Very strange fault of Philips TV SMPS - 715G3812-P02-H20-003U PSU

[Sercan]

Hi Folk,

I have a faulty Philips TV SMPS and this PSU is very difficult for repairing (for me ofcouse  ). PSU is not work for every time but some times is working correctly or just working. After all testing I am not sure working condition. But, when TV opened had been working normally.

First of all, electronic is hobby for me. I can handle easy problems but this situation is hard for me. I am just learning. I have isolation transformer, Rigol DL3021 DC load, Siglent SDS1104X oscilloscope, multimeter and termal camera for testing.

PSU output is S/B, 5V, 12V, 24V. 12V and 24V are not active in standby state. For full working conditions, I did connect 1K resistor S/B to 5V and load 5V with rheostat. At first PSU is working correctly with all outputs.

But after 25 seconds, power LED on PSU is blinking then after this, around 10 seconds its completely off. I tested it many times, it takes 25 seconds each time...

First I set DC load 0.5A between 2.5A and connect it to 12V. It was still working normally even after the LED light was off.

After 12V test, I set 0.3A for 24V. Then after LED light was off I saw that the voltage collapsed to around 11.5V. In this process, the IC901's feedback signal shifts from 40 Hz to 20 Hz.

I am not sure is normal but IC905 is 65-70 degrees celsius on working and I guess is normal. Except this there is no visual problem or thermal issue.

Also I tested all filter capacitors with cheap ESR meter. All in good condition.

My guess is that works normally until some capacitor charges, then it becomes unstable. But I don't know.

I hope, I did tell my problem correctly. What is the problem and how can I solve this problem?