Samsung BN96-03775A Power Supply Problem

[craigc15]

Hi Folks,
I've read through all of the threads associated with this power supply and none of them address my problem. I need help!!!!

The TV sometimes works for hours, but other times it won't stay on at all.

The voltage on the main cap (CP812) is only at 200 volts when the TV won't stay on. It's up around 370 volts when it works.

I've changed just about everything, but it sounds more like a bad connection somewhere. I've scrutinized every inch of the board with a magnifying glass, but can't see any problems. I re-soldered most, if not all of the high-current connections, but no luck.

PLEASE HELP ME!! I need to get this TV fixed and back to the customer.

[tom66]

Re: Samsung BN96-03775A Power Supply Problem

Are you sure about 200V? Not 160V? (which would be a lot more likely.)

Before it fails, if possible, can you measure the voltage and see if it is dropping slowly, or does it drop immediately?

Try heating up things and seeing if you can make the problem occur quicker, this may narrow down the fault location.

[tom66]

Re: Samsung BN96-03775A Power Supply Problem

Schematic of power supply if it's useful:

[craigc15]

Re: Samsung BN96-03775A Power Supply Problem

I get 170 V in standby mode. I get it to stay on by reducing any output load, the easiest is to unplug the inverter plug, and that's when I get the 200 V. So maybe it's because it's operating at a lower output load.
The voltages across the main cap are also all over the place. It is only steady in standby mode.
I was watching the gate drive pin on the PFC MOSFET with my scope and the pulses were square when it was working, but more rounded when it wasn't.

I suspect the problem is in the PFC circuit, but I've checked everything and the mosfet is being triggered and pulsing. It just must be not enough.

I don't think it's heat-related because, as I mentioned, sometimes I will turn it on and it will work for hours just fine.

I have the schematic. Thanks!