Lg 55ga6400 trying to power on but turn off completely

[vinceroger69]

Re: Lg 55ga6400 trying to power on but turn off completely

Thank you! Sadly, eyesight is the first to go when you start to get older! I looked that board over more times than I can count and did not see the F101 plainly (though very small) written there. There is no power on either side of that fuse so as I suspected there is no voltage going to the TCON board. The fuse appears to be good and there is no short to ground on either side of the fuse.

yes my eyesight is going too now im 49 and having wear reading glasses now so do find some repairs on electronics hard, anyway can you trace the feed from the tcon fuse back to the mainboard and post a picture of that area as we need to find where the feed comes from for the fuse.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

Yeah, I'm 50 and it's taking more than one pair of glasses to see these numbers and traces. Guess I really should buy a loop...but I digress...

I managed to trace it back to pins 48-51 on the white connector ribbon cable. I'm not too good with marking up photos but I gave it a good try. I'll explain what I'm seeing in attached pic.

I have traced back from pins 48-51 at the lower left of picture and found that it goes up to the transistor up toward the top of the picture. At that transistor there is continuity from the bottom right leg down to the two caps in the lower right of the picture. There is no continuity between the two bottom pins of the transistor.

When I power up the TV I get 12V on the two caps at the bottom right, which feeds 12V to the bottom right pin of the transistor, 12v on the bottom left pin of the transistor but from a different source than the right pin, and no 12v power out of the top leg of the transistor which goes to my TCON board.

Again, I still have the DRV-ON and P-DIM wires depinned from P201 on the PS board and the small wire connector from the LED driver board to the TCON board disconnected in order to get a backlight.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

I've noticed something else about that cap (C2435) right below the transistor (Q2407) in my pic above that has me puzzled. I've checked it for a short just using my DVOM and it's not shorted but when I power up the TV it's showing 12.46V on right side, same as my 12V rail (expected) and 12.41V on the other side (not expected.) There's not another cap on the board with the same voltage on each side. They're all normal voltage on one side and 0V on the other. Is that normal or is it possible it's just a bad cap on the main board? I don't have any just like it. Can I sub in another cap (I have some electrolytics on hand) there and see what happens?

Edit: I've also checked its ESR and it's 0.22 ohms when all the other caps that look just like it are around 0.15 (again, just checked in circuit.)

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

https://www.badcaps.net/forum/attach...8&d=1534799860

That is is the P-CH MOSFET, 12V is fed to Source pin, Drain pin is the output pin, so right now you also have 12V on the Gate which means the MOSFET is not turned on, the GATE Voltage has to be about 3 ~ 4
less than the Source pin. You can force on that MOSFET by connecting 100 Ohms resistor between Gate and the circuit GND to get 12V at the Drain pin.
If you look at that MOSFET with two pins pointing down, the Left pin is GATE, the Top pin is Drain, the right pin is Source. See T-CON training so you can see how that MOSFET works.
BTW, 49 and 50 years old, heck, I will be 64 this December.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

OK, I was able to turn on the Drain which does power up the TCON board but still no picture or sound. I do hear a click at the TCON board when I do that. I also plugged in the small connector at the LED driver board which causes the backlight to go out but when I turn on the Drain on that MOSFET to power up the TCON board the backlight comes on and stays on even after removing the 100 ohm resistor from the gate. I still have the DRV-ON and P-DIM depinned at P201.

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

Since you have to force on that switched MOSFET, it means the processor is not turning on the MOSFET either, so main board is really bad.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

That's what I was thinking, too. I'm just reading the TCON training document. Thank you for that--very helpful! On page 27 it talks about running the Panel Test. Would it be worthwhile for me to do that just to confirm I get a pattern on the screen? I think this is a 3D set so I'm not sure if it has that feature on the TCON or not...My TCON looks a bit more complicated than the one in the document.

I'm fine with buying a main board for this set but I really want to be right since it's not mine. I'm confident in the assessment that there is a main board failure but is there anything else that could bite me on this if I buy a replacement main board? In other words, is there anything else that could be bad that could have damaged the main board in the first place? There aren't any signs of tampering or moisture damage that I can see anywhere so I'm referring to just some other component failure that would have damaged the main board that could possibly damage a new main board. I once replaced a board on a furnace only to have the new board fry because I forgot to ohm check the blower fan motor and the shorted blower fan motor took out the new board. I may not see as well as I did when I was 30 but I do try to learn from my mistakes!

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

You may want to contact him, he is a member of this site too.
https://www.nickstvs.com/store/p1840/EBT62474404

[vinceroger69]

Re: Lg 55ga6400 trying to power on but turn off completely

All the tests you have done so far is pointing to a bad mainboard on this tv,i would certainly go ahead and replace it if i was repairing it.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

Thanks to both of you! You have been a tremendous help and the documents Bud provided me were also extremely helpful. That TCON training document was great. I wish I'd seen it while I was looking for the power supply traces from the TCON fuse to the main board as it showed exactly where they are.

I will either get this main board fixed or replace it and will update here with the results when I get it done. It doesn't look like Nick has my main board on his list. Mine is an EAX64872104(1.0)

Also, if a mod wants to move this to its own thread I have no issue with that since it didn't turn out to have anything to do with the power supply board as I initially figured it would. My TV is an LG 50LA6970-UE; Symptoms: No picture, no backlight, no display and no sound.

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

Replacement (eBay used/tested) main board installed and TV works.

Thanks again for all your help! I learned a ton from this thread so I really do appreciate it.

John

[budm]

Re: Lg 55ga6400 trying to power on but turn off completely

Thanks for the follow-up.
That was fast. How much is the replacement board? Link to the seller?

[John_Clark]

Re: Lg 55ga6400 trying to power on but turn off completely

I paid $119 plus shipping on eBay from seller "tvpartologist." Some of the regular TV parts shops had listings for less money (around $75) but were all out of stock.