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Old 05-19-2020, 10:01 PM   #1
kent_323is
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Default LG 65UH5500 flashes on and shuts right off

Hi,
I have the LG 65UH55 tv and it has some issues.
Right now it will not come on and stay on. Doing some reading and research on it, it could be the power supply board, LED backlights or main board.

Here's what it does:
1. plug into wall, and the red status light comes on.
2. press power button, and it will very briefly flash up the LG icon on the screen and appears to come on.. but then quickly shuts back down again and the red light comes on again.
3. press power button again, and the red light goes out, and the red light on the main board will light up briefly and go back off while the red light on the power button stays off. Screen and backlight LEDs stays off.
4. Press power button again and main power board will light its red light again and the power button will flash its red light again, and then go off.
Unplugging it from power will then allow you to start at #1 again.

I opened up the back, and I have measured the voltage at the pins on CN2 on the power supply board.
PSON pin is 3.5 and red light is on
13.5v pins are reading 8.6v
80v pins are reading 57v

Does this indicate that the power supply board is bad? I have visually looked at all of the large capacitors, and they seem fine, none of them are bulged.
What else should I check and measure?

Thanks
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Old 05-19-2020, 10:41 PM   #2
neilc6
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Default Re: LG 65UH5500 flashes on and shuts right off

Coincidence someone just messaged me with the same model with the symptom of a quick flash of the LG logo and front red LED flashing. The LG logo flash is a symptom of bad backlights and the protection circuit kicking in and shutting them down. Was a bit unsure since LGs typically stay on with the backlight shut down but I guess not in this model.

https://www.shopjimmy.com/lg-5835-w6...-strips-24.htm

Read the second review. Very difficult repair on a 65" TV with a high chance of cracking the screen since you need to remove the screen to get at the LEDs. Not sure how Shopjimmy rates the skill level as easy. Maybe on a 32" TV.

Last edited by neilc6; 05-19-2020 at 10:50 PM..
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Old 05-20-2020, 08:47 AM   #3
mmartell
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Default Re: LG 65UH5500 flashes on and shuts right off

Can confirm symptoms with exact model I just repaired. One led open.

If you do the repair I would suggest a hardware mod to the led driver to reduce current to the backlight strings. Can help you with that when you get to that point.

57v sounds about right. You need to check those voltages when you power up the set to see how high they spike on each string (six strings) to determine if you have an open led in one of the strings.
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Old 05-20-2020, 09:29 AM   #4
kent_323is
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Default Re: LG 65UH5500 flashes on and shuts right off

So test the voltage on each pin that's listed as 80v on the power supply board?
If an LED string is out, what would it show as voltage?

Is there anyway to unhook the LED strips and verify that the rest of the TV is working okay?

Thanks for the help so far, I did go look at the link for the LED strips, and that seems like my same issue as well.
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Old 05-20-2020, 09:50 AM   #5
kent_323is
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Default Re: LG 65UH5500 flashes on and shuts right off

I measured the output voltages on the 80v pins.
In general for all 5 of them: 70v in standby mode, spikes to 97v when first turning on, and then goes right back down to 65v.

What does that mean?

Thanks
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Old 05-21-2020, 08:43 AM   #6
mmartell
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Default Re: LG 65UH5500 flashes on and shuts right off

Was that with the led connector plugged in ? If you're getting identical readings on all six pins then you may have a different problem.

Wait, five pins ? You should have six, there are six strings !
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Old 05-21-2020, 08:55 AM   #7
mmartell
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Default Re: LG 65UH5500 flashes on and shuts right off

So I looked up a photo of the board we're discussing and I can see now you're testing the voltages at CN401 when you should be testing them at the LED connector, CN403.

Try again with connector plugged in. Red lead to LED1+, black lead to chassis ground and turn on tv. Note the voltage. Lather rinse repeat for all six positive pins.

The set must be in the state where the screen flashes each time you make the measurement so you may have to uplug/replug each time.
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Old 05-21-2020, 09:02 AM   #8
kent_323is
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Default Re: LG 65UH5500 flashes on and shuts right off

Ok, thanks for the clarification. I will re-test and post the results.
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Old 05-22-2020, 10:05 PM   #9
kent_323is
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Default Re: LG 65UH5500 flashes on and shuts right off

I ended up taking it all the way apart, and sure enough, there was a Single bad LED! So I guess next step is ordering new LED strips. I'm going to replace all of them, as it isn't worth doing all this work for a single LED, and then have the next one go out next week or something stupid.
I did find this video on youtube that was super helpful in taking it apart.
https://www.youtube.com/watch?v=5ir7...ature=youtu.be

Thanks for the guidance that it was likely the LED strips.
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Old 05-23-2020, 05:47 PM   #10
mmartell
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Default Re: LG 65UH5500 flashes on and shuts right off

Very good if you'd like to do the current limiting mod in the meantime just let me know.
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Old 05-23-2020, 08:56 PM   #11
kent_323is
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Default Re: LG 65UH5500 flashes on and shuts right off

mmartell,
Yes, what does the current limiting mod entail?
Thanks
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Old 05-23-2020, 09:20 PM   #12
mmartell
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Default Re: LG 65UH5500 flashes on and shuts right off

Remove the led driver board and on the backside remove ONE of the group of three 4.3ohm resistors at location R415a, R417a, R419a, R421a, R423a and R425a for a total of six resistors - one for each led string.

The current to the leds will be reduced by about 30% if I remember correctly. I did not notice much if any change in brightness as a result. YMMV.

Picture courtesy of Shopjimmy.
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File Type: jpg shopjimmy-cov33699501-bottom.jpg (789.9 KB, 6 views)
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