LG 37LF65-ZC No Backlight

[Daveatfernie]

Good Morning All.

After searching for my model number and finding a different inverter set from the one installed in my TV and with a different problem I thought I'd post for help in a nerw thread.

I have an LG 37LF65 which I bought on Friday, The seller said that the TV had sound but no video. It appears to have no damage to the Screen. When powered up I can't see anything with a torch although my light isnt the best and I don't know where the OSD is on these TVs.

I opened the rear and found that there were two leaking caps 680uF @35V on the inverter board and replaced them with good Low ESR caps.

I then powered up the TV and got nothing at all. Green light flashes about 5 times, then stays illuminated green. Nothing on the screen with torch. No flash from the backlight.

I checked all other caps in the TV (main PSU board mainly) and replaced any with an ESR above 0 on my Atlas ESR Meter with a new lower ESR version, Marking each one with a pen mark as I went.

Powered up again and still found nothing.

All voltages coming from the Main PSU board are present and correct to labels, I'm seeing a VSync square wave signal coming from the TCon board@ about 100Hz, 24V is present to the inverter board.

Other voltages are as below:
Option pin from TCON: Seems to be sending Data to the Config chip during startup and shutdown

ADIM
ON/OFF
BRT
Status

All sit low when powered off (and discharge from on fairly slowly)
When powered all lines are pulled high to 24V.

I've checked all 4 of the the FETs and none show short.
The input winding to the transformer is at about 0.2 ohms and the secondary is about 150 Ohms.

I have a 16W 16CFLD4 but i'm sure its just a standard fluorescent tube rather than a CCFL. Will I be able to test my inverter with this?

This all appears (Granted to someone new to this) as looking ok?
Am I looking at a CCFL failure?

[tom66]

Re: LG 37LF65-ZC No Backlight

Place the fluorescent tube near the inverter board outputs. Unplug the outputs. The tube should glow slightly for a few seconds if high voltage is present. You might need to use a neon-bulb (small type) in case a fluorescent is too big. The inverter shuts off after a few seconds if no bulbs are connected, so you will have to repeat the test a few times.

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

Thanks, I'll have a try in the morning (tv is at work). If the lamp lights I assume it's the CCFL?
I did measure a pulse to the primary of the transformer which is momentary (less than a quarter second).

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

Tried the tube next to the transformers, not a peep. I can hear a faint detuned channel noise from the speakers.
Is it worthwhile swapping out the os9966 ic on the inverter board? Do these often go?

[Repairtechlcd]

Re: LG 37LF65-ZC No Backlight

Check for s/c on the os9966 and give it a close inspection for possible burn marks.I wouldn;t be surprised that it failed due to the ripple caused by those two bad caps you had on the invertor boards.

[mmartell]

Re: LG 37LF65-ZC No Backlight

Longshot for you but I've seen it first hand...

See that ribbon cable going into the left side of the t-con ? Unplug it and fire the set back up.

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

The one that has the VSync and option cables? I'll have a go and see what happens....

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

Unfortunately nothing happenned. I assume that the problem is with either the inverter or the tubes themselves. I'm going to order the Inverter controller IC as it's only about £4 from ebay. Failing that I can get a new set of 37" CCFL's for about £20 delivered. Still makes for a cheap TV. (Reconditioned TV's are about £170 on this model).

[mmartell]

Re: LG 37LF65-ZC No Backlight

I should have asked for a closeup of that part of the board. Does that ribbon run from the tcon back to the inverter ?

On mine it controlled a dimming circuit that some LG sets have apparently to cheat on their overall contrast measurements.

[mmartell]

Re: LG 37LF65-ZC No Backlight

Disregard my last post. I see you posted before me.

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

Thanks for the reply but looking at the fact that I don't even get a flash I'm leaning towards the Tubes, I do see a quick peak on the primary side of the transformers but no flash and it seems by all accounts that I shoudl at least see that. I'll have the screen apart and hunt for a cracked tube.

[tom66]

Re: LG 37LF65-ZC No Backlight

There are many tubes in parallel, tubes would be unlikely to cause this. It's possible though that there's a short inside the panel causing this issue.

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

I assumed they'd be in series as there are only two leads from the inverter board rather than a bank of cables as I've seen in other TVs?

[tom66]

Re: LG 37LF65-ZC No Backlight

If it's an LG panel they are all in parallel. One wire goes to one end, the other goes to the other end. EEFL tubes, high reliability.

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

Ah right, How about the driver IC, Are these known to be reliable?

[tom66]

Re: LG 37LF65-ZC No Backlight

They frequently fail.

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

I also assume that I can't swapp EEFLs with CCFLs?

[tom66]

Re: LG 37LF65-ZC No Backlight

Not unless you swapped all of them at once, and were able to replicate the external electrode (versus cold cathode type.)

EEFL vs CCFL:
http://upload.wikimedia.org/wikipedi...L-EEFL.svg.png

Because the electrode to the bulb is external the typical failure mode is mercury sputtering around 150,000-200,000 hrs. A CCFL can last 40,000 hrs to 60,000 hrs (sometimes less, I've had them fail at 15,000 hrs), limited by the internal electrode degrading from the ion depletion and very high temperatures. (Cold cathode just means they aren't deliberately heated, it doesn't mean that they don't get hot!)

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

I'll swap the IC and hope for the best then...

[Daveatfernie]

Re: LG 37LF65-ZC No Backlight

I see on ebay there are a number of inverter boards for the same price as the ic.
Does anyone know if inverters are cross compatible?
My inverter is a single board lc370wu3