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.

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.

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.

Form 4968Printed In U.S.A.

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