70" LG TV Backlight turns off

[bbruzzoni]

I have a 70" LG 4k TV. The model is 70UH6350-UB.BUSMLJR
I got it from someone who said it started to turn off on its own. When I got it, it worked fine for around 3 hours, then the backlight went out. You can still hear the sound and I did the flashlight test on the display and it was still working as well, so it was evident that the backlights went out.
I thought it would be a bad LED, so I successfully opened up the TV, got the LEDs, tested one by one with an LED tester, and they all work just fine, there was nothing wrong with them, all showed the same voltage. So I cleaned the connections all around as I put it back, hoping for some reason it would work or something, but sure enough when I turned it on, it only worked for a few minutes and then the backlights went out again. I had accidentally burned out the power supply as I was doing some testing (whoops, I know) so I replaced with a new one from a trusted ebay seller and the same problem was there, so it can't be the power supply. It's not the LEDs since I tested them. I don't believe it would be the t-con board since ot looks like I have no problems with the display. So I'm thinking it may be the main board, since it has the LED driver I think. I wanted to replace the main board but the only one out there that is being sold is by someone on ebay for $350, and I'm not about to pay that.
Any thoughts? Any help?

[Topcat]

Re: 70" LG TV Backlight turns off

My vote still goes to a defective/leaky LED....

[bbruzzoni]

Re: 70" LG TV Backlight turns off

How could I single it out if so?

[dick_barton]

Re: 70" LG TV Backlight turns off

Turn the backlights down as much as possible (not brightness or contrast) and see if the backlights stay on longer. If they do then it would point to an led problem.
A led tester is useful but only supplies a low current so faulty/intermittent leds can be missed.

[bbruzzoni]

Re: 70" LG TV Backlight turns off

I just turned the backlights down to 0 and the backlights turned off within a few minutes. I turned the tv off, then on again and the backlights turned off within 20 seconds, then within 10 seconds the next 2 times. I left it off for a few minutes, turned it back on and they stayed on for about 8 minutes and just turned off again. I tried again and it lasted for 2 minutes.
I would just replace the backlights to try, but NO ONE is selling these backlights :/
Also, I would replace the singular bad LEDs if I could find them, but I can't tell exactly the type of LED that are on there.

[neilc6]

Re: 70" LG TV Backlight turns off

https://ebay.us/e3XjYZ

[bbruzzoni]

Re: 70" LG TV Backlight turns off

Nice! I just hope it's compatible

[bbruzzoni]

Re: 70" LG TV Backlight turns off

Now I'm trying to see if I can fix the power supply board that I messed up. So I used the LED tester on the LED connections while all the wires were connected and the tv was connected to the outlet. I heard a noise when I put in the LED tester pins on two of the pins from the LED wires. The power supply stopped transferring power. What do you think I may have fried? What would I have to replace, what can I check?

[dick_barton]

Re: 70" LG TV Backlight turns off

Post some photos of your power supply and a close up around the led driver IC.

I would initially check all the 0 ohm resistors in that area.

Edit
Can you compare the resistance of each of the driver transistors Q807 to Q812. Perhaps one is damaged.

[bbruzzoni]

Re: 70" LG TV Backlight turns off

Yes, I actually found one that looks blown, I added a picture of it. I bought the new 103 SMD chip resistor and I'm going to replace it. Hopefully that's the only thing I need to replace

[dick_barton]

Re: 70" LG TV Backlight turns off

Have you compared Q810 resistance against the others to check that if it's not faulty.
What is the part number of Q810