LG 37LC2R, backlight shuts down after 4 seconds

[amach]

Hi everyone,
I have searched the forum but couldn't find a thread with my exact model and problem. Please forgive my language errors.

Without any other worrying sign, after 5 years of impeccable service, my TV stopped showing picture.
The pattern is this: TV being in standby (red light), after turning it on by the remote or the panel buttons, the light flashes 3 times, and turns white-green (as normal). The panel shows fine for 3 to 4 seconds and then blacks-out. Nevertheless, sound goes on. If I put it in standby, I can replay the previous pattern as many times.

At the LG service center they asked for 180 euros to change every board in the set. As this is too much for me right now, I thought I should try to fix it by myself.

After some research before posting, I found the service manual I attach and two posts of this model serie in a french and a german forum. Through google translate, I suppose they cope with faults irrelevant to mine.
I have read this thread, did the hairdryer trick to no avail.
This thread model seem to use the same PSU but differs on main board and kind of problem.
So I read "A guide on how to troubleshoot 2 seconds to black", remarkable work, but I could make use of a "road map" to checking all these parts referred.
I started with capacitors.
Opening up my TV set, in the quest for bulged caps, I didn't find any. As seen on the first photo, here are 3 boards: the main logic, the power supply and the inverter. The PSU board (photos 2-7) uses SAMXON caps which, as I understand from this forum, are of poor quality, but I miss an ESR meter to test them.

With my DMM, I checked the voltages on the both connectors to the inverter.
(Tv set is on, connectors unplugged, black probe on a Gnd pin)
connector socket P205
24V -> 24,93
I-C -> 4,94
BRI -> 3,31

connector socket P204
ERR -> 0
DIM -> 0
I-C -> 4,94
BRI -> 3,31

Does this prove PSU board to be OK, so that I can move on to the inverter?
Or should I recheck with connectors plugged-in?
I'd appreciate your help.

[retiredcaps]

Re: LG 37LC2R, backlight shuts down after 4 seconds

1) 2 seconds to black can be

a) bad caps
b) bad ccfls
c) bad ccfl wiring (especially the return wiring)
d) bad secondary windings on the inverter transformer

2) More details starting with post #19 at

https://www.badcaps.net/forum/showthread.php?t=10419

3) I suggest you measure the resistance of secondary windings.

4) Clear focused pictures of your boards following the instructions below help us help you.

[retiredcaps]

Re: LG 37LC2R, backlight shuts down after 4 seconds

Please forgive my language errors.

Your english is excellent. A lot better than a lot of crap I have been seeing and skipping over (i.e. I don't bother to read or respond) lately from North American based posters.

[budm]

Re: LG 37LC2R, backlight shuts down after 4 seconds

"24V -> 24,93
I-C -> 4,94
BRI -> 3,31

connector socket P204
ERR -> 0
DIM -> 0
I-C -> 4,94
BRI -> 3,31"

Can I see the pictures of theses two connectors and the whole Inverter board? So far it looks like you are getting all the correct voltages and signals to the inverter board. Make sure to use Managed Attachment to up load the pictures.

[tom66]

Re: LG 37LC2R, backlight shuts down after 4 seconds

"The panel shows fine for 3 to 4 seconds and then blacks-out"

This means picture is also good, i.e. if you have a menu up in this time, or channel indicators, volume control etc., it shows?

[amach]

Re: LG 37LC2R, backlight shuts down after 4 seconds

Thanks to all replying.

"The panel shows fine for 3 to 4 seconds and then blacks-out"
This means picture is also good, i.e. if you have a menu up in this time, or channel indicators, volume control etc., it shows?

Everything works as normal besides back-lights. Look at first picture.

"24V -> 24,93
I-C -> 4,94
BRI -> 3,31
connector socket P204
ERR -> 0
DIM -> 0
I-C -> 4,94
BRI -> 3,31"

Can I see the pictures of theses two connectors and the whole Inverter board? So far it looks like you are getting all the correct voltages and signals to the inverter board. Make sure to use Managed Attachment to up load the pictures.

I had attached a few pictures on my original post via "manage attachments" which originally showed up fine, but I can't see them anymore. Perhaps web server rejected them, later, on a second thought, as bigger than expected (2592x1944).
New ones reattached (pics 2 to 5).
I rechecked the 24V line to inverter, on the PSU socket and got 24.8V with lamps off and 24.2V lamps on. The I-C line (sense back to SMPS controller?) stays on 3.575V regardless of lamps.

2) More details starting with post #19 at

https://www.badcaps.net/forum/showthread.php?t=10419

3) I suggest you measure the resistance of secondary windings.

4) Clear focused pictures of your boards following the instructions below help us help you.

I have already mention reading this thread. Sorry about the quality of some of the attached photos, this is the best I can do with my compact camera.

I checked resistance on one of the 20 HV transformers. Pin numbered as on pictures 10 to 12.
(pin,pin)->reading
(1,2)->0L
(1,3)->0.2-0.3 Ohm
(1,4)->3.25 MOhm
(1,5)->3.25 MOhm
(1,6)->0L
(2,x)->0L
(3,4)->3.2 MOhm
(3,5)->3.2 MOhm
(3,6)->0L
(4,5)->0L
(4,6)->0L
(5,6)->0L
Which pins correspond to the secondary?
I have also checked the 5 white SMD fuses on inverter board and they pass for continuity.

Your english is excellent. A lot better than a lot of crap I have been seeing and skipping over (i.e. I don't bother to read or respond) lately from North American based posters.

Thanks

[tom66]

Re: LG 37LC2R, backlight shuts down after 4 seconds

OK, so we have 3~4 seconds to black (called "2 seconds to black")

This is the secondary, I expect.

[amach]

Re: LG 37LC2R, backlight shuts down after 4 seconds

OK, so we have 3~4 seconds to black (called "2 seconds to black")

This is the secondary, I expect.

What baffles me is that I take 0L (auto-scale or manual Ohm) on every single of the 20 transformers secondaries, that is pins (5,6) on my prev post.

[tom66]

Re: LG 37LC2R, backlight shuts down after 4 seconds

Hmm. Can you try removing one of them so we can see how the transformer is wired up.
Also, what range on your meter did you use (or is it autoranging?)

[retiredcaps]

Re: LG 37LC2R, backlight shuts down after 4 seconds

(1,4)->3.25 MOhm
(1,5)->3.25 MOhm
(3,4)->3.2 MOhm
(3,5)->3.2 MOhm

Are you sure that is not 3.25k (kilo) ohm?

A picture of your multimeter might help as well.

[amach]

Re: LG 37LC2R, backlight shuts down after 4 seconds

Hmm. Can you try removing one of them so we can see how the transformer is wired up.
Also, what range on your meter did you use (or is it autoranging?)

Auto-ranging, even tried manual.
I'll try remove one of them, if there is no alternative.

Are you sure that is not 3.25k (kilo) ohm?
A picture of your multimeter might help as well.

First picture shows transformer's primary(?), with my indications of the board pin numbers. Second pic the secondary(?) pins.
Next 3 pics the results at pins (5,6), (1,3) & (1,4).

[budm]

Re: LG 37LC2R, backlight shuts down after 4 seconds

If I remember right, this transformer high voltage winding is on pin 4 and 5, you may want to remove the transfromer so you can see the bottom side. 1 and 3 are for sure is the primary as you can see that they are connected to large pads and to the MOSFETS. You can also see pin 4 is connected to the detection network for lamp open/short detector circuits. You may also want to clean off the flux on the pins that may prevent you from making contact.

[amach]

Re: LG 37LC2R, backlight shuts down after 4 seconds

If I remember right, this transformer high voltage winding is on pin 4 and 5, you may want to remove the transfromer so you can see the bottom side. 1 and 3 are for sure is the primary as you can see that they are connected to large pads and to the MOSFETS. You can also see pin 4 is connected to the detection network for lamp open/short detector circuits. You may also want to clean off the flux on the pins that may prevent you from making contact.

My god, you are absolutely correct! Thank you very much. Pair (1,3) is the primary and (4,5) the secondary. Pin 4 seem to connect also to sense circuit. Pins 2 & 6 seem to stand unconnected.
Sorry, I had been careless on my previous measuring between (4,5) due to surface flux. Now ratings are far from 0L. Here they are for each quad of transformers:

1.737, 1.742, 1.743, 1.728 kΩ

1.742, 1.760, 1.764, 1.757 kΩ

1.745, 1.760, 1.736, 1.753 kΩ

1.750, 1.763, 559, 1.751 kΩ

1.756, 1.756, 1.754, 1.734 kΩ

We are making progress. I'll desolder the faulty part.
Then where can I find a substitute?
Can I temporally disable this transformer-lamp branch?

[retiredcaps]

Re: LG 37LC2R, backlight shuts down after 4 seconds

You may also want to clean off the flux on the pins that may prevent you from making contact.

Good call. I do remember, now, there was a similar case in the past.

[retiredcaps]

Re: LG 37LC2R, backlight shuts down after 4 seconds

1.750, 1.763, 559, 1.751 kΩ

The above is bad.

Is there a part number one the transformer?

You might find it in the service manual?



PS. One retiredcaps' bonus point for a picture of your Fluke 115.

[budm]

Re: LG 37LC2R, backlight shuts down after 4 seconds

@amach, Do you mind if I post the pictures of your board in my album at Photobucket?

[amach]

Re: LG 37LC2R, backlight shuts down after 4 seconds

The above is bad.
Is there a part number one the transformer?
You might find it in the service manual?

PS. One retiredcaps' bonus point for a picture of your Fluke 115.

I already have a service manual (which I attach), but I cannot find the HV transformers. Part number on board is T403.
The prints on it is:
"T51.0059.2108 G1"
"0639 01 8 J"
The inverter board model is vit71010.53 .52 .51

@amach, Do you mind if I post the pictures of your board in my album at Photobucket?

Not at all, please provide me with a link.
Any source for this transformer?

[retiredcaps]

Re: LG 37LC2R, backlight shuts down after 4 seconds

Any source for this transformer?

1) Beg, plead this seller

http://www.ebay.co.uk/itm/261136146214

to measure the secondary winding resistance with a multimeter to see if it is in the 1.75k ohm range.

While the numbers don't match exactly, the first T51.0059 part might be "close enough". That is, the 212 versus 2108 may not be significant?

If it is a good match, I suggest you get 2 because you may be repairing it again in the future.

2) Contact lcdparts.net

http://www.lcdparts.net/t.aspx

and see if they have the exact transformer. They carry a few T51.xxx, but none match your part number. They may have it, but maybe not listed on the website?

3) Sometimes it is cheaper to find/buy the entire board rather than replace one transformer depending on where you live, shipping costs, customs, taxes, etc.

[amach]

Re: LG 37LC2R, backlight shuts down after 4 seconds

The actual part is T51.0059.210B and not .2108. The print is vage but google search had the final word.
I have found it on eBay, 16€ incl. shipping, pricy considering the whole inverter lists for 18€ plus shipping. I am searching and considering my options.
Any way to bypass this transformer-lamp sensing?

[amach]

Re: LG 37LC2R, backlight shuts down after 4 seconds

Just to inform the fellow-forumers that I couldn't find the transformer, anywhere on the internet.
Instead, I purchased form ebay (32 euros incl. ship.) the whole inverter board extracted from a SAMSUNG LE37R74BD or 37WLT66.
My tv now works flawlessly.
Thanks for your advice.