LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

[diif]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Here are some detailed close-ups in original non resized photos:
https://photos.app.goo.gl/v8vkNmFQmjCD4JY8A
I'm going to try the test.

Zero voltage on the contacts marked in red when the main board is powered on: https://photos.app.goo.gl/8A85oQty4MAHEJQr5
Checked all 12V pins still have correct voltage on the 24 pin connector on the mainboard (from PSU).

There won't be any voltage on those points marked in red as they are all ground.

[budm]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

I am looking for the fuse that feed the integrated T-CON circuit on that main board but I could not find it.
At this point it may be better to get the backlights to come on first, if we have backlights but no image on the screen then we can deal with the T-CON section later on.

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Backlight is fully operational when forcing BL-ON pin on PSU using the method budm suggested @post #18.

[budm]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Backlight is fully operational when forcing BL-ON pin on PSU using the method budm suggested @post #18.

Good.
So do you see image on the screen?

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Completely no image, only backlight is on.

[budm]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Completely no image, only backlight is on.

OK.
Do you know if you get sound?
It sound like the main board may have more then one problem, not sure if it is BGA related issue or not.

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

I don't get any sound. Connected antenna cable, tried to put volume up but no reaction. I was able to turn the forced backlight on and off by the remote control but I doubt it will help us a bit. But I haven't tried to force audio high - A-dim is low (0.34V). Should I give it a try?

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

The input may not be set to cable input in order to get sound to begin with...just a thought. No picture on screen is a bad sign and that's usually when I throw in the towel and suggest a MB swap if I can find one and is affordable, unless there's something really obvious like a missing power rail caused by shorted cap, though that never actually happened to me - the main chip usually goes out in some way.

You could try blasting the main chip with some hot air to see if it changes anything, but this never worked for me on TV boards....if anything it made it worse

Side note: some dumb f*** in the joint I work for went as far as to reball the chip on a Samsung board, thus killing it entirely It was most satisfying to watch all that work go to waste, since I knew it from the start

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

I forgot to mention one thing (I just remembered words of my mother-in-law ;-) (not my TV) ). She mentioned that last time when the TV still worked it was difficult to turn it on, tried to power it on several times until in finally worked. Maybe this situation occurred a few times before it stopped working completely. I suspected it might be a bad capacitor but couldn't find any. I should probably measure it but it's not that easy task to check SMD caps, especially when there are sa many caps.
Caps on the PSU seem to be fine.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Measure some of the larger caps around the main IC, see what you get. I expect 3.3v or less, like 1v...out of curiosity

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Danyx: yeah, the last chance. There is not much to lose. It's quite old TV so I don't want to spend money for a new mainboard but I still don't want to give up . At least this is some good experience.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Maybe search for a similar one with a busted screen, sold as non-working and just rip the board from that....I used to do this for a living: buy broken TVs, fix them and sell them. If they were not fixable, I'd part them out and either keep or sell individual components It was going quite well at some point, but alas: the more technology progresses, the more difficult it is to do this...

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

I Found some switch on the mainboard . When pressed it shortly, blue LED slightly flashed and then the TV went to standby mode.
I really have no clue here. Lack of experience so I'll probably have to give up at this point.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Not knowing myself what that switch does, I can only speculate it's a reset switch of sorts.....maybe shorting the SDA-SCL lines of the SPI chip for you. I used to do this with tweezers on TVs that boot-looped or showed signs of firmware corruption - sometimes it got them going....most of the time not...

[dick_barton]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Connect a DVD player or something similar to the SCART input. Switching the DVD player On/ OFF normally switches to the SCART input for the set so you can then check for audio and with a flashlight see if anything is on the screen.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

...check for audio and with a flashlight see if anything is on the screen.

I believe he gets backlight but no picture.

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

I have no backlight and no picture. But with forced backlight on I still see no picture. No audio could be a secondary problem. Probably the whole circuit is prevented to be on. I have 3.3V on error pin so I guess some part of the circuit is purposely disabled by protection circuit or something.
Maybe I should also do a picture check with not forced backlight? (Using a flashlight or some source or light).
Do you guys have any other idea what part of the circuit I should check? Voltage on majority of caps is really quite low, below 3.3V as Dannyx estimated. But I don't know if it means something. This can be normal situation. I measured 12V voltage drop and even 20.5V on some higher voltages caps but again, this probably doesn't help to estimate anything.
I could try to do the foolish try and put the mainboard to the owen but don't think this is wise. At worst I would heat the main chip and / or probably the other chips on 275 degrees Celsius with desoldering heat gun.

Edit: heat sinks on the main chip is tightly in place and a heatsink on the other chip looks good as well.
Anyway I'd need to remove the heat sinks before resoldering the 2 chips but it shouldn't be a problem I guess since I can use CPU heat sink paste for the TV later.

[Dannyx]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

Maybe I should also do a picture check with not forced backlight? (Using a flashlight or some source or light).

Does the power LED make it look like the TV is operating normally, just with no backlight ? If the LED comes on and goes off like I described, it's highly unlikely you get picture.
If you have a hot air gun, I'd say just go for it and hit the main chip - at this point, it's unlikely you can make it worse To maximize you chances of actually changing anything, start by pre-heating the area around the chip for a while and THEN go over the chip itself. This reduces the thermal shock the board receives which can cause it to warp, making the problem worse. I go for around 5 minutes pre-heat, 5 minutes up close, another 5 minutes "outro" where I bring the heat away from the chip slowly so it doesn't cool down too quickly which can again cause issues.....this is on laptop boards at least. Never had success on TV boards, or maybe it's just my technique that needs improving

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

The TV main red led is flashing so backlight doesn't help to anything. I think the normal operation would be indicated by a blue light.
Regarding the chip preheating, It will be challenging to estimate how much to preheat it. But I'll try to do it incrementally, in the way you suggest, then check whether it helped and if not, I'll desolder the chip completely and then position it back on the board and resolder by the heat gun. Not sure if this will be possible since the pads are directly below the chip (I don't see any) and the chip is normally settled down and soldered by machine at exact position. Or maybe the chip is in a socket, need to check this first but I doubt this (don't have the TV opened now and I'm lazy to re-check).

[kubajs]

Re: LG 42ld750 TV not turning on (3.4v on ERROR pin of power supply)

I'll probably desolder the bigger caps first and measure them by ESR meter.