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Hello, I'm Shahid. Sorry to hear that you're having trouble with this but I'll do my best to help. Please reply back when you're ready to proceed.Thank you.
- Do you get any change of volts on PS-ON pin of CN905 connector of power supply board in TV when press power button?
Sorry for delayed reply, I was away from computer.
Since power supply board is getting Power On command from system control board yet no 24V and 12V available that means power supply board is defective. There are two sections of power supplies on that board, one calls standby supply that provides 5V DC output when power cord plugged in and other section calls main power supply that needs PS-ON command from main board to turn On and provide 12V and 24V output, this main section isn't working and probably regulator chips failure occurred and causing the problem. Replacement of power supply board will help to resolve the problem.
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I understand, swapping PC board is the way to repair the unit quickly and to ensure product quality. I do have schematics of this power supply board and can provide you copy if it helps.
Getting the required parts would be tough because now a days manufacturers do not provide spare parts to do component level repair like old days and recommend to replace the pc board.
Q902 & Q903 FETs are SMPS output transistors and IC904 is drive regulator chip that you would need to be replaced. Please reply back if you need copy of service manual of this TV that will help you to repair the power supply board on component level.