LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

[KennyC]

This weekend my LG 42" LCD TV decided to go blank as I was watching it. It then began constant rebooting, accompanied by a "pop" from the speakers (not an electronic/circuitry pop, thank goodness!), and that now repeats for as long as you want while it's plugged in. There is no display. The only response you get from the TV is the power indicator will pulse like a normal startup, then the "pop," and then the repeat. I couldn't find anything quite like that using search, so here I am.

I took a quick video of what it does:
http://www.youtube.com/watch?v=KUCq00gsD8Y
(again, the "pop" is from the speakers, NOT directly from the TV circuitry!)

I haven't opened to TV up to investigate yet - about to do that now. But here are the specs:

LG 42" LCD TV
42LG50-UA
42LG50-UA.BUSHLJM
Manufactured June 2008

I know it's not a whole lot to go on at the moment, but if anyone has any pointers or has seen something like this before, please let me know! Thanks!

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Didn't find anything obvious inside - no bulging caps, no fried components. Forgot to check for blown fuses (oops) but I will. Any hints on where they're all located on this thing?

The only thing I noticed was that there is a small green LED on the main board that does come on when the TV is plugged in. It flickers off and then back on as it reboots. I will say that it seems to reboot right when I believe the screen should be powering up.

Attached are some pics of the internals.

[tibimakai]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

I'm struggling with a 42LG70, mine it's shorted something in the power supply.
There are two fuses, one it's in the center of the board and the other at the red connector.

[t.j.]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

here link to service manuals! the might have something on it in the trouble shooting section~!

just enter lg 42lg50 an it will pop up list of manuals for you!

http://elektrotanya.com/?q=showresult

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

On a whim I decided to unhook the T-Con board from the main board. And...it powered up and stayed powered up! Not that I could see anything, but both the power indicator on the front and the LED on the main board indicate that it's booted "normally" (minus the screen, of course).

Pic of the T-Con board is attached.

So, probably a bad T-Con board? Anything I can do to confirm that, or do I just go ahead and replace it since it's a relatively cheap part?

[Mr Bill]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Check the fuse on the tcon is about all i can recommend. I'm going to go look in my box of goodies and see if i have a tcon. It looks very familiar.

[Mr Bill]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

The ones i have aren't the same. You'll need to match the numbers on the white sticker on the left side. 35-D023305

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Thanks for checking!

Fuse is good. For anyone else reading, I found this thread which has a great reference on this exact T-Con board and how to find and check the fuse.

One thing I did notice after I unhooked the T-Con board and started it up was that the backlight does blink once but doesn't stay on. Not sure if that's significant or not. I just hope it's that the TV is smart enough to realize that the panel isn't working and turns off the backlight and isn't an indication of a bigger problem!

Anyway, I think I'll go ahead and order another T-Con board. It seems like a pretty good gamble at this point.

[Mr Bill]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

I think the tcon has to tell the inverter to turn on via that small flat cable on the side of the tcon.

[budm]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Do you have the LCD panel model number? I need to see the spec and find out if this T-CON has the ERROR signal out put pin or not. Did you check any DC voltage on both ends of the fuse on the T-CON yet?

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Is the picture I posted above have the LCD panel number? If not, I can look again.



10V on both sides of the fuse. Of course the voltage drops to zero as it reboots/resets.

[budm]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

OK, it is Chi Mei model V420H1-L12 Revision C1. Let me see if I can find the spec sheet for it.
OK, I found the spec sheet, the T-CON board only receive signals from the main board, the T-CON does not any feedback pin that goes out to the main board to tell its condition if it is good or bad.
So the T-CON may have short circuits and load down the voltage on the main board, the main board may have current sensing circuits to see how much the T-CON board is pulling the current, if too much current being pulled, the main board tells the circuits to shutdown, that is my theory. You may have to download the TV service manual to verify.
See page 15 of the spec sheet for the pin out of the T-CON board.
On page 8, you have indicate that you are getting 10V at the T-CON fuse, the min VCC for T-CON is 10.8V, typical is 12V which I also expect, not 10V. That may be an indication of the problem, you need to leave the T-CON cable disconnect and check the VCC pin on the main board side to see if you still have 10V also, I expect to see 12V. pin 48~51 are the VCC +12V pins, just trace out those pins on the main board side connector.

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Thanks! I'm getting 12.38V on those 4 pins.

[budm]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

"I'm getting 12.38V on those 4 pins" with T-CON board disconnected?

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Yes, with T-Con disconnected.

[budm]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

So when the T-CON is connected, the voltage drops down to 10 V and also causes the voltage to swing up and down. It looks like you have bad T-CON board, you can try fixing it or replace it.

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

I have a replacement on the way; should be here Thursday or Friday. I'll be sure to report back once I get it in. Thanks again, you've been helpful as always!

[budm]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Keeping my fingers crossed.

[KennyC]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

It's alive!!!

New [to me] T-Con board installed and all is good! Super big thanks to everyone here. I wouldn't have even tried this if it wasn't for this forum.

[tom66]

Re: LG 42" LCD TV 42LG50 - Will not boot, continuous rebooting, no display

Nice to hear you fixed it.