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Ask Shahid Your Own Question
Shahid, Technician
Category: TV
Satisfied Customers: 9357
Experience:  I have been repairing electronic gear since 1993 as a qualified Sony tech
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L42S504---no backlight, sys con seems to least

Customer Question

L42S504---no backlight, sys con seems to least stby. off and on..
No 24 volt developed on power board ...5v OK
No obvious shorts or bulged caps..semiconductor junctions check good on random check..
Where is the 24 derived from? I have no schematic.. ***@******.***
Submitted: 2 months ago.
Category: TV
Expert:  Shahid replied 2 months ago.

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.

Expert:  Shahid replied 2 months ago.

- Do you get any change of volts on PS-ON pin of CN905 connector of power supply board in TV when press power button?

Customer: replied 2 months ago.
toggles from 0 to 3.3 and the led on front panel turns blue...sys con stby is working..
Expert:  Shahid replied 2 months ago.

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.

I hope these info will help. If you have any additional questions, feel free to ask. If you're satisfied, please rate me a 5-star at the top of your page.
Thank you.

Customer: replied 2 months ago.
No offense, but maybe I'm old school before technicians started swapping pc boards...What I need is component level repair guidance. Only the rectifiers for the twelve and the associated circuitry. I don't see how telling someone to swap a board justifies a 26 dollar fee..If you don't have a schematic you shouldn't offer to help..If you had a schematic that would indicate you are a qualified technician, but your answer kind of leads me to believe you are one of ythese new school pseudo techs who are mostly computer key smashers and consider swapping boards a skill...sorry charlie..
Expert:  Shahid replied 2 months ago.

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.