapproximately 10% of normal to thehorizontal output stage

CAUTIONThe HORIZ OUTPUT LOAD TESTSproduce flyback voltages at the

collector of the chassis horizontaloutput transistor and the flyback

secondaries. Do not come in contact with energized circuit points

during the Load Tests.

A.Current – mA1. Remove power from the TV. These

tests should never be performedwith AC power applied to the chas-sis. The load tests can be performedwith the H.O.T. in or out-of-circuit. Ifyou find the H.O.T. is shorted,remove it and proceed with the LoadTests.

2. Connect the RINGER/LOAD TESTleads as follows: black lead to theH.O.T.’s emitter or equivalent con-nection if the H.O.T. is removed, yel-low lead to the H.O.T.’s collector orequivalent connection if the H.O.T. isremoved, and orange to the B+ con-nection on the flyback.

3. Set the HORIZ OUTPUT TEST selec-tor to HORIZ OUTPUT LOAD TESTmA and note the current reading on

The TVA92’s Horizontal Output Tests

Fig. 1: Controls used to perform the HORIZOUTPUT TESTS.

Page 9: Sencore Tech Tips

the LCD. The acceptable range is 5-80mA.

4. A reading of greater than 80mA indi-cates excessive current. To isolate thedefect, disconnect the yellow clip leadfrom the collector and note the cur-rent reading.

a) DC leakage is indicated if thecurrent stays above 15mA. The defectis caused by a DC leakage path on theprimary side of the flyback, such as adamper diode, retrace capacitor orleaky component in the B+ line.

b) AC leakage is indicated if the current falls below 15mA. A defectiveIHVT, horizontal yoke, or secondaryloading can cause a short of this type.

ote: A reading close to or above 250mAndicates a direct DC short to ground,sually caused by a shorted H.O.T. oramper diode.

5. A reading of less than 5mA or dashesindicates an invalid connection or openin the circuit. Be sure the mA readingis within the acceptable range beforeproceeding to the Dynamic Test.

B.Timing – uS1. Set the HORIZ OUTPUT TESTS selec-

tor to HORIZ OUTPUT LOAD TEST uSand note the timing reading on theLCD.

2. The acceptable range is 11.3-15.9uSwith a stable reading.

3. A reading that is stable but outsidethis range indicates a defective timingcomponent.

a) A reading that is greater than15.9uS is likely caused by an openyoke or yoke series capacitor.

b) A reading that is less than11.3uS is likely caused by defectiveretrace timing capacitors, IHVT, orexcessive loading on the secondaries.

Note: A few chassis manufactured byNAP may normally return timing read-ings slightly less than 11.3uS.

4. A reading that is fluctuating indicatesthat the flyback pulse waveshape con-tains ringing or multiple pulses. Thisis likely due to a defective flyback,excessive loading on the secondaries,or leakage.

Note: A few chassis may return areading that fluctuates from normal to0.1-0.5uS. This is due to the differentimpedance in the leads and circuit, not adefect in the set.

5. A reading of dashes (“- - -”) indicatesimproper lead hook-up or an open inthe circuit. Be sure the pulse time iswithin the acceptable range beforeproceeding.

II.Horizontal Output Dynamic TestsThe three sections of the TVA92’sHorizontal Output Dynamic Tests pro-vide a quick and easy B+ voltagemeasurement, input drive test, hori-zontal output waveform analysis, andhorizontal output device substitution.

A. Dynamic B+ and Input DriveMeasurements

The first Dynamic Test enables you tocheck the regulated B+ supplied to thehorizontal output stage and the inputdrive to the base of the H.O.T.

1. Remove the H.O.T. from the chassisand connect the Dynamic Test Leads

S S

OFF 1.5A MAX

0

HORIZ OUTPUTDEVICE SUB & DRIVE

(CURRENT LEVEL)

OFF: TVS HORIZ OUTPUT ACTIVE ON: TVS HORIZ OUTPUT

SUBISTITUTED

••

•••

••

YOKE & FLYBACK

SWITCHING XFORMER

PULSE PPV

PULSE

INPUTDRIVE

DEVICESUBCURRENT

HORIZ OUTPUT TESTS

DCVDYNAMIC TESTSRINGER TESTS

TV

ONTV

OFF

uS

mA

HORIZ OUTPUT

LOAD TEST

TVA92

ASSIS

!

SUB ON OVERLOAD

FLOATING GROUND 1000V ISO

RINGER/LOADTESTS

DYNAMIC TESTS

1500V MAX

(Orange)

g. 2: Connections to perform the HORIZ OUTPUT LOAD TESTS.

Page 10: Sencore Tech Tips

as follows: red to the collector con-nection, blue to the base connection,and black to the emitter connectionor circuit ground.

Note: A few chassis do not connect theH.O.T. emitter to ground. In this case,connect the black clip-lead to circuitground, not the emitter connection.

2. Set the HORIZ OUTPUT TESTS selec-tor to DCV, apply AC power to thechassis, and note the voltage readingon the LCD. This reading shouldclosely match the schematic’s valuefor regulated B+. If the reading doesnot stabilize to this value, turn theHORIZ OUTPUT DEVICE SUB &

DRIVE on just enough so the SUB ONLED lights. This will provide feedbackto the power supply if necessary. TheDCV reading should be near theschematic’s value. If it is not, thepower supply is malfunctioning andshould be repaired before continuing.Turn the DEVICE SUB & DRIVE offbefore continuing.

3. Set the HORIZ OUTPUT TEST selectorto INPUT DRIVE, apply power to thechassis, and note the reading on theLCD. The LCD should read “ON.”Some chassis’ horizontal circuits runoff of a scan derived supply. Withthese types of sets you need to turnthe TV off and back on again watchingthe LCD to see if it will momentarily

flash “ON.” If it reads “ON”, or willmomentarily when the set is power-ing up, a drive signal is present. If itreads “OFF” there is a defect previousto the base of the H.O.T. This defectdoes not necessarily need to berepaired before continuing.

B. Dynamic H.O.T. Sub & DriveThese next steps allow you to substi-tute for the H.O.T. and operate the TVat full voltage without risking anexpensive replacement H.O.T.

1. Set the HORIZ OUTPUT TESTS selec-tor to DEVICE SUB CURRENT.

2. Turn the HORIZ OUTPUT DEVICE SUB& DRIVE slightly on until the SUB ONLED lights and watch the currentreading on the LCD. If the currentexceeds 500mA, turn the HORIZDEVICE SUB & DRIVE off. There islikely a defect in the circuit that needsto be repaired before continuing. Ifthe current stays below 500mA turnthe knob quickly to the 12 or 1 o’clockposition (higher for larger sets).Adjust the HORIZ OUTPUT DEVICESUB & DRIVE control to get normalhorizontal deflection without“foldover” in the center of the CRTdisplay. The current reading may nowbe over 1A depending on the size ofthe set.

3. With the HORIZ OUTPUT TESTSselector, check the DCV to see that thepower supply is regulating and thePULSE PPV and PULSE TIME uS tomeasure the amplitude and width ofthe horizontal output pulse to be surethat the horizontal output stage isoperating properly. Repair any prob-lems before continuing.

C.Dynamic Horizontal OutputParameter MeasurementsThis final step monitors the horizontalcircuit’s operation at full voltage soyou can be sure that it is workingproperly with the H.O.T. installed.

1. Install a good H.O.T. and reconnectthe Dynamic Tests leads as describedabove.

Note: Be sure the HORIZ OUTPUT DEVICESUB & DRIVE knob is in the OFF position.

2. Apply AC power to the chassis.3. With the HORIZ OUTPUT TESTS

selector, check DCV, PULSE PPV, andPULSE TIME uS for correct values.

TEST: NORMAL RANGE BAD RANGE

mA 5-80 mA <5 mA or >80 mA

µS 11.3 – 15.9 µS <11.3 µs or >15.9 µS

Table 1: HORIZ OUTPUT LOAD TESTS Good/Bad ranges.

Table 2: Possible HORIZ OUTPUT LOAD TEST readings and likely causes.

HORIZ LOAD TEST READOUTS MOST LIKELY mA µS CAUSES

---- ---- • Improper Connections• Open Flyback• Open Output Stage Circuit Paths

BAD ---- • Severe B+ Supply Short Or Leakage Path• < 5 mA = Open Flyback Or Circuit Path

GOOD ---- • Open Flyback• Improper “Collector” Connection• Open Ringer/Load Fuse

GOOD GOOD • No Severe Loading Or Timing Defects

BAD GOOD • Severe B+ Leakage And/Or FlybackSecondary Short Or Leakage Path

• Flyback Transformer

GOOD BAD • Defective Output Timing Components• Flyback Transformer• Severe Flyback Secondary Short Or

Leakage Path

BAD BAD • Severe B+ Leakage• Flyback Secondary Short Or Leakage Path• Flyback Transformer• Defective Output Timing Components

NOTE: Fluctuating µS readout values indicate abnormal flyback pulse ringing or timing.

Page 11: Sencore Tech Tips

#231

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S3200 Sencore Drive, Sioux Falls, SD 57107

1-605-339-0100 • www.sencore.com

Form #6905Printed In U.S.A.

For More Information, Call Toll Free 1-800-SENCORE

(736-2673)

RINGS mAuS KVPP

RINGS mAuS KVPP

- +

0

OFF 1.5A MAX

0

VERTICAL YOKEDRIVE LEVEL

HORIZ OUTPUTDEVICE SUB & DRIVE

(CURRENT LEVEL)

OFF: TVS HORIZ OUTPUT ACTIVE ON: TVS HORIZ OUTPUT

SUBISTITUTED

HORIZ OUTPUT TESTS

••

•••

••

YOKE & FLYBACK

SWITCHING XFORMER

PULSE PPV

PULSE TIME uS

INPUTDRIVE

DEVICESUBCURRENT

HORIZ OUTPUT TESTS

DCVDYNAMIC TESTSRINGER TESTS

TV

ONTV

OFF

uS

mA

HORIZ OUTPUT

LOAD TEST

TVA92

VERTICAL YOKEDRIVE OUTPUT

50V 1A

DISCONNECT YOKE FROM CHASSIS

!

SUB ON OVERLOAD

FLOATING GROUND 1000V ISO

RINGER/LOADTESTS

DYNAMIC TESTS

1500V MAX

130

Fig. 3: Connections to perform the HORIZ OUTPUTDYNAMIC TESTS.

SYMPTOM PROBABLE CAUSESB+ = 0 Volts • Open Fuses • Bad B+ Supply • Shorted B+ Path

Low B+ Volts • B+ Power Supply Regulation • Low AC Voltage

High B+ Volts • B+ Power Supply Regulation• Open Loads On B+ Supply

Pulse PPV = 0 V • No B+ • No Input Drive • Open HOT• Open Flyback Primary

Low Pulse PPV • Leaky Retrace Capacitor or HOT• Flyback Loading • Reduced Value Of Yoke Capacitor• Bad Yoke • Low B+ • Insufficient Input Drive

High Pulse PPV • Retrace Capacitors • Flyback Shorted Turn• High B+ (regulator)

Pulse Time = 0 µS • No B+ • No Input Drive • Open HOT• Open Flyback Primary

Pulse Time < 11.3 µS • Flyback Loading • Flyback Shorted Turn• Retrace Capacitors

Pulse Time >15.9 µS • Yoke • Yoke Series Capacitor

Multiple Pulse Times • Flyback Loading • Flyback Shorted Turn • Leaky HOT Damper Diode, Yoke, Retrace Capacitors, Yoke OrYoke Capacitor

Input Drive “ON” • Drive present to base of HOT

Input Drive “OFF” • No Drive To Base Of HOT

HOT = Horizontal Output Transistor

ig. 4: Possible HORIZ OUTPUT DYNAMIC TEST indications and their possible causes.

Page 12: Sencore Tech Tips

Capacitors continue to be found in electronic circuits in record breakingnumbers. In fact, the number of capacitors used in the manufacturing ofelectronic circuits continues to rise eachyear. In 1997, U.S. factories sold over 50 billion capacitors. As these capacitorsage or are stressed by circuit voltagesand heat many will fail causing impropercircuit operation.

Finding a bad capacitor and replacing itto restore normal circuit operation ischallenging. First, you must identifywhich capacitor is suspect. Second, thecapacitor must be unsoldered, removed,tested and reinstalled if good orreplaced. These steps can be time consuming and you also risk damage tothe circuit board traces or the capacitor.In fact, many manufacturers suggestreplacement of surface mount capacitorswhen unsoldered and removed. Time and money is wasted if the removedcapacitor is good or a replacement doesn’t fix the problem.

The Sencore LC103 “ReZolver” providesa patent pending test of capacitors whilestill soldered in-circuit. The in-circuitcapacitor analyzing test determines if thecapacitor is good, bad, or if it should beremoved for further tests. This Tech Tipcovers how to test capacitors in-circuit

with the LC103’s In-Circuit CapacitorGood/Bad test and explains how to interpret the test results.

In-Circuit Capacitor TestingChallenges

Obtaining meaningful and reliable testresults when analyzing a capacitor in-circuit has many complications. Firstyou must make an electrical connectionto each of the capacitor’s test leads and maintain a stable connection whileperforming the tests. Second, you mustperform analyzing tests that determinewith a high reliability if the capacitor isgood or if it may have a defect andshould be removed for further testing.You also need the flexibility to test a wide range of capacitors found in today’scircuits to be comprehensive. Finally, youneed a simplified solution to interpretingthe in-circuit capacitor test readouts toavoid confusion.

The first challenge is simply making connection to an in-circuit capacitor.Electrical connections to each of thecapacitors leads while soldered in-circuitis complicated by a wide range of capacitor types, values, sizes andmechanical lead basings. Since mostcapacitors do not expose enough leadlength for clip lead connections whensoldered in-circuit, connections must bemade on the solder side of the circuitboard. Surface mount capacitors arealready mounted directly to the solderside. Connection to the soldered side ofthe circuit board requires 2 sharp probetips. Connecting to each of the capacitorlegs requires a hand for each probe,leaving no hands to operate a test instrument. Even if you were able to holdeach probe with one hand, reaching or

looking at the test instrument could easily cause you to slip off the capacitorresulting in improper measurements,frustration and potential circuit damage.

Sencore has overcome these mechanicaldifficulties with the innovative AdjustableIn-Circuit Test Probe. The Adjustable In-Circuit Test Probe (AP291) joins twoprobe tips and provides an adjustablespacing wheel. The probe mechanicallyadjusts providing the versatility to fit thelead spacing of capacitors ranging fromsurface mount to large electrolytics.

The angled tips provide ease in probingsurface mount electrolytic capacitors. A push button switch conveniently located on the test probe enables theLC103’s in-circuit capacitor test to avoidprobe slippage. For most applications,the probe can be adjusted and connectedto the in-circuit capacitor with one hand.In addition, the LC103 beeps when thefirst complete measurement is completeand the readings are momentarily frozenon the LC103 display after the test button is released to be sure you have sufficient time to view the in-circuit test result.

Fig. 2: A push button switch conveniently located on the test probe enables theLC103’s in-circuit capacitor test.

S

Understanding The LC103’s In-Circuit Capacitor Test

#2

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#232

232#232

#232

Fig. 1: The Sencore LC103 “ReZolver”provides a patent pending in-circuitcapacitor test to reduce servicing timeand expense.

IN-CIRCUIT OUT-OF-CIRCUITCOMPONENT TESTS

CAPACITORGOOD/BAD

INDUCTORGOOD/BAD

CAPACITORVALUE

CAPACITORESR

CAPACITORLEAKAGE

INDUCTORRINGER

DIELECTRICABSORPTION

INDUCTORVALUE

Page 13: Sencore Tech Tips

While capacitors fail in several ways a combination of two common measurements, capacitor value and equivalent series resistance (ESR), candetermine if a capacitor is likely good orsuspect in-circuit with a high level of reliability. Aluminum Electrolytic capacitors and tantalum capacitors commonly fail from increased ESR priorto changing value and increasing in leakage. Other capacitor types commonlychange value. Testing both value and ESRprovides the most comprehensive andaccurate in-circuit test results. An ESRtester alone would mistakenly report ashorted capacitor or circuit short as good.Likewise, a capacitor value test alonewould miss capacitors with excessive ESR.

Accurate in-circuit capacitor testing can be hindered by the presence of components in parallel with the capacitor.Capacitance, resistance, inductance andsemiconductor junctions in parallel withthe capacitor may influence the accuracyand reliability of in-circuit capacitor tests.At times the parallel components mayhave little effect on the accuracy of thetests but at other times the parallel components cause significant changes to the test results. It is important to know when the parallel components are effecting the in-circuit capacitor measurements.

The LC103’s In-Circuit Capacitor test function measures the capacitance valueand ESR of an in-circuit capacitor. Thesemeasurements are simultaneously displayed in the COMPONENT TESTRESULTS display. Capacitance value measurements range from 0.002 uF to20,000 uF.Capacitor ESRmeasurements aredisplayed forcapacitors rangingin value from 0.02uF to 20,000 uF.Measurement volt-ages are below PN forward bias volt-ages so the testsare unaffected bysemconductor junctions.

The LC103’s In-Circuit Capacitor test per-forms several sophisticated tests to deter-mine if parallel components are present which may be effecting the accuracy of the in-circuit capacitancevalue and ESR measurements. The testsinclude a test to determine how muchcurrent is needed to hold a capacitorcharge. Current exceeding the originalcharging current by 20% indicates parallel resistance that can impact thecapacitor value test. A second test uses aselection of test frequencies and analyzesthe Xc of the circuit. A capacitance value is determined and compared to acapacitance value determined with an RC time constant value measurement.Large differences in the capacitance values indicate parallel components which would impact the in-circuit measurement accuracy.

Fig. 3: The LC103 analyzes the capacitor forparallel components that would alter theaccuracy of the in-circuit test results.

It can be difficult to determine if a capacitor ESR readout is normal or not as capacitor ESR values vary among different capacitor types and also varywith the capacitor’s value and voltage

rating. A calculator would be needed to determine if the measured capacitancevalue is within a normal tolerance.

The LC103 provides Good/Bad test analy-sis with every in-circuit capacitor test tohelp determine if the capacitor value andESR is within a normal range. ESR evaluations are based upon maximumallowable limits established by component manufacturers and theElectronic Industries Association (EIA).Capacitor measured values are automatically compared to maximum and minimum values calculated from theentered value and tolerance of the capacitor being tested.

LC103 In-Circuit Capacitor Testing

The LC103 offers two alternatives forGood/Bad testing a capacitor with the In-Circuit Capacitor Good/Bad Test function.You may perform a basic Good/Bad test ofthe capacitor or a complete EIA Good/Badtest. Both testing alternatives perform thesame analyzing tests but use different ref-erences for Good/Bad interpretation. Thedisplay readouts vary slightly dependingupon the test alternative.

To perform a basic Good/Bad check applypower to the LC103 and attach and zerothe test probe. Connect the Adjustable In-Circuit test probe to the capacitor legs and push & hold the front panelCAPACITOR GOOD/BAD push-buttonswitch or the small push-button switchon the In-Circuit Test Probe. The testresults are shown in the COMPONENTTEST RESULTS display.

Chart 1: The LC103 performs a basic Good/Bad test of the capacitor or a complete EIA Good/Bad test. Both testing alternatives perform the same in-circuit analyzing tests but use different references for Good/Bad interpretation.

In-Circuit Capacitor Test Component Parameters Good/Bad Judgement Factors

Basic Good/Bad Check None Measured capacity and 50VTantalum ESR Chart if >1 µF Ceramic (10Ω) if <1uF

EIA Good/Bad Test Capacitor type, value, Based on the EIA chart for entered capacitor type tolerance, rated voltage Measured capacity versus entered value/tolerance

Page 14: Sencore Tech Tips

The display readouts shown during the basic Good/Bad check include thecapacitance value, capacitor ESR and a“GOOD??” or “BAD??” or “SUGGESTREMOVAL” display readout. ESR is notdisplayed for capacitor values below 0.02uF. The good or bad evaluation is basedupon the ESR measurement and the measured capacitance value. For measured capacitance values over 1 uF,the measured ESR is compared to themaximum ESR values for a similar valuetantalum capacitor as determined by theEIA. For measured capacitor values lessthan 1 uF, a 10 ohm good/bad referenceis used. ESR values of 10 ohms or moreare considered “BAD??” while less than10 ohm are considered “GOOD??.”

Question marks accompany both thegood or bad readouts during a basicGood/Bad check because the LC103 cannot compare the measured capacitancevalue to the rated value of the capacitorbeing tested. When you see the questionmarks, remember to check the LC103’scapacitance measurement to the capaci-tor’s rated capacitance value to determineif it is within a normal tolerance.

Note: Double Layer Lytics and High RDouble Layer capacitor values are beyondthe range and testing capability of the In-Circuit Capacitor Good/Bad test. TheIn-Circuit Capacitor Good/Bad test shouldnot be used on these capacitor types.

To Perform an In-Circuit Capacitor -Basic Good/Bad Check:

1. Apply Power to the LC103.

2. Connect the In-Circuit Ajustable TestProbe to the LC103’s TEST LEAD jack.

3. Perform the Lead Zero Adjustment.

4. Connect the probe tips to the capacitorleads.

5. Push & hold the In-Circuit CAPACITORGOOD/BAD push-button or test probepush-button.

6. Read the COMPONENT TEST RESULTSdisplay

A complete EIA Good/Bad test evaluatesboth the measured in-circuit capacitancevalue and ESR.

The display readouts shown during the EIA Good/Bad test includes the capacitance value, capacitor ESR and a“GOOD” or “BAD” or “SUGGESTREMOVAL” indicator. An ESR measurement readout is not displayed for capacitor values below 0.02 uF. Thegood or bad evaluation is based uponboth the measured capacitance value and measured ESR. The measured capacitance value is compared to theentered value and tolerance. The measured ESR is compared to the maximum ESR determined by the EIA for the entered capacitor type. If the measured capacitance value is out-of-tolerance and/or the ESR exceeds the maximum determined by the EIA, a “BAD” readout is indicated. If the capacitance value is within the rated tolerance and the ESR is below a maximum EIA level, a “GOOD” readoutis indicated.

To Perform an In-Circuit Capacitor - EIAGood/Bad Test:

1. Apply Power to the LC103.

2. Connect the In-Circuit Adjustable TestProbe to the LC103’s TEST LEAD jack.

3. Perform the Lead Zero Adjustment.

4. a.Enter the capacitor - Component TypeExample: Push the “ALUMINUMLYTIC” push-button.

b. Enter the capacitor value.Example: Push the 2, 2, 0, uF, push-buttons.

c. Enter the capacitor value tolerance.Example: Push the 2, 0, +%, -%,push-buttons.

d.Enter the capacitor’s rated voltage.Example: Push the 5, 0, V, push-buttons.

5. Connect the probe tips to the capacitorleads.

6. Push & hold the In-Circuit CAPACITORGOOD/BAD push-button or the testprobe’s push-button.

7. Read the COMPONENT TEST RESULTSdisplay

Understanding the “SUGGESTREMOVAL” In-Circuit CapacitorGood/Bad Test Readout

A “SUGGEST REMOVAL” message issometimes displayed during either the in-circuit capacitor basic Good/Bad test or EIA Good/Bad test. This message indicates that the LC103’s tests haveidentified components in parallel with the capacitor being measured and that theparallel components are influencing theaccuracy of the capacitor value and/orESR measurements. For an accurate evaluation of the capacitor’s value and/orESR the capacitor must be unsolderedfrom the circuit and tested with theLC103’s out-of-circuit capacitor tests.

Most “SUGGEST REMOVAL” messagesare accompanied by capacitor value andESR test readouts. These readouts may not be accurate because of parallel components but are often helpful in

I M P O R TA N T

Do not hold-in the CAPACITORGOOD/BAD switch or Adjustable TestProbe push-button switch while con-necting the Test Probe to an in-circuitcapacitor. The LC103 circuitry may bedamaged because capacitor dischargeprotection is lost.

Fig. 4: The Adjustable In-Circuit Test Probeprovides reliable in-circuit connections andpush-button test ease.

Page 15: Sencore Tech Tips

determining if the capacitor likely has aproblem. Occasionally, the readings mayhelp you avoid removal and testing time.For example, a capacitor value readoutthat is much higher than the rated valueof the capacitor is likey caused by acapacitor in parallel with the one beingtested. If the schematic shows a capacitorin parallel with the one being tested thatresults in a total capacitance near the displayed value, the value of the capacitorbeing tested is likely fine. At other times,you may know from previous experiencewhat to expect for capacitance and ESRreadouts with the in-circuit capacitor testsacross a particular capacitor.

#232

#232#232

#232

#2

32

#2

32

For More Information, Call Toll Free 1-800-SENCORE

(1-800-736-2673)

Form #6939Printed In U.S.A.

S3200 Sencore Drive, Sioux Falls, SD 57107

www.sencore.com

Fig. 5: The “SUGGEST REMOVAL” readout indicates there arecomponents in parallel with the capacitor being measured thatwill influence the test results.

Page 16: Sencore Tech Tips

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Today's computer, medical, security,design and industrial video display moni-tors operate at a host of different horizon-tal resolutions or scanning frequencies.Many change modes to display video atseveral scan frequencies while othersadapt to display a range of horizontal scanfrequencies or resolutions.

All CRT based video displays have hori-zontal stages including a horizontal outputstage. Some use two horizontal outputstages, one to produce high voltage andanother to produce horizontal yoke cur-rent. The frequency of the horizontal out-put stage(s) must match the video's hori-zontal sync or scan frequency. Despite thewide range of operating frequencies anduses, several basic horizontal output cir-cuit configurations are common.

This Tech Tip examines the common hori-zontal output stage configurations foundin multi-frequency CRT video displays. Itprovides details on how these stagesoperate and what to expect for normalvoltages and waveforms to improve yourtroubleshooting.

The Basics Of A Horizontal OutputStage

All horizontal output stages have 4 keycomponents and require two essentialinputs. These are listed below.

All horizontal output stages operate in asimilar manner. A power supply voltage(B+ voltage) is applied to the stage, typi-cally to one side of the transformer or coil.(See Fig. 1.) The power supply providesthe current to energize the transformer orcoil. When energized, alternating sawtoothcurrents in the transformer primary or coilwinding are produced. Induced voltagesfrom the transformer or coil develop highvoltage and/or yoke current.

An H.O.T. (horizontal output transistor) pro-vides a path for current to energize thetransformer primary or coil winding. Thetransistor's emitter-to-collector currentpath is switched fully on or off by an inputdrive signal. Horizontal output stage tran-sistors can be a conventional bipolar typeor newer N-channel enhancement MOSFET.

A retrace or timing capacitor (Ct) is usedto tune or time the horizontal output stage.With the H.O.T. switched open, the timingcapacitor forms an LC tuned circuit withthe transformer or coil winding. The tim-ing capacitor slows the rate of the trans-former or coil's collapsing magnetic fieldcontrolling the level of induced voltage.

A damper diode parallels the horizontaloutput transistor. The diode is biased onwith induced voltage from the flyback orcoil during a critical time in the outputstage cycle. The damper diode's conduc-tion path prevents reverse breakdown cur-rent in the horizontal output transistor

resulting in transistor heating and failure.The damper diode permits energy storedin the transformer or coil at the end of theoutput stage cycle to be returned to the B+supply. The damper diode is a fast switch-ing high voltage, high current diode.

High Voltage or Deflection OnlyHorizontal Output Stages

Multi-frequency video display monitorsmay have one or two horizontal outputstages. A display with one horizontal out-put stage combines the yoke and flybacktransformer into a circuit which producesboth high voltage and horizontal yokedeflection current simultaneously. When avideo display contains two horizontal out-put stages, one horizontal output stage isresponsible for producing high voltagewhile a second output stage produces hor-izontal yoke current.

A multi-frequency video display is morelikely to have two horizontal output stagesif the CRT size is greater than 15 inches.Separate HV and deflection horizontal out-put stages are rarely found in monitorswith CRT sizes less than 15 inches or intelevisions because of the added cost.Larger CRT display monitors requiringhigher yoke current and larger currentchanges to accommodate multi-frequencyoperation are more likely to have separatehigh voltage and deflection output stages.Design of a horizontal output stage to sat-isfy the yoke current requirements andmaintain reliable operations is simplifiedwhen separated from the high voltagegenerating horizontal output stage.Separating the output stages allows eachto operate with much less current (power)

UNDERSTANDING HORIZONTAL OUTPUTSTAGES OF COMPUTER MONITORS

Four Key Components: Two Essential Inputs:1.Horizontal Output transistor 1. B+ Power Supply Voltage2.Transformer Primary or Coil 2. Input Signal Drive 3.Retrace Timing Capacitor4.Damper Diode

Page 17: Sencore Tech Tips

requirements than if combined.

A typical high voltage only horizontal out-put stage consist of a flyback transformer,timing or retrace capacitor, damper diodeand horizontal output transistor(s) asshown in Fig. 1. Either a bipolar transistor,MOSFET transistor or paralleled MOSFETtransistors may be used.

Because of the reduced current in a highvoltage only horizontal output stage com-pared to a combination stage, a MOSFEThorizontal output transistor can be usedreliably. In cases where the current is stillsubstantial, matched paralleled MOSFETtransistors may be used to divide theH.O.T. conduction current for increasedreliability. Considering the reduced costs ofMOSFETs and drive components, the costof a mosfet compared to a bipolar horizon-tal output transistor is slightly better.

Two important differences exist whenusing a MOSFET horizontal output transis-tor compared to a bipolar type. First, theflyback voltage pulses induced into theoutput stage must be reduced becauseMOSFETs have a lower breakdown voltagerating than bipolar transistors. Secondly,the input drive signal and driver circuitsmust be different to match the differencesin the MOSFET's transistor's operatingcharacteristics. For these reasons bipolarand MOSFET output transistors cannot beinterchanged.

Today’s MOSFET horizontal output transis-tors typically have a maximum voltage rat-ing from drain to source of either 800 voltsor 1000 volts. In comparison, bipolar tran-

sistors have a maximum voltage ratingcollector-to-emitter of 1500 volts.Typically, flyback pulses in MOSFET outputstages are at least 100 volts under theirmaximum rated voltage or less than 900volts peak-to-peak. In comparison, flybackpulses in a bipolar transistor output com-monly exceed 1000 VPP. Lesser inducedvoltages are compensated for with a differ-ent flyback transformer turns ratio to pro-duce the needed high voltage.

A MOSFET transistor is a voltage operateddevice while a bipolar transistor is a currentoperated transistor. Switching a bipolartransistor on requires that the drive pro-duce base current of several hundred mil-liamps. The base drive current switches thetransistor fully on enabling it to conductcollector currents of several amps. A MOS-FET output transistor turns fully on whenpositive voltages greater than 4 volts areapplied to the gate. The input signal typical-ly ranges from near 0 volts (H.O.T. off) tobetween 5 and 15 volts (H.O.T. on). Whenswitched on the MOSFET transistor reducesits drain-to-source resistance path to lessthan 2 ohms permitting peak currents tobuild in the flyback primary winding.

Because MOSFET and bipolar output tran-sistors have different input drive require-ments, the horizontal driver stages for eachare considerably different. Driver stages forbipolar output transistors use an amplifierand current stepup transformer to producethe needed drive current to the bipolartransistor's low impedance base. Driverstages for MOSFET output transistors usean amplifier to provide a changing voltageto the MOSFET'S high impedance gate.

High Voltage Only HorizontalOutput Stage Operation

Operation of the high voltage only horizon-tal output stage is fundamental to all hori-zontal output stages. When the H.O.T. isdriven on by the drive signal, B+ currentincreases through the H.O.T. collectorenergizing the transformer or coil winding.The current is opposed by the inductanceincreasing at a near constant rate reachinga peak of several amps. The magnetic fieldbuilds in the transformer or coil's core dur-ing this time inputting the power requiredto produce high voltage. The B+ supplyvoltage, coil inductance, H.O.T. conductiontime, beta and base current drive all effectthe level of energizing current buildup.

When the H.O.T. is switched open, the tim-ing capacitor (Ct) is effectively placed intothe circuit forming an LC resonant circuit.Immediately after the H.O.T. is switchedoff, the magnetic field of the transformer orcoil begins to collapse. The collapsingmagnetic field causes current to flowthrough the low impedance of the B+ sup-ply capacitor charging Ct. This is the begin-ning of the retrace time and correspondswith horizontal sync. As the timing capaci-tor charges, a rising voltage is produced atthe collector or drain of the output transis-tor. The voltage reaches its peak as themagnetic field is fully collapsed.

The timing capacitor performs a criticalfunction in slowing down the rate of thecollapsing magnetic field. If the capacitorvalue decreases or is opened, the field col-lapses much more rapidly producing amuch higher induced voltage, several

+V

H.O.T.

DDamper

1 CRetrace/TimingCapacitor

T

+V

B+Volts

FlybackTransformer

C TD1

OptionalParallelH.O.T.

DriverAmp.

H.O.T.B+Volts

FlybackTransformer

Retrace/TimingCapacitor

DamperDiode

BIPOLAR MOSFET

Fig: 1: Typical bipolar and MOSFET high voltage only horizontal output stages and their driver stage.

Page 18: Sencore Tech Tips

thousand volts or more. The induced volt-age would produce excessive high voltageand/or deflection and quickly damage thehorizontal output transistor. Because ofthis key role in controlling the flyback orkickback voltage, the timing capacitor isoften called a "safety capacitor".

After the magnetic field has completelycollapsed, Ct begins to discharge, causingcurrent flow back into the transformer orcoil in the opposite direction. A magneticfield builds, but with the opposite magnet-ic polarity. This action completes the sec-ond part of retrace and corresponds to thefalling portion of the voltage waveform orpulse at the collector or drain of the H.O.T.

When Ct has completely discharged themagnetic field of the transformer or coilbegins to collapse. The collapsing fieldinduces a voltage with a polarity that for-ward biases the damper diode. Thedamper diode conducts producing aninductive circuit similar to when the H.O.T.was conducting. The damper diode allowsthe magnetic energy of the transformer orcoil winding to decay at a controlledinductive rate returning energy (current)back to the B+ supply capacitor. As themagnetic field is nearly fully collapsed thehorizontal output transistor is turned onand the cycle repeats.

Deflection Only Horizontal OutputStage

A deflection only horizontal output stage isresponsible for producing yoke deflectioncurrent. This horizontal output stage con-tains the basic components of a horizontaloutput stage plus components which

comprise the yoke's current path. A typi-cal deflection horizontal output stage isshown in Fig. 2. Because of the yoke'shigh current requirements, bipolar outputtransistors are used. A high frequency coilor transformer replaces the flyback trans-former.

The horizontal yoke and its series compo-nents parallel the timing or retrace capac-itor of the horizontal output stage.Included in the yoke's current path isalways a linearity coil and an "S-shaping"capacitor. These components shape therise and fall of the alternating current inthe yoke to produce a linear and uniformdeflection on the CRT. The series compo-nents can be arranged in any order. Othercomponents that may be found in the hor-izontal yoke's current path are a pincush-ion transformer and efficiency or widthcontrol transformer or coil (not shown inFig. 2).

Operation of a deflection only horizontaloutput stage is the same as described forthe high voltage only horizontal outputstage, but with the additional path for yokecurrent. To produce yoke current that is insync with the video's horizontal retracetime, a common horizontal drive signaloriginating from the locked horizontaloscillator feeds the separate HV and deflec-tion output stages. The common horizontaldrive synchronizes the high voltage anddeflection output stages to produce flybackpulses at nearly the same time.

Current for the horizontal yoke is derivedfrom the output stage's retrace or timingcapacitor. When the H.O.T. is turned on,the bottom side of the S-shaping capacitorconnects to the top of the linearity coil.Because the S-shaping capacitor is fullycharged from the previous cycle, it beginsto discharge through the horizontal outputtransistor. The resulting current flow pro-duces an expanding magnetic field in thelinearity and yoke coils. The polarity of theincreasing current deflects the CRT's elec-tron beam from the center to the right. Atthe same time, B+ power supply currentflows through the H.O.T. to energize thetransformer or coil winding.

When the horizontal output transistor isswitched open, the retrace timing capaci-tor effectively is placed in parallel with theyoke and its series components increasingthe resonant frequency or rate of current

change in the yoke. The yoke's magneticfield rapidly collapses producing currentwhich charges the retrace timing capacitorand S-shaping capacitor. Because of thedifference in capacitor values, most of theenergy is returned to Ct. Correspondingwith this time, is the collapsing magneticfield of the B+ transformer or coil whichreplenishes or fully charges the retracecapacitor. You may recall this is the risingedge of the inductive "kickback" voltagepulse at the collector of the output tran-sistor. Now, fully charged, Ct becomes thecurrent source for the yoke for theremainder of the cycle. This time corre-sponds to the 1st part of retrace when theCRT's electron beam is quickly returned tothe center of the display.

During the 2nd part of retrace, Ct and theS-shaping capacitor produce dischargingcurrent through the yoke in the oppositedirection. The current rises to a peakbuilding the magnetic field in the yoke andquickly moving the electron beam fromthe center of the CRT to the left. Also, dur-ing this time, Ct is energizing the B+ sup-ply transformer or coil.

When the retrace capacitor and the S-shap-ing capacitor are fully discharged, theyoke's magnetic field begins to collapse.This corresponds with the collapsing mag-netic field of the transformer or coil ener-gized by the B+ supply. The induced voltageforward biased the damper diode into con-duction. The circuits timing now agreeswith the timing during the right trace timewhen the H.O.T. was conducting. Theyoke's collapsing magnetic field returnsenergy to the circuit charging the S-shap-ing capacitor. Yoke current moves theCRT's electron beam slowly from the rightto the center of the CRT. When the yokesmagnetic field is collapsed, the damperdiode stops conducting. This correspondswith the beginning of the H.O.T.'s conduc-tion and the cycle repeats.

Indirect Flyback Driven – HV OnlyHorizontal Output Stages

In most high voltage horizontal outputstages the flyback primary winding is in thedirect current path of the horizontal outputtransistor. However, there are several non-conventional horizontal output stage con-figurations in which the flyback primary isnot in the H.O.T.'s conduction path.

H.O.T.

D1 C T

B+ Volts

Coil or Transformer

S-Capacitor

Horiz.Yoke

LinearityCoil

Fig. 2: Basic deflection only horizontal output stage.

Page 19: Sencore Tech Tips

For example, Fig. 3 shows a high volt-age only horizontal output stage inwhich the flyback transformers primarycurrent is provided by the retracecapacitor Ct. This configuration is near-ly the same as the deflection only hori-zontal output stage of figure 2. The onlydifference is that the yoke is replaced bythe flyback transformer primary wind-ing. Recall that when the H.O.T. isswitched on by gate drive, B+ supplycurrent flows to energize the coil andproduce an expanding magnetic field.When the H.O.T. is switched off, thecoil's magnetic field induces voltageand charging current to Ct. Ct thenbecomes the supply or current sourceof flyback current. Current alternates inthe tuned circuit including the flybacktransformer primary in the same man-ner as described earlier for the deflec-tion only horizontal output stage.

Combination High Voltage andDeflection Horizontal OutputStages

A combination high voltage and deflec-tion horizontal output stage produceshigh voltage and yoke current simulta-neously. There are 3 common combina-tion horizontal output stage configura-tions found in multi-frequency displaymonitors. They include:

1. Single Damper type2. Split or Dual Damper type3. Emitter Driven type

Single Damper CombinationHorizontal Output Stage

A common combination horizontal out-put stage is the single damper horizontal

output stage. The single damper combi-nation horizontal output stage is usedin the majority of television receivers.The single damper output stage pro-duces high voltage and deflection withthe fewest parts and component costs.It offers good performance and reliabil-ity for single frequency operation. Thesingle damper output stage is also pop-ular in computer display monitors thatoperate over only a few frequencymodes or a limited operating frequencyrange.

The single damper diode output stagecan be recognized by the fact it has onlyone damper diode. (See Fig. 2). Thediode is placed from the H.O.T. collectorto emitter. The damper diode may be adiscrete component or integrated intothe bipolar horizontal output transistor.

In a single damper diode horizontal out-put stage, the flyback transformer pri-mary winding connects from the collec-tor of the output transistor to the B+power supply. The yoke and seriescomponents connect between theH.O.T. collector and emitter or ground.The timing capacitor provides energy toproduce yoke current much like thedeflection only horizontal output stage.

Operation of the single damper horizon-tal output stage is identical to thedeflection only horizontal output stageexplained earlier. The only exception isthat the coil between the collector andB+ supply is replaced with the primarywinding of a flyback transformer.Conduction of the H.O.T. energizes theprimary of the flyback transformer.When the H.O.T. is switched off the col-lapsing magnetic field of the flybacktransformer charges Ct. The charge inCt produces current in the yoke and fly-back transformer primary. The damperdiode shunts the H.O.T. and is biasedinto conduction by the induced voltagefrom the flyback and yoke to returnmagnetic energy to the power supply.

While the single damper diode combi-nation output stage reliably produceshigh voltage and deflection current withthe fewest components and costs, it islimited in multi-frequency applications.This is because any changes to the out-put stage to increase high voltagewould also increase yoke current and

vice versa. With extreme horizontal fre-quency changes it becomes difficult tochange the operating parameters toboth establish normal high voltage andproper yoke deflection current simulta-neously.

For a complete explanation of a singledamper diode combination horizontaloutput stage request Sencore Tech Tip#207.

Split Damper CombinationHorizontal Output Stage

A popular combination horizontal out-put stage found in multi-frequencyvideo displays uses two damper diodes.The damper diodes are placed in seriesfrom the collector to the emitter of theH.O.T. (See Figure 4) This horizontaloutput stage also uses two timingcapacitors placed in series between theH.O.T.'s collector and emitter. A con-nection in the middle of the damperdiodes and timing capacitors splits theoutput.

The split damper diode and timingcapacitor configuration provides ameans to control the level of yokedeflection current while not impactingthe flyback current and resulting highvoltage. It furthermore provides amethod of achieving pincushion correc-tion and other dynamic modifications ofthe horizontal yoke current.

An understanding of the operation of asplit damper horizontal output stagecan be gained by analyzing the currentsand resulting voltages during 4 timesduring the horizontal cycle, startingwith the conduction time of the H.O.T.(See Fig. 5). When the H.O.T. isswitched on by base drive, currentincreases through the flyback primary

C T

B+ Volts

Coil

15mH

Flyback Transformer

Cs

Fig. 3: Indirect current drive to the flyback transformer.

B+ Volts

S-Shaping

LinearityCoil

Yoke

D1

D2CT2

CT1

B+Filter

Capacitor

FlybackTransformer

Multiplier

Fig. 4: A split damper combination horizontal output stage.

Page 20: Sencore Tech Tips

creating an expanding flyback magneticfield. Current is also supplied from the S-shaping capacitor charged from the previ-ous cycle. Current flows from the S-shap-ing capacitor through the bottom damper(D2), H.O.T. linearity coil and yoke. Yokecurrent deflects the CRT's electron beamsfrom center to the right of the picture.

When the H.O.T. is switched off, the mag-netic fields in the flyback and yoke col-lapse. The flyback's collapsing magneticfield produces induced voltage and charg-ing current to timing capacitors CT1 andCT2. Values of Ct1 and Ct 2 are chosen soapproximately 80-90% of the charge isdelivered to CT1 and 10-20% to CT2. Theyoke’s induced voltage produces chargingcurrent to CT1 and the S-shaping capaci-tor. The difference in capacitor valuesreturns the greatest charge to CT1. Thisportion of the cycle is the 1st part ofretrace which quickly returns the CRTbeam from the right to the center of thepicture.

With the flyback and yoke magnetic fieldsfully collapsed, capacitors CT1 and CT2begin to discharge. Capacitor CT1, nowfully charged, supplies discharge currentalong with the lesser charged S-shapingcapacitor to the horizontal yoke. The yokecurrent moves the CRT's electron beamfrom the center to the left completingretrace. Capacitors CT1 and CT2 producecurrent through the flyback primary, butin the opposite direction, producing anexpanding magnetic field.

When the timing capacitors are dis-charged, the flyback’s magnetic field col-lapses biasing on the damper diodes.Diodes D1 and D2 conduct providing acurrent path for the magnetic energy of

the flyback to recharge the power supplyfilter capacitor. The collapsing magneticfield of the yoke charges the S-shapingcapacitor with current flowing throughD1. Yoke current moves the CRT electronbeams from the left slowly to the center.When the yoke magnetic field is fully col-lapsed, the S-shaping capacitor nears fullcharge. The H.O.T. is then switched on torepeat the horizontal cycle.

Emitter Driven CombinationHorizontal Output Stage

Most horizontal output stages in multi-frequency display monitors are singledamper or split damper type, but occa-sionally a modification to these will be

encountered. One such change is found inseveral computer display monitors fromGateway and several other manufacturers.The modification changes the manner inwhich the H.O.T. is switched on and off bythe input drive signal. (See Fig. 6) Allother operations of the output stage arethe same as the single damper combina-tion output stage.

The emitter driven horizontal output stageplaces a switching MOSFET at the emitterlead of the bipolar horizontal output tran-sistor to ground. The horizontal drive sig-nal is applied to the gate of the MOSFETturning its source-to-drain conductionpath on or off. Effectively the emitter leadof the H.O.T. is connected to ground whenthe MOSFET is switched on. The supply

Fig. 5: Operation of a Split Damper Combination Horizontal output stage cycle.

B+ Volts

LinearityCoil

D1

D2CT2

CT1

Multiplier

Shaping Capacitor

++ ++

Yoke

LinearityCoil

Yoke

++ ++

++ ++

CT2

Charge20%

Charge80%

CT2

B+ Volts

+ +

+ +CT2

CT1

B+ Volts

D2

D1

B+ Volts

++ ++

++ ++

Charge

Discharge

A B C D

B+ Cap.

B+ Cap.

B+ Cap.

++ ++

CT1

Charge

B+ Volts

CT2

CT1

FlybackTransformer

Multiplier

Damper

Base Drive Transformer

S-Shaping Capacitor

Yoke

LinearityCoil

PincushionTransformer

MOSFETEmitterDrive

TransistorHorizDrive

+12V

BiasResistor

H.O.T.1KΩ

Fig. 6: Emitter driven combination horizontal output stage.

Page 21: Sencore Tech Tips

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S3200 Sencore Drive, Sioux Falls, SD 57107

1-605-339-0100 • www.sencore.com

Form #6955Printed In U.S.A.

voltage to the base of the H.O.T. permits alow level of base current enabling theH.O.T. to start conducting flyback primarycurrent. As the current slowly increases inthe flyback primary, the base drive trans-former induces voltage to the base lead.The voltage draws additional base currentenabling the H.O.T. to produce additionalcollector current. In this manner, theinductive base drive dynamically increasesthe base current to produce the increasingcollector current. The base drive trans-former also improves the switching speedto turn the H.O.T. off. This H.O.T. switchingmethod provides more efficient H.O.T.operation reducing its heat and improvingstage reliability.

Emitter Output Horizontal OutputStage (HV or Deflection Only)

Most HV or deflection only horizontal out-put stages include the flyback or energiz-ing coil between the H.O.T.'s collector andthe B+ power supply voltage. This pro-duces a voltage or flyback pulse output atthe collector or drain of the H.O.T. A raredeviation of this typical horizontal outputstage results when the flyback or coil isplaced at the emitter lead of the H.O.T.(See Fig. 7) The emitter outputs current tothe flyback or coil, thus an emitter outputhorizontal output stage. This emitter out-put configuration can be used as a highvoltage or deflection only horizontal outputstage or combination horizontal outputstage.

The flyback primary or coil winding leadsto ground typically through a voltage regu-lation stage (not shown) which regulatesthe ground side of the current path. Adamper diode and timing capacitor parallelthe H.O.T. The yoke and its series compo-nents draw energy from Ct to producedeflection.

Operation of an emitter output type hori-zontal output stage is nearly the same asthose with the flyback or coil in the collec-tor lead. However, because the emitter isnot grounded and contains the inductivecomponent to ground, DC voltages andflyback induced voltages are developed atthe emitter in reference to circuit ground.For this reason, the stage's output isseemingly at the emitter of the H.O.T.

To better understand the emitter output,consider the horizontal cycle. During theconduction time of the horizontal outputtransistor, the emitter is effectively con-nected to the B+ supply voltage at the col-lector. Current flows through the flyback orcoil winding to energize the stage. Whenthe transistor is switched opened, thecoil's magnetic field collapses producingcurrent to charge Ct. The voltage at theemitter decreases with the induced voltagein the flyback or coil winding as Ctcharges. When the magnetic field is fullycollapsed, Ct discharges through the fly-back or coil winding. This causes the volt-age at the emitter to increase or becomeless negative. The charging and discharg-ing action of Ct results in a negative going

induced voltage pulse at the emitter of theH.O.T. in respect to circuit ground.

During damper diode time, the collapsingmagnetic field returns energy to the circuitproducing current flow though the damperdiode. The current charges the B+ powersupply capacitor. During damper diodetime, the B+ supply voltage is switched tothe emitter.

This configuration produces no waveformat the collector of the H.O.T. as it is con-nected to the B+ supply voltage. A DC volt-age measurement at the collector readsthe B+ supply voltage. Because of the con-figuration and switching action of thestage, a DC voltage and waveform at theemitter reflect the normal operation of thestage. Negative going flyback pulses ofseveral hundred volts peak-to-peak aretypical. The DC voltage at the emitterreflects the B+ supply voltage to the outputstage. This is determined by a regulationstage typically along the ground currentpath on the input side of the flyback or coil.

For More Information, Call Toll Free 1-800-SENCORE

(736-2673)

B+ Volts

Multipler

FlybackTransformer

B+ Volts

Coil or FlybackTransformer

S-Shaping Capacitor

LinearityCoil

DCV

Pulses

Yoke

Emitter Output HV Deflection Only

Fig. 7: Typical Emitter Output high voltage or deflection only horizontal output stages.


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