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Horizontal output stage symptomsand component failures are oftencaused by problems in the horizontaldriver stage. The horizontal driverstage is seldom suspected, and oftenwhen it has a defect, the scopewaveform at the base of thehorizontal output transistor looksnormal. Precious service time isoften lost and horizontal output stagecomponents are needlessly replacedor destroyed.

This Tech Tip explains how thehorizontal driver stage works andhow the drive current output to thebase of a bipolar horizontal outputtransistor can be controlled in a widerange multi-frequency video display.It also covers typical driver stagesymptoms and explains how toanalyze the horizontal driver stagewith the HA2500's Horiz. Driver Test.

Understanding Horizontal DriverStages

The horizontal driver stage outputs adrive signal to turn the horizontaloutput transistor (H.O.T) on and off.The drive signal consists of a positiveoutput bias voltage to forward biasthe base/emitter junction and anegative output bias voltage toreverse bias the base/emitterjunction. The positive output voltageproduces base current turning on theH.O.T. The negative output voltagequickly rids the base of currentturning off the H.O.T.

The horizontal driver stage, used to

drive a bipolar horizontal outputtransistor, is commonly a discretetransistor amplifier stage. Theamplifier stage consists of five mainelements as shown in Fig. 1. Thebasic driver stage includes: 1) transistor, 2) transformer, 3) primary resistor 4) RC networkand, 5) primary filter capacitor.

The driver transistor is a mediumpower transistor that operates withvoltages from 10 to 100 volts. Thetransistor may be a conventionalbipolar or newer MOSFET type. Thedriver transistor operates as an on oroff switch. When driven on by thebase or gate drive signal, thetransistor permits current flow in thedriver transformer primary. Whendriven off by the base or gate drivesignal, the current path is opened. The driver transformer couples the

output of the driver stage to thehorizontal output transistor. Thedriver transformer providesimpedance matching and currentstep up to drive the H.O.T. A typicaldriver transformer turns ratio is 30:1from the primary to secondary. Thisratio matches the high impedance ofthe driver amplifier to the lowbase/emitter junction impedance ofthe horizontal output transistor. Itfurther steps up the drivertransformer primary current toproduce a much higher secondaryH.O.T. base current.

A primary resistor is part of thecurrent path with the driver transistorand transformer primary. The resistorestablishes the level of drivertransformer primary current alongwith the power supply voltage to thedriver stage. An increase or decrease

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Understanding The HA2500's Horiz Driver Test

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supply current limiting resistor, drivertransformer characteristics andresistor between the drivertransformer and base of the H.O.T. It isalso in part affected by the gain of thedriver transistor and its base current.The driver stage components aredetermined by designers to satisfy thecurrent requirements of the H.O.T. andthe horizontal output stage.

Driver stages for bipolar HV onlyhorizontal output stages typicallyproduce base drive currents rangingfrom 150 to 600 mA. Driver stages ofcombination HV/Defl. output stagesand deflection only output stagestypically produce higher drive currentsranging from 350 to 900 mA.

Horiz. Driver Test mA readouts lessthan the typical mA range shown inthe chart indicate a likely weak orinsufficient current drive for thehorizontal output transistor. Horiz.Driver Test mA readouts greater thanthe typical range usually do notindicate a problem unless driver stagecomponents have been improperlysubstituted. Improper substitution canresult in higher than normal driveroutput current and contribute tohorizontal output transistor failures.

Note Specific Chassis Horiz.Driver Test Readings

Added benefits of the Horiz. DriverTest can be realized by recording the

normal mA level of a working chassis.On chassis that switch output drivecurrent levels, be sure to select themonitor's highest and lowest operatingfrequencies or resolutions and recordthe drive current readings. Then in thefuture, you can reference the normalmA drive current for that chassis whenconfronted with horizontal problems.You can also more accurately identifyhorizontal driver stage problems thatare beginning to weaken the drivebefore they cause major problems.

Isolating Intermittent HorizontalDrive Problems

Intermittent horizontal drive problemscan be frustrating as they often causerepeat H.O.T. failures while you areattempting to troubleshoot thesymptoms. Also, the waveformobserved on an oscilloscope at thebase of the H.O.T. may or may notchange much and you cannot be sureif a change was caused by thehorizontal driver or horizontal outputstage.

The HA2500's Horiz. Driver Testreadout helps you quickly determine ifthe current output of the driver stageis intermittent. Simply perform theHoriz. Driver Test and monitor thecurrent readout for a short period oftime. Watch the mA readout for anybig fluctuations or abrupt changes. Anormal current drive output changesthe readout by only a few microamps.

An intermittent output drive currentcauses substantial changes in the mAoutput.

To isolate intermittent drive problems,try introducing vibrations, heating,stimuli or cooling stimuli to thehorizontal driver stage andcomponents while watching the mAreadout. After isolating and correctingintermittents, use the Horiz. DriverTest to confirm a constant mAreadout. Using the Horiz. Driver Testprevents damage to replacementhorizontal output transistors.

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

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circuit board to open the circuit pathto the base.

The Horiz. Driver Test is performedusing the Dynamic Test Leads.Connect the blue test lead clip to theoutput of the driver stage at thesecondary side of the horizontal drivertransformer. This corresponds to thetransformer side of the opened pathbetween the transformer and base ofthe horizontal output transistor (seeFig. 5). Connect the black test lead clipto the horizontal output stage groundtypically at the H.O.T.'s emitter orbottom of the driver transformersecondary. Connect the yellow testlead clip to the collector of thehorizontal output transistor.

When performing the Horiz. DriverTest, it is important to connect to thecircuit point equivalent to the base ofthe horizontal output transistor.Resistors located between the driver

transformer and the base of the H.O.T.impact the level of base drive current.Always connect the blue clip of theDynamic Tests Lead to the base sideof the resistors if it is unsoldered andlifted from the circuit board.

The Horiz. Driver Test is an active testrelying on the chassis horizontaldriver stage to produce an output.With no drive output from the driverstage, the test reads little or no mAdrive current. To produce an output,the horizontal driver stage must havea horizontal drive input signal fromthe horizontal oscillator and a powersupply voltage and be operational. Toconfirm the horizontal driver stage isproducing an output, apply AC voltageto the chassis and perform the Baseor Gate Dynamic Tests. No horizontaldrive is indicated by low PPV readingsand no frequency readout. Isolate thecause of no drive output beforeperforming the Horiz. Driver Test.

To Perform The Horiz. Driver Test:

1.Remove AC voltage to the chassis.

2.Unsolder the H.O.T. or remove oropen the base circuit path.

3.Connect the DYNAMIC TEST leads:Blue clip to driver transformersecondary. Black clip to horizontalstage ground. Yellow clip tocollector.

4.Set DYNAMIC TESTS switch to“Horiz. Driver Test.”

5.Apply AC voltage to the chassis.

6.Push & Hold the Horiz. Driver TestOr Sub Drive test button.

7.Read the mA current drive.

8.Compare reading to Typical mARange chart (see the HA2500 PullChart or Chart 1 in this Tech Tip).

Interpreting The Horiz. DriverTest mA Readout

The mA readout of the Horiz. DriverTest indicates the level of output basedrive current the chassis horizontaldriver stage can produce. The amountof output drive current varies betweenchassis depending on the amount ofcollector current which must beproduced by the horizontal outputtransistor. The base current peakmultiplied by the beta of thehorizontal output transistor mustsatisfy the peak buildup in the H.O.T.'scollector current. The buildup incollector current is determined by thehorizontal output stage.

The amount of driver stage outputcurrent is determined by the driverstage's power supply voltage, power

Chart 1: Typical ranges of current expected during the Horiz. Driver Test.

Fig. 1: Basic components of a horizontal driver stage and resulting base currents in a bipolarhorizontal output stage.

Fig. 5: Dynamic Test Lead connections for performing the Horiz. Driver Test.

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in the resistor value directly affectsthe primary current and the driverstage's output. Often two resistors inparallel are used.

A bypass filter capacitor connectsfrom the voltage input side of thedriver transformer primary to ground.The filter capacitor effectivelyincreases the transformer primarycurrent by charging and dischargingcurrent through the transformerprimary.

An RC network parallels the drivertransistor from the collector or drainto ground. The network protects thetransistor from induced voltages. Thenetwork has little impact on thehorizontal driver stage's outputcurrent. In some driver stages, the RCnetwork is placed across the drivertransformer primary winding.

An input horizontal signal to the baseor gate of the horizontal drive stage isneeded for normal operations. Thedrive signal is typically a square waveat the horizontal frequency. During aportion of the input drive waveform,the driver transistor is biased on and

current flows through the transistor,driver transformer primary, andprimary resistor. The discharge actionof the bypass filter capacitor producesadditional current in the drivertransformer primary. Current builds inthe transformer primary producing anexpanding magnetic field and inducedvoltage to the secondary. The inducedvoltage produces secondary currentthat depletes the horizontal outputtransistor base of conduction carriersswitching the horizontal outputtransistor off.

When the driver transistor is biasedoff by the remaining portion of thehorizontal drive input, current ceasesin the driver transistor. The magneticfield in the primary of the drivertransformer collapses inducing voltageinto the driver transformer secondaryto forward bias the base/emitterjunction of the horizontal outputtransistor. The driver stage andtransformer action produce anincreasing secondary base current inthe horizontal output transistor. Thebase current increases to a level thatbiases on the horizontal outputtransistor and continues to increase to

permit the buildup of collector currentfor the horizontal output stage.

Changing Driver Stage OutputCurrent In A Multi-FrequencyMonitor

Multi-frequency CRT displays operatethrough a range of many differenthorizontal frequencies. The horizontaldriver stage must produce the properH.O.T. base current through the entirehorizontal frequency range. Monitorswith a wide horizontal operating rangemay change the horizontal driverstage to increase or decrease the basedrive current for the H.O.T. At loweroperating resolutions or frequencies,the base current may be increasedcompared to higher operatingfrequencies.

The reason for this is that the H.O.T.has slightly different collector currentrequirements depending on thehorizontal frequency. At loweroperating frequencies, the horizontaloutput transistor conducts longerreaching higher collector currentpeaks to produce the required highvoltage and/or deflection. Therefore, a

higher level of base drive current isrequired to reach the needed collectorcurrent. A driver stage designed tosupply sufficient base drive currentfor the highest operating frequencyoverdrives the H.O.T. producing morethan the required base current at amuch lower operating frequency. Thisresults in a longer turn off time of theH.O.T causing added powerdissipation and heating of the H.O.T.To optimize operation of the H.O.T.and reduce transistor heating, thedriver stage may be switched betweentwo or more output drive currentlevels.

Two methods of switching the driverstage output current level to the baseof the H.O.T. are shown in Fig. 2A.The first method involves changingthe level of primary current in thedriver transformer. This can be doneby changing the power supply voltageapplied to the horizontal driver stageor by varying the value of the primaryresistor. Increasing the power supply voltage increases the drivertransformer's primary current and theoutput base drive current. Decreasingthe power supply voltage to the driverstage decreases the output base drivecurrent.

The voltage to the horizontal driverstage can be selected or varied usingpower supply regulating methods. A pass transistor (as shown in Fig.2A) outputs voltage at its emitteraccording to the voltage establishedat its base. Switching converterssimilar to those used by highvoltage/deflection regulators may alsobe used.

Varying the primary resistor valuealso changes the transformer primarycurrent. The resistance value can bechanged by placing a transistor inparallel with the primary resistor. By controlling the transistor'sconductance, the resistor value canbe changed to increase or decrease

the driver transformer's primarycurrent and output current.

A second method of changing thedriver stages output drive current isshown in Fig. 2B. Placing a resistor inseries with the base current path ofthe horizontal output transistorreduces the base current to therequired level. By switching in a lowervalue resistor for lower horizontalfrequencies, the base drive current isboosted. The resistor can be switchedin or out of the base current path witha MOSFET transistor.

Understanding Horizontal DriverStage Symptoms

The output of the horizontal driverstage is essential for normal operationof the H.O.T. and horizontal outputstage. As with any bipolar transistor,it is the base current that becomescritical to normal operation of theH.O.T. It is the horizontal driver stagethat produces the proper drive voltageand resulting base current in thebipolar horizontal output transistor.

Consider the normal operation of theH.O.T. When on, the H.O.T.'s collectorcurrent rises producing expandingmagnetic fields in the flybacktransformer and/or yoke (see Fig. 1).The collector current rises to a peakof several amps before the H.O.T. isswitched off. The magnetic energyproduced by the collector currentproduces normal high voltage and/ordeflection.

The base current produced by thehorizontal driver stage determineshow much collector current can beproduced by a bipolar horizontaloutput transistor. The H.O.T.'smaximum collector current isdetermined by multiplying the basecurrent by the transistor's currentgain or beta. If the base current is notproduced by the driver stage, theH.O.T. cannot be turned on and the

horizontal output stage remainsinactive.

If the base current is reduced fromnormal, the H.O.T. with normal gaincannot produce the required collectorcurrent. The symptom depends on theseverity of the current decrease. If thebase current is reduced only slightly,little if any change in high voltageand/or deflection results. At somelevel of base current reduction, thecollector current peak is limitedcausing the high voltage and/ordeflection to decrease. The flybackpulses at the collector or draindecrease from normal, indicating adecrease in the flyback and/or yokecurrents. A severe reduction in basedrive current often results in quickheating and failures of the H.O.T.

The reverse bias drive voltage appliedto the base of the H.O.T. transistorswitches the H.O.T. off to stopcollector current. Switching off theH.O.T. as it reaches a current peak ofseveral amps and base current ofseveral hundred milliamps is notsimple. The base region of thetransistor must be evacuated ofcharged carriers before collectorcurrent ceases. Reduced base drivecurrent slows the transistor turn offleading to increased H.O.T. powerdissipation, heating, and prematurefailure. This failure commonly occurswith reductions in the driver stage'sbase current long before decreases inthe high voltage or deflection occurs.

Intermittents in the horizontal driverstage can be another cause ofhorizontal output transistor failure.Intermittents cause a fluctuation inthe driver stage's output base drivecurrent. These current changes causeabrupt current variations as thehorizontal output transistor is drivenon and off. Combined with the highcurrents and voltages produced bythe horizontal output stage, the H.O.Tis often damaged.

When To Use The HA2500'sHoriz. Driver Test

The HA2500's Horiz Driver Testmeasures the output drive currentcapability of the horizontal driverstage. This is the current drive tothe base of a bipolar horizontaloutput transistor. If there is normalhigh voltage and/or deflection,normal Collector DCV/uS/PPVDynamic Tests, and the bipolarhorizontal output transistor is notoverheating, there is little reason tosuspect the horizontal driver stage.

When horizontal related symptomsexist, the horizontal driver stageshould be considered suspect. Usethe Horiz. Driver Test when there isgood cause to suspect thehorizontal driver stage may haveinsufficient drive output current.

You should suspect the horizontaldriver stage and perform the Horiz.Driver Tests when:

1. The original problem was adefective bipolar horizontal outputtransistor and the HA2500's LoadTests indicate normal.

2. The horizontal output transistor isdefective or repeatedly fails but areplacement seems to restorenormal operation.

3. The horizontal output stageproduces insufficient high voltageand/or deflection (Reduced CollectorPPV) but the DCV seems normal.

4. The chassis seems to operatenormally (Collector Or DrainDCV/uS/PPV meter tests indicatenormal), but the horizontal outputtransistor gets too hot often failingwithin hours, days, or weeks ofreplacement.

5. The horizontal output transistorburns out or gets hot when thechassis horizontal driver stage isused but is fine when using theHA2500's Base Sub Drive.

How The Horiz. Driver TestWorks

To accurately measure the driverstage's output current capability,the Horiz. Driver Test simulates thebase/emitter junction of a bipolarhorizontal output transistor. Thetest uses a low barrier shottkeydiode and small value resistor (see Fig. 4). The polarity of theshottkey diode permits drivecurrent output produced by thedriver stage to flow through theresistor during the positive cycle.This closely simulates how thepositive drive output cycle of thedriver stage produces base currentin the horizontal output transistor.The low forward bias voltage of theshottkey diode permits a moresensitive current measurement tobetter test the driver stage's outputcurrent drive capability.

Current through the small valueresistor produces a voltage that isrelative to the amount of potentialoutput drive current during thepositive cycle. A metering circuitmeasures the voltage andcalculates the average positiveconduction current. This current isthe base drive current that isavailable from the driver stage tothe horizontal output transistor.

How To Perform The Horiz.Driver Test

Performing the Horiz. Driver Testrequires that the horizontal outputtransistor be removed or that thebase lead of the horizontal outputtransistor be opened. This permitsall the drive current from the driverstage to be routed to the HA2500'smeasurement circuit for accuratedrive current measurements. Itfurther prevents any chance ofdamaging the chassis' horizontaloutput transistor. Before performingthe Horiz. Driver Test, remove ACvoltage to the chassis and unsolderthe H.O.T. or open the circuit pathleading from the driver transformerto the base of the horizontal outputtransistor. There is commonly ajumper, small resistor, inductor,and/or diode between thetransformer and H.O.T. base.Unsolder one lead of thecomponent(s) and lift from the

Fig. 3: The Horiz. Driver Test is adynamic test that measures the outputdrive current capability of thehorizontal driver stage.

Fig. 2: Two methods in multi-frequency monitors used to control the horizontal driver stage's output drive current.

in the resistor value directly affectsthe primary current and the driverstage’s output. Often two resistors inparallel are used.

A bypass filter capacitor connectsfrom the voltage input side of thedriver transformer primary to ground.The filter capacitor effectivelyincreases the transformer primarycurrent by charging and dischargingcurrent through the transformerprimary.

An RC network parallels the drivertransistor from the collector or drainto ground. The network protects thetransistor from induced voltages. Thenetwork has little impact on thehorizontal driver stage’s outputcurrent. In some driver stages, the RCnetwork is placed across the drivertransformer primary winding.

An input horizontal signal to the baseor gate of the horizontal drive stage isneeded for normal operations. Thedrive signal is typically a square waveat the horizontal frequency. During aportion of the input drive waveform,the driver transistor is biased on and

current flows through the transistor,driver transformer primary, andprimary resistor. The discharge actionof the bypass filter capacitor producesadditional current in the drivertransformer primary. Current builds inthe transformer primary producing anexpanding magnetic field and inducedvoltage to the secondary. The inducedvoltage produces secondary currentthat depletes the horizontal outputtransistor base of conduction carriersswitching the horizontal outputtransistor off.

When the driver transistor is biasedoff by the remaining portion of thehorizontal drive input, current ceasesin the driver transistor. The magneticfield in the primary of the drivertransformer collapses inducing voltageinto the driver transformer secondaryto forward bias the base/emitterjunction of the horizontal outputtransistor. The driver stage andtransformer action produce anincreasing secondary base current inthe horizontal output transistor. Thebase current increases to a level thatbiases on the horizontal outputtransistor and continues to increase to

permit the buildup of collector currentfor the horizontal output stage.

Changing Driver Stage OutputCurrent In A Multi-FrequencyMonitor

Multi-frequency CRT displays operatethrough a range of many differenthorizontal frequencies. The horizontaldriver stage must produce the properH.O.T. base current through the entirehorizontal frequency range. Monitorswith a wide horizontal operating rangemay change the horizontal driverstage to increase or decrease the basedrive current for the H.O.T. At loweroperating resolutions or frequencies,the base current may be increasedcompared to higher operatingfrequencies.

The reason for this is that the H.O.T.has slightly different collector currentrequirements depending on thehorizontal frequency. At loweroperating frequencies, the horizontaloutput transistor conducts longerreaching higher collector currentpeaks to produce the required highvoltage and/or deflection. Therefore, a

higher level of base drive current isrequired to reach the needed collectorcurrent. A driver stage designed tosupply sufficient base drive currentfor the highest operating frequencyoverdrives the H.O.T. producing morethan the required base current at amuch lower operating frequency. Thisresults in a longer turn off time of theH.O.T causing added powerdissipation and heating of the H.O.T.To optimize operation of the H.O.T.and reduce transistor heating, thedriver stage may be switched betweentwo or more output drive currentlevels.

Two methods of switching the driverstage output current level to the baseof the H.O.T. are shown in Fig. 2A.The first method involves changingthe level of primary current in thedriver transformer. This can be doneby changing the power supply voltageapplied to the horizontal driver stageor by varying the value of the primaryresistor. Increasing the power supply voltage increases the drivertransformer’s primary current and theoutput base drive current. Decreasingthe power supply voltage to the driverstage decreases the output base drivecurrent.

The voltage to the horizontal driverstage can be selected or varied usingpower supply regulating methods. A pass transistor (as shown in Fig.2A) outputs voltage at its emitteraccording to the voltage establishedat its base. Switching converterssimilar to those used by highvoltage/deflection regulators may alsobe used.

Varying the primary resistor valuealso changes the transformer primarycurrent. The resistance value can bechanged by placing a transistor inparallel with the primary resistor. By controlling the transistor’sconductance, the resistor value canbe changed to increase or decrease

the driver transformer’s primarycurrent and output current.

A second method of changing thedriver stages output drive current isshown in Fig. 2B. Placing a resistor inseries with the base current path ofthe horizontal output transistorreduces the base current to therequired level. By switching in a lowervalue resistor for lower horizontalfrequencies, the base drive current isboosted. The resistor can be switchedin or out of the base current path witha MOSFET transistor.

Understanding Horizontal DriverStage Symptoms

The output of the horizontal driverstage is essential for normal operationof the H.O.T. and horizontal outputstage. As with any bipolar transistor,it is the base current that becomescritical to normal operation of theH.O.T. It is the horizontal driver stagethat produces the proper drive voltageand resulting base current in thebipolar horizontal output transistor.

Consider the normal operation of theH.O.T. When on, the H.O.T.’s collectorcurrent rises producing expandingmagnetic fields in the flybacktransformer and/or yoke (see Fig. 1).The collector current rises to a peakof several amps before the H.O.T. isswitched off. The magnetic energyproduced by the collector currentproduces normal high voltage and/ordeflection.

The base current produced by thehorizontal driver stage determineshow much collector current can beproduced by a bipolar horizontaloutput transistor. The H.O.T.’smaximum collector current isdetermined by multiplying the basecurrent by the transistor’s currentgain or beta. If the base current is notproduced by the driver stage, theH.O.T. cannot be turned on and the

horizontal output stage remainsinactive.

If the base current is reduced fromnormal, the H.O.T. with normal gaincannot produce the required collectorcurrent. The symptom depends on theseverity of the current decrease. If thebase current is reduced only slightly,little if any change in high voltageand/or deflection results. At somelevel of base current reduction, thecollector current peak is limitedcausing the high voltage and/ordeflection to decrease. The flybackpulses at the collector or draindecrease from normal, indicating adecrease in the flyback and/or yokecurrents. A severe reduction in basedrive current often results in quickheating and failures of the H.O.T.

The reverse bias drive voltage appliedto the base of the H.O.T. transistorswitches the H.O.T. off to stopcollector current. Switching off theH.O.T. as it reaches a current peak ofseveral amps and base current ofseveral hundred milliamps is notsimple. The base region of thetransistor must be evacuated ofcharged carriers before collectorcurrent ceases. Reduced base drivecurrent slows the transistor turn offleading to increased H.O.T. powerdissipation, heating, and prematurefailure. This failure commonly occurswith reductions in the driver stage’sbase current long before decreases inthe high voltage or deflection occurs.

Intermittents in the horizontal driverstage can be another cause ofhorizontal output transistor failure.Intermittents cause a fluctuation inthe driver stage’s output base drivecurrent. These current changes causeabrupt current variations as thehorizontal output transistor is drivenon and off. Combined with the highcurrents and voltages produced bythe horizontal output stage, the H.O.Tis often damaged.

When To Use The HA2500’sHoriz. Driver Test

The HA2500’s Horiz Driver Testmeasures the output drive currentcapability of the horizontal driverstage. This is the current drive tothe base of a bipolar horizontaloutput transistor. If there is normalhigh voltage and/or deflection,normal Collector DCV/uS/PPVDynamic Tests, and the bipolarhorizontal output transistor is notoverheating, there is little reason tosuspect the horizontal driver stage.

When horizontal related symptomsexist, the horizontal driver stageshould be considered suspect. Usethe Horiz. Driver Test when there isgood cause to suspect thehorizontal driver stage may haveinsufficient drive output current.

You should suspect the horizontaldriver stage and perform the Horiz.Driver Tests when:

1. The original problem was adefective bipolar horizontal outputtransistor and the HA2500’s LoadTests indicate normal.

2. The horizontal output transistor isdefective or repeatedly fails but areplacement seems to restorenormal operation.

3. The horizontal output stageproduces insufficient high voltageand/or deflection (Reduced CollectorPPV) but the DCV seems normal.

4. The chassis seems to operatenormally (Collector Or DrainDCV/uS/PPV meter tests indicatenormal), but the horizontal outputtransistor gets too hot often failingwithin hours, days, or weeks ofreplacement.

5. The horizontal output transistorburns out or gets hot when thechassis horizontal driver stage isused but is fine when using theHA2500’s Base Sub Drive.

How The Horiz. Driver TestWorks

To accurately measure the driverstage’s output current capability,the Horiz. Driver Test simulates thebase/emitter junction of a bipolarhorizontal output transistor. Thetest uses a low barrier shottkeydiode and small value resistor (see Fig. 4). The polarity of theshottkey diode permits drivecurrent output produced by thedriver stage to flow through theresistor during the positive cycle.This closely simulates how thepositive drive output cycle of thedriver stage produces base currentin the horizontal output transistor.The low forward bias voltage of theshottkey diode permits a moresensitive current measurement tobetter test the driver stage’s outputcurrent drive capability.

Current through the small valueresistor produces a voltage that isrelative to the amount of potentialoutput drive current during thepositive cycle. A metering circuitmeasures the voltage andcalculates the average positiveconduction current. This current isthe base drive current that isavailable from the driver stage tothe horizontal output transistor.

How To Perform The Horiz.Driver Test

Performing the Horiz. Driver Testrequires that the horizontal outputtransistor be removed or that thebase lead of the horizontal outputtransistor be opened. This permitsall the drive current from the driverstage to be routed to the HA2500’smeasurement circuit for accuratedrive current measurements. Itfurther prevents any chance ofdamaging the chassis’ horizontaloutput transistor. Before performingthe Horiz. Driver Test, remove ACvoltage to the chassis and unsolderthe H.O.T. or open the circuit pathleading from the driver transformerto the base of the horizontal outputtransistor. There is commonly ajumper, small resistor, inductor,and/or diode between thetransformer and H.O.T. base.Unsolder one lead of thecomponent(s) and lift from the

Fig. 3: The Horiz. Driver Test is adynamic test that measures the outputdrive current capability of thehorizontal driver stage.

Fig. 4: Block diagram of the HA2500 Horiz. Driver Test.Fig. 2: Two methods in multi-frequency monitors used to control the horizontal driver stage’s output drive current.

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in the resistor value directly affectsthe primary current and the driverstage's output. Often two resistors inparallel are used.

A bypass filter capacitor connectsfrom the voltage input side of thedriver transformer primary to ground.The filter capacitor effectivelyincreases the transformer primarycurrent by charging and dischargingcurrent through the transformerprimary.

An RC network parallels the drivertransistor from the collector or drainto ground. The network protects thetransistor from induced voltages. Thenetwork has little impact on thehorizontal driver stage's outputcurrent. In some driver stages, the RCnetwork is placed across the drivertransformer primary winding.

An input horizontal signal to the baseor gate of the horizontal drive stage isneeded for normal operations. Thedrive signal is typically a square waveat the horizontal frequency. During aportion of the input drive waveform,the driver transistor is biased on and

current flows through the transistor,driver transformer primary, andprimary resistor. The discharge actionof the bypass filter capacitor producesadditional current in the drivertransformer primary. Current builds inthe transformer primary producing anexpanding magnetic field and inducedvoltage to the secondary. The inducedvoltage produces secondary currentthat depletes the horizontal outputtransistor base of conduction carriersswitching the horizontal outputtransistor off.

When the driver transistor is biasedoff by the remaining portion of thehorizontal drive input, current ceasesin the driver transistor. The magneticfield in the primary of the drivertransformer collapses inducing voltageinto the driver transformer secondaryto forward bias the base/emitterjunction of the horizontal outputtransistor. The driver stage andtransformer action produce anincreasing secondary base current inthe horizontal output transistor. Thebase current increases to a level thatbiases on the horizontal outputtransistor and continues to increase to

permit the buildup of collector currentfor the horizontal output stage.

Changing Driver Stage OutputCurrent In A Multi-FrequencyMonitor

Multi-frequency CRT displays operatethrough a range of many differenthorizontal frequencies. The horizontaldriver stage must produce the properH.O.T. base current through the entirehorizontal frequency range. Monitorswith a wide horizontal operating rangemay change the horizontal driverstage to increase or decrease the basedrive current for the H.O.T. At loweroperating resolutions or frequencies,the base current may be increasedcompared to higher operatingfrequencies.

The reason for this is that the H.O.T.has slightly different collector currentrequirements depending on thehorizontal frequency. At loweroperating frequencies, the horizontaloutput transistor conducts longerreaching higher collector currentpeaks to produce the required highvoltage and/or deflection. Therefore, a

higher level of base drive current isrequired to reach the needed collectorcurrent. A driver stage designed tosupply sufficient base drive currentfor the highest operating frequencyoverdrives the H.O.T. producing morethan the required base current at amuch lower operating frequency. Thisresults in a longer turn off time of theH.O.T causing added powerdissipation and heating of the H.O.T.To optimize operation of the H.O.T.and reduce transistor heating, thedriver stage may be switched betweentwo or more output drive currentlevels.

Two methods of switching the driverstage output current level to the baseof the H.O.T. are shown in Fig. 2A.The first method involves changingthe level of primary current in thedriver transformer. This can be doneby changing the power supply voltageapplied to the horizontal driver stageor by varying the value of the primaryresistor. Increasing the power supply voltage increases the drivertransformer's primary current and theoutput base drive current. Decreasingthe power supply voltage to the driverstage decreases the output base drivecurrent.

The voltage to the horizontal driverstage can be selected or varied usingpower supply regulating methods. A pass transistor (as shown in Fig.2A) outputs voltage at its emitteraccording to the voltage establishedat its base. Switching converterssimilar to those used by highvoltage/deflection regulators may alsobe used.

Varying the primary resistor valuealso changes the transformer primarycurrent. The resistance value can bechanged by placing a transistor inparallel with the primary resistor. By controlling the transistor'sconductance, the resistor value canbe changed to increase or decrease

the driver transformer's primarycurrent and output current.

A second method of changing thedriver stages output drive current isshown in Fig. 2B. Placing a resistor inseries with the base current path ofthe horizontal output transistorreduces the base current to therequired level. By switching in a lowervalue resistor for lower horizontalfrequencies, the base drive current isboosted. The resistor can be switchedin or out of the base current path witha MOSFET transistor.

Understanding Horizontal DriverStage Symptoms

The output of the horizontal driverstage is essential for normal operationof the H.O.T. and horizontal outputstage. As with any bipolar transistor,it is the base current that becomescritical to normal operation of theH.O.T. It is the horizontal driver stagethat produces the proper drive voltageand resulting base current in thebipolar horizontal output transistor.

Consider the normal operation of theH.O.T. When on, the H.O.T.'s collectorcurrent rises producing expandingmagnetic fields in the flybacktransformer and/or yoke (see Fig. 1).The collector current rises to a peakof several amps before the H.O.T. isswitched off. The magnetic energyproduced by the collector currentproduces normal high voltage and/ordeflection.

The base current produced by thehorizontal driver stage determineshow much collector current can beproduced by a bipolar horizontaloutput transistor. The H.O.T.'smaximum collector current isdetermined by multiplying the basecurrent by the transistor's currentgain or beta. If the base current is notproduced by the driver stage, theH.O.T. cannot be turned on and the

horizontal output stage remainsinactive.

If the base current is reduced fromnormal, the H.O.T. with normal gaincannot produce the required collectorcurrent. The symptom depends on theseverity of the current decrease. If thebase current is reduced only slightly,little if any change in high voltageand/or deflection results. At somelevel of base current reduction, thecollector current peak is limitedcausing the high voltage and/ordeflection to decrease. The flybackpulses at the collector or draindecrease from normal, indicating adecrease in the flyback and/or yokecurrents. A severe reduction in basedrive current often results in quickheating and failures of the H.O.T.

The reverse bias drive voltage appliedto the base of the H.O.T. transistorswitches the H.O.T. off to stopcollector current. Switching off theH.O.T. as it reaches a current peak ofseveral amps and base current ofseveral hundred milliamps is notsimple. The base region of thetransistor must be evacuated ofcharged carriers before collectorcurrent ceases. Reduced base drivecurrent slows the transistor turn offleading to increased H.O.T. powerdissipation, heating, and prematurefailure. This failure commonly occurswith reductions in the driver stage'sbase current long before decreases inthe high voltage or deflection occurs.

Intermittents in the horizontal driverstage can be another cause ofhorizontal output transistor failure.Intermittents cause a fluctuation inthe driver stage's output base drivecurrent. These current changes causeabrupt current variations as thehorizontal output transistor is drivenon and off. Combined with the highcurrents and voltages produced bythe horizontal output stage, the H.O.Tis often damaged.

When To Use The HA2500'sHoriz. Driver Test

The HA2500's Horiz Driver Testmeasures the output drive currentcapability of the horizontal driverstage. This is the current drive tothe base of a bipolar horizontaloutput transistor. If there is normalhigh voltage and/or deflection,normal Collector DCV/uS/PPVDynamic Tests, and the bipolarhorizontal output transistor is notoverheating, there is little reason tosuspect the horizontal driver stage.

When horizontal related symptomsexist, the horizontal driver stageshould be considered suspect. Usethe Horiz. Driver Test when there isgood cause to suspect thehorizontal driver stage may haveinsufficient drive output current.

You should suspect the horizontaldriver stage and perform the Horiz.Driver Tests when:

1. The original problem was adefective bipolar horizontal outputtransistor and the HA2500's LoadTests indicate normal.

2. The horizontal output transistor isdefective or repeatedly fails but areplacement seems to restorenormal operation.

3. The horizontal output stageproduces insufficient high voltageand/or deflection (Reduced CollectorPPV) but the DCV seems normal.

4. The chassis seems to operatenormally (Collector Or DrainDCV/uS/PPV meter tests indicatenormal), but the horizontal outputtransistor gets too hot often failingwithin hours, days, or weeks ofreplacement.

5. The horizontal output transistorburns out or gets hot when thechassis horizontal driver stage isused but is fine when using theHA2500's Base Sub Drive.

How The Horiz. Driver TestWorks

To accurately measure the driverstage's output current capability,the Horiz. Driver Test simulates thebase/emitter junction of a bipolarhorizontal output transistor. Thetest uses a low barrier shottkeydiode and small value resistor (see Fig. 4). The polarity of theshottkey diode permits drivecurrent output produced by thedriver stage to flow through theresistor during the positive cycle.This closely simulates how thepositive drive output cycle of thedriver stage produces base currentin the horizontal output transistor.The low forward bias voltage of theshottkey diode permits a moresensitive current measurement tobetter test the driver stage's outputcurrent drive capability.

Current through the small valueresistor produces a voltage that isrelative to the amount of potentialoutput drive current during thepositive cycle. A metering circuitmeasures the voltage andcalculates the average positiveconduction current. This current isthe base drive current that isavailable from the driver stage tothe horizontal output transistor.

How To Perform The Horiz.Driver Test

Performing the Horiz. Driver Testrequires that the horizontal outputtransistor be removed or that thebase lead of the horizontal outputtransistor be opened. This permitsall the drive current from the driverstage to be routed to the HA2500'smeasurement circuit for accuratedrive current measurements. Itfurther prevents any chance ofdamaging the chassis' horizontaloutput transistor. Before performingthe Horiz. Driver Test, remove ACvoltage to the chassis and unsolderthe H.O.T. or open the circuit pathleading from the driver transformerto the base of the horizontal outputtransistor. There is commonly ajumper, small resistor, inductor,and/or diode between thetransformer and H.O.T. base.Unsolder one lead of thecomponent(s) and lift from the

Fig. 3: The Horiz. Driver Test is adynamic test that measures the outputdrive current capability of thehorizontal driver stage.

Fig. 2: Two methods in multi-frequency monitors used to control the horizontal driver stage's output drive current.

TT-223 7/1/99 1:18 PM Page 1

Horizontal output stage symptomsand component failures are oftencaused by problems in the horizontaldriver stage. The horizontal driverstage is seldom suspected, and oftenwhen it has a defect, the scopewaveform at the base of thehorizontal output transistor looksnormal. Precious service time isoften lost and horizontal output stagecomponents are needlessly replacedor destroyed.

This Tech Tip explains how thehorizontal driver stage works andhow the drive current output to thebase of a bipolar horizontal outputtransistor can be controlled in a widerange multi-frequency video display.It also covers typical driver stagesymptoms and explains how toanalyze the horizontal driver stagewith the HA2500's Horiz. Driver Test.

Understanding Horizontal DriverStages

The horizontal driver stage outputs adrive signal to turn the horizontaloutput transistor (H.O.T) on and off.The drive signal consists of a positiveoutput bias voltage to forward biasthe base/emitter junction and anegative output bias voltage toreverse bias the base/emitterjunction. The positive output voltageproduces base current turning on theH.O.T. The negative output voltagequickly rids the base of currentturning off the H.O.T.

The horizontal driver stage, used to

drive a bipolar horizontal outputtransistor, is commonly a discretetransistor amplifier stage. Theamplifier stage consists of five mainelements as shown in Fig. 1. Thebasic driver stage includes: 1) transistor, 2) transformer, 3) primary resistor 4) RC networkand, 5) primary filter capacitor.

The driver transistor is a mediumpower transistor that operates withvoltages from 10 to 100 volts. Thetransistor may be a conventionalbipolar or newer MOSFET type. Thedriver transistor operates as an on oroff switch. When driven on by thebase or gate drive signal, thetransistor permits current flow in thedriver transformer primary. Whendriven off by the base or gate drivesignal, the current path is opened. The driver transformer couples the

output of the driver stage to thehorizontal output transistor. Thedriver transformer providesimpedance matching and currentstep up to drive the H.O.T. A typicaldriver transformer turns ratio is 30:1from the primary to secondary. Thisratio matches the high impedance ofthe driver amplifier to the lowbase/emitter junction impedance ofthe horizontal output transistor. Itfurther steps up the drivertransformer primary current toproduce a much higher secondaryH.O.T. base current.

A primary resistor is part of thecurrent path with the driver transistorand transformer primary. The resistorestablishes the level of drivertransformer primary current alongwith the power supply voltage to thedriver stage. An increase or decrease

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supply current limiting resistor, drivertransformer characteristics andresistor between the drivertransformer and base of the H.O.T. It isalso in part affected by the gain of thedriver transistor and its base current.The driver stage components aredetermined by designers to satisfy thecurrent requirements of the H.O.T. andthe horizontal output stage.

Driver stages for bipolar HV onlyhorizontal output stages typicallyproduce base drive currents rangingfrom 150 to 600 mA. Driver stages ofcombination HV/Defl. output stagesand deflection only output stagestypically produce higher drive currentsranging from 350 to 900 mA.

Horiz. Driver Test mA readouts lessthan the typical mA range shown inthe chart indicate a likely weak orinsufficient current drive for thehorizontal output transistor. Horiz.Driver Test mA readouts greater thanthe typical range usually do notindicate a problem unless driver stagecomponents have been improperlysubstituted. Improper substitution canresult in higher than normal driveroutput current and contribute tohorizontal output transistor failures.

Note Specific Chassis Horiz.Driver Test Readings

Added benefits of the Horiz. DriverTest can be realized by recording the

normal mA level of a working chassis.On chassis that switch output drivecurrent levels, be sure to select themonitor's highest and lowest operatingfrequencies or resolutions and recordthe drive current readings. Then in thefuture, you can reference the normalmA drive current for that chassis whenconfronted with horizontal problems.You can also more accurately identifyhorizontal driver stage problems thatare beginning to weaken the drivebefore they cause major problems.

Isolating Intermittent HorizontalDrive Problems

Intermittent horizontal drive problemscan be frustrating as they often causerepeat H.O.T. failures while you areattempting to troubleshoot thesymptoms. Also, the waveformobserved on an oscilloscope at thebase of the H.O.T. may or may notchange much and you cannot be sureif a change was caused by thehorizontal driver or horizontal outputstage.

The HA2500's Horiz. Driver Testreadout helps you quickly determine ifthe current output of the driver stageis intermittent. Simply perform theHoriz. Driver Test and monitor thecurrent readout for a short period oftime. Watch the mA readout for anybig fluctuations or abrupt changes. Anormal current drive output changesthe readout by only a few microamps.

An intermittent output drive currentcauses substantial changes in the mAoutput.

To isolate intermittent drive problems,try introducing vibrations, heating,stimuli or cooling stimuli to thehorizontal driver stage andcomponents while watching the mAreadout. After isolating and correctingintermittents, use the Horiz. DriverTest to confirm a constant mAreadout. Using the Horiz. Driver Testprevents damage to replacementhorizontal output transistors.

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

(736-2673)

S3200 Sencore Drive, Sioux Falls, SD 57107Fax: 1-605-339-0317 www.sencore.com

circuit board to open the circuit pathto the base.

The Horiz. Driver Test is performedusing the Dynamic Test Leads.Connect the blue test lead clip to theoutput of the driver stage at thesecondary side of the horizontal drivertransformer. This corresponds to thetransformer side of the opened pathbetween the transformer and base ofthe horizontal output transistor (seeFig. 5). Connect the black test lead clipto the horizontal output stage groundtypically at the H.O.T.'s emitter orbottom of the driver transformersecondary. Connect the yellow testlead clip to the collector of thehorizontal output transistor.

When performing the Horiz. DriverTest, it is important to connect to thecircuit point equivalent to the base ofthe horizontal output transistor.Resistors located between the driver

transformer and the base of the H.O.T.impact the level of base drive current.Always connect the blue clip of theDynamic Tests Lead to the base sideof the resistors if it is unsoldered andlifted from the circuit board.

The Horiz. Driver Test is an active testrelying on the chassis horizontaldriver stage to produce an output.With no drive output from the driverstage, the test reads little or no mAdrive current. To produce an output,the horizontal driver stage must havea horizontal drive input signal fromthe horizontal oscillator and a powersupply voltage and be operational. Toconfirm the horizontal driver stage isproducing an output, apply AC voltageto the chassis and perform the Baseor Gate Dynamic Tests. No horizontaldrive is indicated by low PPV readingsand no frequency readout. Isolate thecause of no drive output beforeperforming the Horiz. Driver Test.

To Perform The Horiz. Driver Test:

1.Remove AC voltage to the chassis.

2.Unsolder the H.O.T. or remove oropen the base circuit path.

3.Connect the DYNAMIC TEST leads:Blue clip to driver transformersecondary. Black clip to horizontalstage ground. Yellow clip tocollector.

4.Set DYNAMIC TESTS switch to“Horiz. Driver Test.”

5.Apply AC voltage to the chassis.

6.Push & Hold the Horiz. Driver TestOr Sub Drive test button.

7.Read the mA current drive.

8.Compare reading to Typical mARange chart (see the HA2500 PullChart or Chart 1 in this Tech Tip).

Interpreting The Horiz. DriverTest mA Readout

The mA readout of the Horiz. DriverTest indicates the level of output basedrive current the chassis horizontaldriver stage can produce. The amountof output drive current varies betweenchassis depending on the amount ofcollector current which must beproduced by the horizontal outputtransistor. The base current peakmultiplied by the beta of thehorizontal output transistor mustsatisfy the peak buildup in the H.O.T.'scollector current. The buildup incollector current is determined by thehorizontal output stage.

The amount of driver stage outputcurrent is determined by the driverstage's power supply voltage, power

Chart 1: Typical ranges of current expected during the Horiz. Driver Test.

Fig. 1: Basic components of a horizontal driver stage and resulting base currents in a bipolarhorizontal output stage.

Fig. 5: Dynamic Test Lead connections for performing the Horiz. Driver Test.

TT-223 7/1/99 1:18 PM Page 1

Horizontal output stage symptomsand component failures are oftencaused by problems in the horizontaldriver stage. The horizontal driverstage is seldom suspected, and oftenwhen it has a defect, the scopewaveform at the base of thehorizontal output transistor looksnormal. Precious service time isoften lost and horizontal output stagecomponents are needlessly replacedor destroyed.

This Tech Tip explains how thehorizontal driver stage works andhow the drive current output to thebase of a bipolar horizontal outputtransistor can be controlled in a widerange multi-frequency video display.It also covers typical driver stagesymptoms and explains how toanalyze the horizontal driver stagewith the HA2500's Horiz. Driver Test.

Understanding Horizontal DriverStages

The horizontal driver stage outputs adrive signal to turn the horizontaloutput transistor (H.O.T) on and off.The drive signal consists of a positiveoutput bias voltage to forward biasthe base/emitter junction and anegative output bias voltage toreverse bias the base/emitterjunction. The positive output voltageproduces base current turning on theH.O.T. The negative output voltagequickly rids the base of currentturning off the H.O.T.

The horizontal driver stage, used to

drive a bipolar horizontal outputtransistor, is commonly a discretetransistor amplifier stage. Theamplifier stage consists of five mainelements as shown in Fig. 1. Thebasic driver stage includes: 1) transistor, 2) transformer, 3) primary resistor 4) RC networkand, 5) primary filter capacitor.

The driver transistor is a mediumpower transistor that operates withvoltages from 10 to 100 volts. Thetransistor may be a conventionalbipolar or newer MOSFET type. Thedriver transistor operates as an on oroff switch. When driven on by thebase or gate drive signal, thetransistor permits current flow in thedriver transformer primary. Whendriven off by the base or gate drivesignal, the current path is opened. The driver transformer couples the

output of the driver stage to thehorizontal output transistor. Thedriver transformer providesimpedance matching and currentstep up to drive the H.O.T. A typicaldriver transformer turns ratio is 30:1from the primary to secondary. Thisratio matches the high impedance ofthe driver amplifier to the lowbase/emitter junction impedance ofthe horizontal output transistor. Itfurther steps up the drivertransformer primary current toproduce a much higher secondaryH.O.T. base current.

A primary resistor is part of thecurrent path with the driver transistorand transformer primary. The resistorestablishes the level of drivertransformer primary current alongwith the power supply voltage to thedriver stage. An increase or decrease

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supply current limiting resistor, drivertransformer characteristics andresistor between the drivertransformer and base of the H.O.T. It isalso in part affected by the gain of thedriver transistor and its base current.The driver stage components aredetermined by designers to satisfy thecurrent requirements of the H.O.T. andthe horizontal output stage.

Driver stages for bipolar HV onlyhorizontal output stages typicallyproduce base drive currents rangingfrom 150 to 600 mA. Driver stages ofcombination HV/Defl. output stagesand deflection only output stagestypically produce higher drive currentsranging from 350 to 900 mA.

Horiz. Driver Test mA readouts lessthan the typical mA range shown inthe chart indicate a likely weak orinsufficient current drive for thehorizontal output transistor. Horiz.Driver Test mA readouts greater thanthe typical range usually do notindicate a problem unless driver stagecomponents have been improperlysubstituted. Improper substitution canresult in higher than normal driveroutput current and contribute tohorizontal output transistor failures.

Note Specific Chassis Horiz.Driver Test Readings

Added benefits of the Horiz. DriverTest can be realized by recording the

normal mA level of a working chassis.On chassis that switch output drivecurrent levels, be sure to select themonitor's highest and lowest operatingfrequencies or resolutions and recordthe drive current readings. Then in thefuture, you can reference the normalmA drive current for that chassis whenconfronted with horizontal problems.You can also more accurately identifyhorizontal driver stage problems thatare beginning to weaken the drivebefore they cause major problems.

Isolating Intermittent HorizontalDrive Problems

Intermittent horizontal drive problemscan be frustrating as they often causerepeat H.O.T. failures while you areattempting to troubleshoot thesymptoms. Also, the waveformobserved on an oscilloscope at thebase of the H.O.T. may or may notchange much and you cannot be sureif a change was caused by thehorizontal driver or horizontal outputstage.

The HA2500's Horiz. Driver Testreadout helps you quickly determine ifthe current output of the driver stageis intermittent. Simply perform theHoriz. Driver Test and monitor thecurrent readout for a short period oftime. Watch the mA readout for anybig fluctuations or abrupt changes. Anormal current drive output changesthe readout by only a few microamps.

An intermittent output drive currentcauses substantial changes in the mAoutput.

To isolate intermittent drive problems,try introducing vibrations, heating,stimuli or cooling stimuli to thehorizontal driver stage andcomponents while watching the mAreadout. After isolating and correctingintermittents, use the Horiz. DriverTest to confirm a constant mAreadout. Using the Horiz. Driver Testprevents damage to replacementhorizontal output transistors.

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

(736-2673)

S3200 Sencore Drive, Sioux Falls, SD 57107Fax: 1-605-339-0317 www.sencore.com

circuit board to open the circuit pathto the base.

The Horiz. Driver Test is performedusing the Dynamic Test Leads.Connect the blue test lead clip to theoutput of the driver stage at thesecondary side of the horizontal drivertransformer. This corresponds to thetransformer side of the opened pathbetween the transformer and base ofthe horizontal output transistor (seeFig. 5). Connect the black test lead clipto the horizontal output stage groundtypically at the H.O.T.'s emitter orbottom of the driver transformersecondary. Connect the yellow testlead clip to the collector of thehorizontal output transistor.

When performing the Horiz. DriverTest, it is important to connect to thecircuit point equivalent to the base ofthe horizontal output transistor.Resistors located between the driver

transformer and the base of the H.O.T.impact the level of base drive current.Always connect the blue clip of theDynamic Tests Lead to the base sideof the resistors if it is unsoldered andlifted from the circuit board.

The Horiz. Driver Test is an active testrelying on the chassis horizontaldriver stage to produce an output.With no drive output from the driverstage, the test reads little or no mAdrive current. To produce an output,the horizontal driver stage must havea horizontal drive input signal fromthe horizontal oscillator and a powersupply voltage and be operational. Toconfirm the horizontal driver stage isproducing an output, apply AC voltageto the chassis and perform the Baseor Gate Dynamic Tests. No horizontaldrive is indicated by low PPV readingsand no frequency readout. Isolate thecause of no drive output beforeperforming the Horiz. Driver Test.

To Perform The Horiz. Driver Test:

1.Remove AC voltage to the chassis.

2.Unsolder the H.O.T. or remove oropen the base circuit path.

3.Connect the DYNAMIC TEST leads:Blue clip to driver transformersecondary. Black clip to horizontalstage ground. Yellow clip tocollector.

4.Set DYNAMIC TESTS switch to“Horiz. Driver Test.”

5.Apply AC voltage to the chassis.

6.Push & Hold the Horiz. Driver TestOr Sub Drive test button.

7.Read the mA current drive.

8.Compare reading to Typical mARange chart (see the HA2500 PullChart or Chart 1 in this Tech Tip).

Interpreting The Horiz. DriverTest mA Readout

The mA readout of the Horiz. DriverTest indicates the level of output basedrive current the chassis horizontaldriver stage can produce. The amountof output drive current varies betweenchassis depending on the amount ofcollector current which must beproduced by the horizontal outputtransistor. The base current peakmultiplied by the beta of thehorizontal output transistor mustsatisfy the peak buildup in the H.O.T.'scollector current. The buildup incollector current is determined by thehorizontal output stage.

The amount of driver stage outputcurrent is determined by the driverstage's power supply voltage, power

Chart 1: Typical ranges of current expected during the Horiz. Driver Test.

Fig. 1: Basic components of a horizontal driver stage and resulting base currents in a bipolarhorizontal output stage.

Fig. 5: Dynamic Test Lead connections for performing the Horiz. Driver Test.