RCA RLED4778A No Backlight no Pic No Response

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Should there be about 65ohms between any rails ? I'm getting that between the 1.8v supply and ground.

[tom66]

Re: RCA RLED4778A No Backlight no Pic No Response

that would generally indicate a shorted CPU to me.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

I don't see a separate micom to control standby functions. Do cheaper sets like this use the main processor for that ?

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

I ask because standby seems to work. When power button is pushed light turns blue and set is unresponsive until power plug is removed and reinserted.

Would be a strange fault for the processor to work and then freeze up, no ?

[tom66]

Re: RCA RLED4778A No Backlight no Pic No Response

Yes, this uses one processor to do both, and it is odd. It is also plausible the regulator is bad.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

I tore the heatsink off it. It is a Renesas RA... uses 1.2, 1.8 and 3.3v supplies. The question is are the lower voltages switched on at power up ? Will check that when I can.

I remember the voltages at the regulators testing good.

[tom66]

Re: RCA RLED4778A No Backlight no Pic No Response

1.2V is the core, 3.3V is IO.
1.8V might be for analog interface. It's odd for it to use both 1.2V & 1.8V.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

That info is according to their website. Can't remember the full part number atm.

Now that I think about it I believe the 1.8v and the 1.2v lines share the same low resistance to ground. Is there anything in common between the two other than the processor ?

[tom66]

Re: RCA RLED4778A No Backlight no Pic No Response

Capacitors, and sometimes 1.8V will be used by memory or other ICs on the board.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Crap. A shorted mlcc is nearly impossible to find isn't it ?

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Ordered a main board. Hoping for compatibility as it's not an exact match as per sticker data.

Will post back...

[martino]

Re: RCA RLED4778A No Backlight no Pic No Response

I'm sure that will fix it,as it sounds very strongly like a bad voltage regulator to me on the main board...
Vizi......oh....oh..... are notorious for the exact same problem.....0.12 volt out of spec on one regulator and nothing else then powering on,non responsive after start up....

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

This board was suffering a short, likely in the main processor.

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Replacement main board fixed the tv. I measured the five regs on the good board vs the bad board and they are identical. Well, one reg was 0.02v lower can't see that causing a problem...

This working board also showed the same low(ish) resistance to ground from the reg outputs so now I don't really know what to think as to what the actual failure was.

[KevinK]

Re: RCA RLED4778A No Backlight no Pic No Response

I have the same TV on my bench with the same problem - it initializes occasionally when power is applied, will turn "on" according to the power LED, but won't turn off.

I'm new to flat screen TV repair but not to electronics. The supply voltages are present and within spec when the machine is turned on but there's no other sign of activity.

mmartell, can you tell me where you found your board, and if it's an RCA board? If it's a Sceptre or some other board, does that make any difference beyond the startup screen?

I did see the other thread about the Sceptre TV.

Thanks,

Kevin

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

I found the board on ebay and it was an RCA though I was prepared to use another brand hoping that the only difference would be the splash screen.

These are rare boards. Search for the "12035..." number (going from memory). Then match the sticker on the back as best you can.

[KevinK]

Re: RCA RLED4778A No Backlight no Pic No Response

Update:

I froze the board for a while (in the freezer) which didn't help so I baked it at 300ºF for 10 minutes. That was 2 days ago and it's worked fine ever since!

Kevin

[ferous]

Re: RCA RLED4778A No Backlight no Pic No Response

Standby light lit and set responds to power up by changing red standby light to blue. Other than that no indication of any life. Will not go back into standby and power must be cycled at the mains to go back into standby.

All voltages present at PS after power up.

I'm thinking main/driver board/tcon in that order.

This is an LG panel and probably other internals. One and a half years old.

Need advice on how to narrow this down as I'm not familiar with failure modes on this set and there are no schematics/manuals that I can find.

Pics are of PS, main, driver board. Tcon buried atm.

Hello I'm Dominic
Looks like we all have the same problem, searched with all my contacts and still no luck, any luck on your end?

[mmartell]

Re: RCA RLED4778A No Backlight no Pic No Response

Read the previous 3 or 4 posts to find out two different resolutions.

[Jeep16]

Re: RCA RLED4778A No Backlight no Pic No Response

Update:

I froze the board for a while (in the freezer) which didn't help so I baked it at 300ºF for 10 minutes. That was 2 days ago and it's worked fine ever since!

Kevin

I found that freezing the processor caused the display to come on. I tried Kevin's suggestion of baking the board at 300F, but two caps just popped and now I am looking for a new board (I might try replacing the caps if the board is not available.) Maybe should have tried 250F?