Identifying Television SMPS ProblemsMany of todays television receivers useswitch mode power supplies (SMPS,“choppers” or “switchers”) to regulate theDC voltage that powers the horizontaloutput stage. The SMPS also providesmost of the low voltages that wereproduced by the scan-derived supplies.The SMPS provides excellent B+regulation and removes that extra loadthat the scan-derived supplies placed onthe horizontal stages.

This Tech Tip explains how to determine ifthe startup or shutdown problem iscaused by the SMPS or by the horizontaloutput stage. Refer to Tech Tip #203 forinformation on how the SMPS used intelevision receivers work, and Tech Tip#205 fo r f u r t he r i n fo rma t i on ontroubleshooting television SMPS.

NOTE: The horizontal output stage is atype of switched mode power supply. Inthis Tech Tip, however, when we refer toSMPS we are referring to the supply thatprovides the DC to the horizontal outputtransistor (H. O. T.)

Troubleshooting Overview

The SMPS also provides a means ofsafety shutdown should the horizontaloutput stage produce excessive highvoltage or draw too much current.Protection circuits shut down the SMPSwhen either of these conditions occurs.This can cause a problem for a servicersince a startup or shutdown condition canbe caused by a problem in either thehorizontal output stages or in the SMPS

Troubleshooting the horizontal circuits ina chassis that uses a SMPS to supply Btto the horizontal output transistor involvestroubleshooting a power supply that ispart of another power supply. A dead setmight be caused by a bad horizontal stageor by a bad SMPS. The set might run for ashort time, and then shut down - this, too,could be caused by problems in either thehorizontal circuits or the SMPS.

The procedures in Tech Tip #131explained how to service the horizontalstartup and shutdown stages in olderconventional chassis that did not use aSMPS. Many of those procedures apply totoday’s chassis that use a SMPS to powerthe horizontal stages. There are two majordifferences, however. First, you can notlonger use your PR57 Variable AC PowerSupply to reduce the DC to the horizontaloutput stages. This is because the B+ isnow provided by the SMPS and a SMPSwill provide a constant output voltage asthe applied AC input voltage is changed.

Fig. 1: The regulated DC for the horizontal output stage is produced by a SMPS. The SMPS receives unregulated DC from the AC fine.

The second difference in troubleshootingstartup and shutdown conditions is aresult of how the protection circuitsproduce shutdown. Shutdown was usuallyachieved by removing the drive signal tothe horizontal output transistor, or byturning off the B+ regulator. In chassisthat use a SMPS however, shutdown isachieved by turning off the SMPS.

Most SMPS chassis have a secondary (or“standby”) SMPS which powers theremote control circuits. Since the mainpower supply won’t start if the remotecontrol is dead, check the secondarysupply and the remote control circuitsbefore testing the main supply and thehorizontal output stages. See Tech Tip#205 for more information.

The Key Test Points

Just as the horizontal output transistor(H.O.T.) provides the most important testpoint when testing the horizontal stages,the SMPS output transistor (S.O.T.)provides the best starting point whentroubleshooting a switch mode powersupply. Collect as many clues as possiblefrom both test points the first fewseconds after you apply power to separates t a r t u p p r o b l e m s f r o m s h u t d o w nproblems.

In the following steps you will monitor theoutput of the SMPS S.O.T. with Channel Aof your Waveform Analyzer and thehorizontal retrace pulse (collector of theH.O.T.) with Channel B. You will presetthe Waveform Analyzer controls so thatyou can see any signal, even if it is onlypresent. in the circuit for a moment.

W A R N I N G

This procedure ties the “hot” and“isolated” grounds together. It isabsolutely essential that the chassisbe connected to an isolationtransformer, such as the SencorePR57 AC POWERITE.

Observe safety precautions to avoidshock hazards. Some signals can beover 1200 volts. The input of yourmeasuring instruments must beprotected to a higher voltage thanthis. Using the Waveform Analyzerwith its supplied probes, providesprotection to 2500 volts.

Follow these steps:

1. Connect the Channel A probe to theterminal on the switching power transistorwhich connects to the primary of theswitching transformer (The collector if thetransistor is a bipolar type, or the drain ifa FET is used).

2. Connect the Channel A probe ground tothe ground of the switching supply’sprimary winding (normally the “hot”ground).

3. Connect the Channel B probe to thecollector of the H.O.T.

4. Connect the Channel B probe ground tothe ground of the horizontal output stage(normally isolated or “cold” ground).

NOTE: for correct measurements youmust connect BOTH grounds. This will notdamage the chassis, as long as it isplugged into an isolation transformer. Ifyou don’t make both ground connectionsthe switcher transformer wil l causewaveform distortion in the waveforms.

5. Preset the controls on your testinstruments as follows:

PR57:a. Set for a 117 volt output before

connecting the chassisB. Select the “1.5 Amp” range.c.Turn off the PR57 and connect the

chassis.

Waveform Analyzer:a. Set the Ch.A VOLTS/DIV to “100”.b. Set the Ch.B VOLTS/DIV to “200”.c. Set both INPUT COUPLING switches

to “AC”.d. Set the TIMEBASE-FREQ switch to

“10 microsecond”.e. Set the TRIGGERING controls:

SOURCE to “CH B”.MODE to “Auto”.POLARITY to "+".LEVEL to "0".

f. Press the “A&B” CRT SELECTORbutton.

g. Position the Ch. A trace on thesecond major CRT marking from thetop of the screen.

h. Position the Ch. B trace 1/2 of amajor CRT marking from the bottomof the screen.

i. Press the Channel A “DCV” DIGITALREADOUT button.

Now you are ready to observe the eventsthat take place as soon as you applypower to the chassis. The first thing youneed to know is whether a waveformappears in either of the test points.

Analyzing Startup Conditions

Watch for results on your WaveformAnalyzer during each step of applyingpower. Each step provides valuable cluesto help you get right to the problem. Thesesteps are outlined in the Trouble Treeshown in Figure 2, and are explained inmore detail in the following steps.

Unregulated DC To SMPSThere should be about 150-160 voltspresent at the drain (or collector) lead ofthe SMPS output device with the PR57turned on and the chassis turned off. Ifthis voltage is present, you know that theswitcher’s unregulated DC supply isworking. If the voltage is low or missingmove to the Trouble Tree in Figure 3which isolates problems related to theunregulated DC supply, an open SMPStransformer primary winding, or a shortedswitching output device.

Move your probe to the input side of theprimary winding (the end connected tounregulated DC) helps isolate the cause. Ifyou see full DC on the input terminal thetransformer primary or some other seriesconnection is open. If you see a lowerthan normal DC the switching transistor(or a parallel component) is likely shorted.

If your troubleshooting points to theunregulated DC supply don’t forget tocheck for a blown fuse. An open fuse canindicate that one of the power supplybridge diodes is shorted, or that there is ashort in the switching supply. Check thediodes before moving on. If the input fuseis blown and the unregulated powersupply diodes are good, suspect a shortedswitcher transistor.

Confirm Proper B+ Voltage To H.O.T.If the unregulated DC voltage is present atthe S.O.T., press the Channel B DCVbutton to measure the B+ voltage at thehorizontal output device when you turnthe chassis on. (This is the regulated DC

-WARNING _____NEVER defeat the safety shutdowncircuits when troubleshooting shutdownproblems as this may allow excessivehigh voltage to develop. Excessive highvoltage will cause a severe shock hazardand can damage circuits throughout thechassis, as well as the test equipmentconnected to the chassis.

defective shutdown circuit. Informationon how to troubleshoot shutdownproblems is covered in other Tech Tips.

Horizontal RunningIf you observe pulses at the collector ofthe horizontal output transistor and theyremain, the “dead set” symptom is notcaused by a horizontal problem. Look forproblems in the video amplifiers or one ofthe low voltage switcher power suppliesthat feeds other circuits.

Fig. 2: Follow these steps to determine if a startup or shutdown problem is caused bythe horizontal output stages or by the switch mode power supply.

output of the SMPS). Confirm that thisvoltage is correct. Normally it is about130 VDC, but check the schematic for theexact amount. If this voltage is notc o r r e c t i t w i l l c a u s e s h u t d o w n .Troubleshoot the SMPS if the regulatedBt voltage is incorrect.

Horizontal ShutdownIf the regulated DC output of the SMPS iscorrect carefully observe the CRT on yourWaveform Analyzer as you turn on thechassis power. Pay special attention tothe Channel B (H.O.T.) trace (the one nearthe bottom of the screen) while glancingat the Channel A (S.O.T.) trace soon after.

If horizontal pulses (and DC voltage)appear and then disappear, the chassis isstarting up and then going into shutdown.Most chassis that use a SMPS turn off theSMPS when a shutdown condition occursin the horizontal stages. Therefore, the DCvoltage output from the SMPS will goaway. A few chassis, however, mayachieve shutdown by removing thehorizontal drive. If so, the pulses at theH.O.T. will go away but the DC voltage willremain. In either case, the shutdownproblem is likely caused by a defect in the

horizontal stage. Possible problemsinclude a retrace time that is too short,(defective timing capacitor), excessivePPV (an open scan derived load if thechassis uses scan derived supplies), or a

Fig. 4 : Follow these steps to locatehorizontal startup problems.

Horizontal B+ OK, No H.O.T. PulsesIf there is proper B+ voltage at theco l l ec to r o f t he ho r i zon ta l ou tpu ttransistor but no pulses, the SMPS isworking. Follow the procedures in theTrouble Tree in Figure 4, which takes youthrough horizontal startup problems.These steps involve checking forhorizontal drive. If there is no drive, checkfor a startup voltage at the horizontaloscillator.

Separate The SMPS From TheHorizontal Output Stage

Fig. 3 Follow these steps to locateproblems in the unregulated DC supply.

If there is proper unregulated DC at thedrain (or collector) of the switching

device, but there are no pulses, or onlymomentary pulses, you need to determineif the SMPS is defective, or if it is beingloaded down or shut down by a defect inthe horizontal stage. To do this you needto substitute a known load in place of thehorizontal output, as shown in Figure 5. A60 watt light bulb provides a suitablesubstitute load.

To substitute for the H.O.T. load:

1. Turn off the PR57

2. Disconnect the collector of thehorizontal output transistor from theprimary of the flyback transformer.

3. Locate the point where the feedbackpath connects to the SMPS DC output.

4. Connect a one side of a 60 watt lightbulb to the SMPS DC voltage line afterthe feedback takeoff point.

Do not connect the tight bulb in serieswith the flyback primary. This winding isnot designed to handle a continuouscurrent and may be damaged.

5. Connect the other side of the light bulb

to the ground (emitter) of the horizontaloutput stage.

After you have connected the substituteload, turn on both your PR57 and thechassis. If the SMPS is operating the lightbulb will light. The fact that the SMPSoperates with a substitute load indicatesthat something in the horizontal sectionwas likely causing it to shut down.

Also check the opto-isolators that couplethe startup and shutdown signals back tothe switcher from the circuits on theisolated side of the switcher transformer.These isolating components often couplethe regulator control signal as well as theshutdown signal and a problem in theregulator loop can also cause shutdown.

If the SMPS still has no pulses at theS.O.T. (but the unregulated DC is correct)with the substitute load, you need totroubleshoot the SMPS as explained inTech Tip #205.

Testing the Components

Many problems will be related to badcapacitors. Use your LC102 Z Meter tomake the usual tests of value and leakage

at rated voltage to find most capacitorfailures. But don’t forget that equivalentseries resistance (ESR) in electrolyticcapacitors is much more critical in thehigh frequencies involved with a SMPSthan with other, low frequency powersupplies. Always test ESR when yoususpect a capacitor problem, even thoughthe capacitor has good value and leakage.

If you suspect either the horizontal flybackor the SMPS flyback transformer, use theZ Meter Ringer test to check for shorts,shorted turns, or open windings.

Sometimes a transistor tests good at thelow test voltages applied by a transistortester, but breaks down (emitter tocollector ) under load. If you suspect thata transistor is breaking down use your ZMeter leakage power supply to test foremitter-collector breakdown.

Similarly, if you suspect that a zener diodemay be defective, use your Z Meter powersupply to check for correct regulation.Details on using your Z Meter powersupply to test these components arefound in Tech Tip #112.

Fig. 5: Open the B+ to the H.O.T. and use a 60 watt fight bulb to substitute for thehorizontal output load on the

Form 5433Pr in ted In U.S.A.