TV PSU giving me nightmares

[Archimedes]

Please help

I have a Sansui SV2938T (actually an EC2938) TV that had an intermittent problem. It would be on for a while and then automatically turn off. I initally thought it was the switching IC STRS6309 that was causing the problem so I replaced it.

The problem is now the various voltages are almost a third of what they should be as soon as the PSU is reconnected to the main board. If however the 142V line is unloaded all the various voltages return to normal.

I tested the current being drawn at the 142V side when the PSU is connected to the main board and this amounts to a mere 146mA. The various voltages all seem to collapse as soon as the 142V side is connected to a load. If however any of them are left connected to the main board and only the 142V side taken off their respective voltages are correct.

As soon as the 142V side is loaded the voltages are as follows:
142V side = 56V
32V side = 14V
12V side = 4.5V

I have acquired a service repair manual over the Internet but this manual is not very helpful. It looks to be a problem with the regulation but just where to look is the issue.

I am attaching the schematic and if possible can someone please advise where the problem could be. I have already replaced all of the transistors as well as the opto. The

[Symon]

The output filter electrolitics are a favorite point of failure in switch mode supplies.

Radial capacitors freqently they show they are failing by slight swelling of the top of the can. It should be perfectly flat.

[Ghianni]

The output filter electrolitics are a favorite point of failure in switch mode supplies

Yes! Check all electrolitics and replace with low ESR ones.

[Archimedes]

I have already replaced the electrolytic on the 142V line but that did not help. I have carefully checked the other electrolytics and there vent caps are still as flat as when new. Could the transformer be a culprit?

[gfiandy]

Hi,

My guess would be that either the error amplifer IC802 or the feedback opto isolator has failed. So the voltage is correct off load but as soon as you draw a load there is no feedback signal to correct the output.

The other possibility is that the input bridge rectifier has lost one of its diodes. In this case it would half wave rectify which would not provide enough current on load. However I think the error amp is more likely.

You could take some measurements to see what is fed back from the error amp.

I am not a specialist with switch mode supplies so I may be wrong.

You would be better off taking this to the PSU section or trying another forum specialising in switch mode supplies where you would probably get more informative replies.

Regards,
Andy.

[Ghianni]

I have carefully checked the other electrolytics and there vent caps are still as flat as when new

You have to check the caps with an ESR meter to be sure of their condition. Most of the times they "look" OK but they don't !

[sharpi31]

My Loewe Xelos 32" CRT has a very similar problem. After it is first switched on, it will switch itself into standby after a couple of minutes. It repeates this two or three times, then works fine for as long as it is left switched on.

Not investigated it myself as I'd resigned myself to buying an HD flatscreen (as soon as led backlight LCDs come down in price).

I'll be interested if you find a fix though....

[infinia]

With your o-scope monitor the PWM with the +135 disconnected and gradually increase the loading on it with a couple of 10W test resistors. The PWM is controlled by 3 output levels and CPU shutdown sig.. Look at IC802 part#?

pay atten Q802 and zener a regulator to supply 8V holdup time for CPU. ( if normal 8V output fails then Q802 is hot all of the time and load may be too much).

[Ghianni]

A possible cure is to check the 814, 815 caps for capacitance and ESR change and then may be change both. Just a thought.

[Low Orbit]

Originally Posted by Archimedes

I have already replaced the electrolytic on the 142V line but that did not help. I have carefully checked the other electrolytics and there vent caps are still as flat as when new. Could the transformer be a culprit?

Sorry about the late reply. I had a telly that was doing the same thing.

Had it fixed by a knowledgeable bloke at work - turned out to be a faulty diode - very common fault it would seem.

I'll have a chat with him today and see which one(s).

Cheers.