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The second difference in troubleshootingtartup and shutdown conditions is aesult of how the protection circuitsroduce shutdown. Shutdown was usuallychieved by removing the drive signal tohe horizontal output transistor, or byurning off the B+ regulator. In chassishat use a SMPS however, shutdown ischieved by turning off the SMPS.

Most SMPS chassis have a secondary (or standby) SMPS which powers theemote control circuits. Since the mainower supply wont start if the remoteontrol is dead, check the secondaryupply and the remote control circuitsefore testing the main supply and theorizontal output stages. See Tech Tip205 for more information.

The Key Test Points

ust as the horizontal output transistor H.O.T.) provides the most important test

point when testing the horizontal stages,he SMPS output transistor (S.O.T.)provides the best starting point whenroubleshooting a switch mode power upply. Collect as many clues as possiblerom both test points the first feweconds after you apply power to separatet a r tu p p ro b lem s f ro m sh u td o w n

problems.

n 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 presethe Waveform Analyzer controls so that

you can see any signal, even if it is onlypresent. in the circuit for a moment.

WARNING

This procedure ties the hot andisolated grounds together. Itisabsolutely essential that the chassis

be 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 your measuring instruments must beprotected to a higher voltage thanthis. Using the Waveform Analyzer with its supplied probes, providesprotection to 2500 volts.

Follow these steps:

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

2. Connect the Channel A probe ground tothe ground of the switching supplysprimary winding (normally the hotground).

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 you

must connect BOTH grounds. This will not damage the chassis, as long as it is

plugged into an isolation transformer. If you dont make both ground connectionsthe switcher transformer will 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 trace1/2 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 ConditionsWatch for results on your Waveform Analyzer 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 of the SMPS output device with the PR57turned on and the chassis turned off. If this voltage is present, you know that theswitchers 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. If you see full DC on the input terminal thetransformer primary or some other seriesconnection is open. If you see a lower than normal DC the switching transistor (or a parallel component) is likely shorted.

If your troubleshooting points to theunregulated DC supply dont 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 power supply 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., pre ss the Channel B DCVbutton to measure the B+ voltage at thehorizontal output device when you turnthe chassis on. (This is the regulated DC

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- WARNING ____NEVER defeat the safety shutdcircuits when troubleshooting shutdproblems as this may allow exceshigh voltage to develop. Excessive voltage will cause a severe shock hazand can damage circuits throughout chassis, as well as the test equipmconnected to the chassis.

defective shutdown circuit. Inforon how to troubleshoot shutproblems is covered in other Tech Ti

Horizontal RunningIf you observe pulses at the collecthe horizontal output transistor andremain, the dead set symptom caused by a horizontal problem. Loproblems in the video amplifiers or the low voltage switcher power su

that feeds other circuits.

Fig. 2: Follow these steps to determine if a startup or shutdown problem is caused by the 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 notco r rec t i t w i l l cau se sh u td 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 your Waveform Analyzer as you turn on thechassis power. Pay special attention tothe Channel B (H.O.T.) trace (the one near the 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 locahorizontal startup problems.

Horizontal B+ OK, No H.O.T. PulsesIf there is proper B+ voltage aco l l ec to r o f t h e h o r i zo n ta l otransistor but no pulses, the SMPworking. Follow the procedures i

Trouble Tree in Figure 4, which takethrough horizontal startup probThese steps involve checkinghorizontal drive. If there is no drive,for a startup voltage at the horioscillator.

Separate The SMPS From THorizontal Output Stage

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

If there is proper unregulated DC drain (or collector) of the swi

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evice, but there are no pulses, or onlymomentary pulses, you need to determine

the SMPS is defective, or if it is beingaded down or shut down by a defect ine horizontal stage. To do this you needsubstitute a known load in place of the

orizontal output, as shown in Figure 5. A0 watt light bulb provides a suitablebstitute load.

o substitute for theH.O.T. load:

Turn off the PR57

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

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

Connect a one side of a 60 watt light

bulb to the SMPS DC voltage line after the feedback takeoff point.

o not connect the tight bulb in seriesith the flyback primary. This winding isot designed to handle a continuousurrent and may be damaged.

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 capacitor failures. But dont forget that equivalentseries resistance (ESR) in electrolyticcapacitors is much more critical in thehigh frequencies involved with a SMPSthan with other, low frequency power supplies. 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 transistor tester, but breaks down (emitter tocollector ) under load. If you suspect thata transistor is breaking down use your ZMeter leakage power supply to test for emitter-collector breakdown.

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

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

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