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

[Dannyx]

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

The chip is most definitely BGA (ball grid array), not socketed, so do not attempt to remove it off the board entirely ! You will not be able to put it back in place on the balls ! Heat it enough so the solder balls melt and that's it - that's the most you can do without a BGA machine. You have to hit it pretty hard so the balls start melting. You want the air to be hot enough to make the balls liquid, but not hot enough so they combine and form a big blob underneath (game over in that case). The chip itself is probably failed in some way, so I doubt even a proper reball would get it going....unless someone else confirms otherwise...

[kubajs]

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

Alright. Anyway the chances I fix this are already pretty small but I'll try my best . I'll wait a few days in case somebody comes with any good idea including the idea to throw the TV from the window .

[kubajs]

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

Hi guys, the happy end is here. Today I took a heat gun and heated the main chip and other smaller chips at 275° C (or 527° F). Not for very long and with heat sink on the main chip so there is a high probability the main chip wasn't the root of the issue as it highly probably hasn't reached the tin melting point (one side completely covered by the big heatsink so I doubt the tin below the chip could melt).
Therefore I believe there was some cold connection on one of the other smaller chips (or close resistors or caps) - the smaller chips were definitely heated sufficiently. The TV was turned on about 2 hours after the heating so it looked like the temp was already in normal. It runs flawlessly, picture on, audio on. If the issue happens again, i'll give it some more re-heating.

Thanks everybody for your great advices guys, that helped me a lot! And I learned something new.

[Dannyx]

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

Oh yeah, that's another thing to do when "reflowing": remove any heatsinks from the chip you're attacking...
It could still be a BGA issue with the main chip, despite it not reaching the point where the balls would start to melt - the heat may have caused the board to warp just enough that it got it working but may fail again in the near future...no losses I guess - it still counts as a bad motherboard, so at least you know for sure you can replace it without worries. I would only charge diagnostics for such a "repair" or perhaps nothing at all if the customer happens to be a girl I'm interested in

SMD caps are unlikely to cause issues and so are other passives like resistors or SOT23 transistors (the small 3 pin devices). They do not run hot, so they're unlikely to become loose, plus they do not use BGA soldering which is most prone to failure....

My money is on these RAM modules (in red) or on the main chip...

[kubajs]

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

I know I should remove the heat sink but was just lazy, would do so in case the TV didn't turn on. We'll see, the TV works now and I haven't "fixed" it for a typical customer (it's a family customer and the loss was only on my side - the time I spent with that TV + one 680uF cap I replaced on the PSU before) so no worries. If it stops working again, I'll give it more appropriate treatment.
The ram modules were among the components I heated the most so you could be pretty close with the estimation.
Good point with the hot running components, I'll focus on it more the next time.

The heat sink is probably attached by screws from the bottom of the motherboard, right?
Do you know what is located under the smaller heat sink above the RAM modules? Any secondary chip? BGA as well?

[Dannyx]

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

Neither heatsink appears to be fastened to the board mechanically - they're glued to the top of the chips with that white stuff and yes, both chips are very likely BGA. I haven't seen a QFP (quad flat pack - visible leads on all 4 sides) main IC on a TV board in ages now, except on maybe some Vestel chassis. Removing a glued-on heatsink is quite difficult - takes a lot of force and can't be put back on afterwards without thermal glue...I only remove them if I have no other option and don't care about whether I can put it back on or not - I've collected hundreds of heatsinks like these from dead boards. I'd insert a beefy screwdriver under the heatsink and pry up as hard as I can - in most cases, the glue is so hard that the chip rips off the board too along with most traces and balls, which is not a problem since I don't care at that point, but in the case of a board that still has a chance to live you don't want this to happen, so I suggest going really gently AND heating up the heatsink beforehand to hopefully melt the glue enough that it gives. Use a thin flat pry tool and try to cut your way through the glue between the chip and the heatsink - don't pry up.

The silver chip which is partially visible in the middle (no idea why they wouldn't use a proper heatsink that covers ALL of it ! !) is the main chip (CPU if you want) and the one above that and to the right is a FRC IC if I'm not mistaken, but others are free to correct me on this (frame-rate converter). On other sets this resides on a separate T-con board which sits between the main board and the panel - this one appears to have the panel cables plugged straight into the main board, which explains why the FRC is on here as well...

[kubajs]

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

Good to know, thanks for the explanation. I'm not planning to remove the heatsink now as the TV works fine so far. We'll see later.
I had the doubts about removing the heatsing therefore I haven't rather touched it, looks like it was a good choice.