LG 65UH652T won't turn on

[spurf]

I have had this TV for nearly 6 years from new. It has been connected to a smart powerboard the whole time so the power to the TV is turned off when the AV Receiver turns off. Was working fine one day and then the following day wouldn't power on. The red standby LED lights up but when I press power on the remote or manually the red LED turns off after a second or so and then comes straight back on. I've connected the TV to a power meter and the TV is only drawing ~1W even after the power button is pressed so not much is happening.

I took off the back and was hoping to find an obvious fault but everything looks pristine, fuses have continuity and none of the caps looked domed or have leaked. Testing the power board voltages on standby has the 13.2V rails oscillating between 11.65 and 11.85 approx every 2 seconds. The LED+ on the black and white leads are also oscillating between 95.9 and 96.1 at the same frequency. Pressing the power button briefly drops the 13.2V rail to 5V but goes back to the previous value. The green led on the power board doesn't light up.

I tried reflowing the mainboard in the oven without any change. I ended up ordering a replacement mainboard but unfortunately it behaves the same.

I followed the instructions found on this link:

https://masterelectronicsrepair.blog...wer-board.html

Applying 3V externally to PWR_ON and DRV_ON makes the LED backlight blink on and off and the green LED will start blinking. The voltages with the jig connected oscillate between 11.8 to 13.6V but dips briefly to 10.5V when the LEDs flash. If I also add the 12V 21W lightbulb as per the link to any of the 13.2V rails the LED backlight stops blinking and the bulb pulsates at a regular interval (see video). The 13.2V rail drops down to between 0 and 3V with the pulsation of the light. The light bulb behaves this way whether or not the 3V is applied to the PWR_ON and DRV_ON connectors. There is a faint ticking coming from the board on standby that gets louder when the light bulb is connected and is the same frequency as the light pulsation.

I'm guessing something has failed on the powerboard but don't know where to start. Not keen on getting another power board as they are expensive and also mindful they will be the same age and might be about to fail the same way. Any advice for a noob would be greatly appreciated.

[nomoresonys]

heat up those capacitors by the connector with a hairdryer, then see if it comes on if only for a bit, if not heat the rest of the caps and try again and check all the diodes. Some good how to videos on youtube if needed for the diode checking.

[spurf]

Originally Posted by nomoresonys

heat up those capacitors by the connector with a hairdryer, then see if it comes on if only for a bit, if not heat the rest of the caps and try again and check all the diodes. Some good how to videos on youtube if needed for the diode checking.

Are you talking about the little ones on the top right? I tried the hair-dryer already but not sure if I got them hot enough. Do you know what temperature we are aiming for as I have an IR thermometer to measure. I've checked the diodes and they all have ~0.5v drop across them

[Diah]

it sound you have BL issue. test each line alone on white/black socket at start to see the high and low reading.

[nomoresonys]

for sure do Diah's test, if no clear conclusion, maybe try your jig with the backlights unplugged from powerboard and see if the voltage is any different.

[Storyteller12]

It’s the backlights 100%.
I don’t even troubleshoot anything else on these sets. It’s always the backlights

[Storyteller12]

Hopefully you didn’t damage the main board cooking it in the oven.

[dskall]

I would say you have a short or bad cap. 13v rail should be able to output 4amps. What is the number on the controller on back of board. ic?00.

[spurf]

Originally Posted by Storyteller12

It’s the backlights 100%.
I don’t even troubleshoot anything else on these sets. It’s always the backlights

Sorry for the noob question but how do you test the backlights? Also, the TV behaves the same even when the black and white leads are disconnected from the power board, i.e. a brief blink of the red LED and the power consumption doesn't shift from 1W. Does this make the backlights unlikely to be the issue?

[spurf]

Originally Posted by Storyteller12

Hopefully you didn’t damage the main board cooking it in the oven.

I baked the original mainboard not the replacement. If I can get the TV to power up I'll retest the old board to see if it still works

[spurf]

Originally Posted by dskall

i would say you have a short or bad cap. 13v rail should be able to output 4amps. What is the number on the controller on back of board. Ic?00.

ic801

[dskall]

What is part# on the controller at ic801?

[spurf]

It's hard to make out as it's covered in some sort of lacquer. Here is a photo:

[dskall]

The rectangular controller on primary side.

[spurf]

Oh I see. That one is IC100. Even harder to make out due to thicker globs of lacquer:

[nomoresonys]

Originally Posted by spurf

Sorry for the noob question but how do you test the backlights? Also, the TV behaves the same even when the black and white leads are disconnected from the power board, i.e. a brief blink of the red LED and the power consumption doesn't shift from 1W. Does this make the backlights unlikely to be the issue?

did you test those voltages again tho? not just the power consumption.

[spurf]

Originally Posted by nomoresonys

did you test those voltages again tho? not just the power consumption.

Retested this morning and the voltages are unchanged whether the black and white LED cables are connected or not. There seems to be a substantial amount of AC on the LED+ and 13.2V lines. It is present with or without the jig.

Here is a video testing the 13.2V line with the jig.

[dskall]

Why are you measuring in AC?

[spurf]

Originally Posted by dskall

Why are you measuring in AC?

Trying to get a quantitative read on why the voltage is so unstable. I had retested in DC but got the same readings as I had in my opening post.

[dskall]

This all is a little bit convoluted. From what I read your problem is you basically have no 13v. correct? What is the voltage DC on the main capacitor in standby and when you press the power button? And I really need to know what the part numbers are on those 2 controllers on the back.