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Old 11-16-2021, 12:19 PM   #1
pauldow
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Default LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

On this TV, the standby LED is lit. When I press the power button, the LED goes out for 1 second, and it comes back on.
I tested the LED strips, and they're all OK. I checked the larger capacitors on the power supply along with the fuses.
I disconnected the cable between the power supply and main board, and no LEDs illuminate.
Here's the voltage readings I got. The second column is the reading I get when pressing the power button. Any voltage in that column changes back to the first column value after about a second.

I don't think it's worth spending a lot of time on this. I would just like to find out which board is likely to be the one failed. Hopefully it's the power supply since it looks like the processor board is a bit difficult to find, and therefore expensive.

Thank you for any help,
Paul
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Last edited by pauldow; 11-16-2021 at 12:37 PM..
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Old 11-16-2021, 12:23 PM   #2
vinceroger69
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

I think this is one of the models that suffers with bad bga soldering on the ic under the black heatsink on the mainboard
https://www.nickstvs.com/store/p1803/EBT62214703.html

Last edited by vinceroger69; 11-16-2021 at 12:26 PM..
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Old 11-16-2021, 01:18 PM   #3
pauldow
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

Thanks for the reply Vince,
I tried my heat gun, but no change in symptoms. I may not have gotten it fully up to 200 deg C though. It's tough getting a temperature reading off the reflective label. It seems to never go up that last 25 deg. I'll give it one more try.
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Old 11-16-2021, 01:25 PM   #4
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

Best you power on your board disconnected from main and check that you have all your voltages stable and correct.
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Old 11-16-2021, 03:17 PM   #5
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

Looking at the picture of your main board the red marker pen numbers etc may mean it's been repaired before do you know, the background of this tv have you had it from new. Like above post best start from the beginning doing voltage tests etc.
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Old 11-17-2021, 10:11 AM   #6
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

Thanks for diagnostic suggestions.
I removed the cables between the power supply and main board, and I just get the 3.5 volts on the appropriate pins. All others are 0.
I removed the heat sink, wrapped the board in aluminum foil, and cut out a hole where that IC is. I put in in a toaster oven for 10 minutes, and then gave it a brief shot from my heat gun. I was able to see 200 deg. C. There is no change in the symptoms though.

This was a dump find, so it is entirely possible that the main board was already worked on and it was given up. I've had this sitting around for probably a year. Now that colder weather has returned, I went to work on my backlog of clutter around the house.
It looks like the cost/benefit on this one may not be worth it.

Sorry it's not better news.
Paul
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Old 11-17-2021, 10:45 AM   #7
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

200c ? Lead free solder melts at 217c.
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Old 11-17-2021, 05:00 PM   #8
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

Your chart shows 1.4v on the 3.5v pins. Possible short. You need to supply 3.3v to power on to get board to power up.
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Old 11-17-2021, 09:13 PM   #9
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

I fixed a 42LM6400 with these symptoms, it was the BGA solder of the main chip, or the fcBGA of the chip itself, using a heat gun at 420șC slowly circling the edges and center, it lasted two more years and failed again, this time trying to re-heat the solder did not work, it would sometimes not even start or power on only for 1 minute, so I swapped the main board, working for who knows how long.

If you decide to replace the main board, get one from the same inch model, a 47LM6400 main did not work on the 42 inch model, lots of vertical stripes on the screen.
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Old 11-18-2021, 07:20 PM   #10
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

OK. You guys have convinced me to give it a couple more tries. I'll work on it again Friday.

Thanks,
Paul
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Old 11-19-2021, 01:37 AM   #11
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

Hi, i don't understand if the set is already been serviced by other people, if yes then there's the possibility of a bad reflow work done with popcorn effect, so the main chip must be replaced, it is surely almost that it is the fault origin, i think if oven cure is made there isn't possibility that another try will work.. using oven for food after this job is health unsafe

Last edited by Davi.p; 11-19-2021 at 01:39 AM.. Reason: boh
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Old 11-22-2021, 01:48 PM   #12
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

I tried heating it again up to 225 deg C, and there's still no change. I do use a separate toaster oven from the dump for this kind of thing.
I have had some success with other devices with reheating, but as you say, it looks like someone already tried and failed with this one.
From the top, I don't see any cracks in the IC, but there could be something bad underneath, or internally.

Looks like another learning experience.
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Old 11-22-2021, 02:58 PM   #13
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

is it yours? if you have bought it, like i do with mines, a good idea is to keep a file on the own smart phone with a list of questions, first of them: have you already serviced it? Much important.. the second: it comes from a smokers house?? Bleaaaah!!
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Old 11-23-2021, 10:22 AM   #14
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Default Re: LG 55LM6400-UA Standby LED Blinks off. No sound or Picture

This was a dump find, so I don't know its history.
It didn't smell of smoke. Tar should have good electrical insulating resistance. :-) It's like in the days when smoking was allowed on airplanes. The tar would seal the seams in the pressurized fuselage joints. It was a maintenance headache, but it was easier on the pressurization systems.
And cats too. I've opened up computers that have sat on the floor for years, and I was able to peel the dust and fur off the heat sinks. Those were easy to diagnose why they stopped working.

Next up is a Samsung 60" plasma. The words "Smart TV" and then 4 icons show up. Then it turns off. I'll search the history before asking about it though.
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