LG 37LF65 backlight goes off

[Chron-O-John]

Hi,

I have a problem with my TV and I am hoping you can help me.
I've got it from someone else and don't how anything about the process of the problem (if it got worse over time or something else)

The backlight on my LG 37LF65 is not working. When i turn on the TV the backligt is lit for about half a second and then it goes dark. The panel itself is working. If the TV is cold (like outside cold ~ 15° Celsius / 60° Farenheit) it is working fine. I left it running for about 15 minutes.

If the TV is in a warm enviroment it is not working. This is happening when it's turned on:
http://youtu.be/fkQuIPwiJqs
(Don't mind the cat )
When the light is on I can hear a silent buzzing/humming sound.

I've checked all the Electrolytic Radial Caps on the Power board and on the Inverter boards using an ESR-Meter. All of them seem ok.
There's one 1uF cap on the powerboard which has ~5ohms ESR (I think it is C123) is that OK or too high?

All the voltage readings seem to be OK (at least with CCFLs off)
I measured the 24V going to the inverter boards (on the top) and the other connectors on the side.
The 19V outlet is only 18.5, is this enough?

Next thing I'll measure will be the MOSFETs (but I have to desolder them, right?)

Oh yea and i have the following Equipment:
Fluke 77II multimeter
Capacitor meter (masuring the capacity)
ESR meter
Soldering stuff

I think i could borrow an oscillator from my parents, if necessary, but I have no clue about how to operate it

[tom66]

Re: LG 37LF65 backlight goes off

18.5V is fine for 19V... something like 16 or 17V would be bad. Is 24V good even when set fails to work?

On this board (https://www.badcaps.net/forum/attach...8&d=1381253497), there is ON/OFF and DIM on connector. Check ON/OFF signal when TV works and when it fails.

Also, check ERROR OUT, again when failed and when working. If it changes state when the set fails, there is a problem.

[Chron-O-John]

Re: LG 37LF65 backlight goes off

Hmm... where is dimm?

Here are the readings when its not working:

ON/OFF: 4V
VBR: 3.3V
VIN: 24,5V

Error Out: 0,317V

I'll update this post as soon as it is cold enogh for the TV to work.

[tom66]

Re: LG 37LF65 backlight goes off

DIM = VBR on this panel. But from your measurements looks like bad inverter or tubes as everything else appears OK.

What you can try is getting a hairdryer (or hot air gun) and heating up one inverter or the other, and see if it fails when you heat one board or the other.

[Chron-O-John]

Re: LG 37LF65 backlight goes off

Hmm... I left the Tv in a room, window open. Didn't work. I guess it is too warm nowdays.
Any other advice for now?

[tom66]

Re: LG 37LF65 backlight goes off

Can you get hold of freeze spray, or canned air? Reversed canned air gives freeze spray.

[Chron-O-John]

Re: LG 37LF65 backlight goes off

Sorry for not answering for such a long time.
I haven't had much time last months.

So I've bought some freeze-Spray. Cooling down the Left Inverter board makes it run again.

What should i look for next?

Thanks any Merry Christmas!

[tom66]

Re: LG 37LF65 backlight goes off

Which is the left inverter?

[selldoor]

Re: LG 37LF65 backlight goes off

Which is the left inverter?

Fourth picture post 1


Chron-o John

Can you narrow down the area you are cooling.

Perhaps try reflowing the joints on the inverter transformer

[Chron-O-John]

Re: LG 37LF65 backlight goes off

After leaving the unit alone for some time, i tried to narrow down the area with cold spray.

It didnt work. Now cooling down the right board (see post one for pic) seems to do the trick.

I am pretty sure that i didn't cool down the right board during the first try. I'll investigate further and write as soon as i know more.

I'll also try reflowing as soon as i identified the faulty board.

UPDATE!
Ok, update guys. It doesn't seem to be any of the Inverter boards, I think I followed the wrong track all the time.
The reason it worked after cooling down one of the bords, is simply the fact that I turned if off and on again several times.

Plugging out the Power cable and leaving the TV alone for a while sort of "resets" it.
When I plug it in again I have to try about 30 times then is comes on.
If i continue turning it off and on i'll see the following:
https://www.youtube.com/watch?v=21gb...ature=youtu.be
(sorry for the bad quality next time I'll use my mobile phone which has a much better video-quality)
Hope you can help me!

UPDATE2:
Ok, now I am completely confused wether its the Inverter board or not.
I gave it another try, turned it off an on for about 30 times - didn't work.

Cooled the right board, worked after the 5th try or so. 3rd and 4th try the picture was visible for about 3 seconds. 5th try it stayed visible.

Thanks!