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Old 01-22-2019, 03:59 AM   #1
Daveatfernie
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Default LG 50PX990 Plasma Powering itself off

Good morning

I have an intermittent 50 inch PX990 plasma TV which I think has electronics based on the 50PK990.
Recently it's started turning itself off after being on for 10-15 minutes and gets to the point where it will only stay on for a few minutes before turning off.
When it turns off and you try to turn it on you can hear the relays clicking in but it clicks off immediately. Leaving it for a few minutes (perhaps to cool or discharge) allows the TV to be powered up for a little while.


I've repaired quite a few LCDs previously but the back of a plasma is a different kettle of fish. I had a look at the guides from the links but none of them seemed to be for an intermittent fault as mine is.


I've tried to take clear pics but if you need closeups of anything please let me know.

Thanks in advance
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Old 01-22-2019, 05:25 AM   #2
Daveatfernie
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Default Re: LG 50PX990 Plasma Powering itself off

I managed to find the supply voltages sticker in the chassis so I'll have a look tonight to see what they look like compared to the PSU output.
There seems to be a wealth of spare boards if required on e-bay but I'd prefer to not have the board in a skip.
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Old 01-22-2019, 09:27 AM   #3
aaronwt6
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Default Re: LG 50PX990 Plasma Powering itself off

Based on my experience with similar LG boards in both plasma and LCD TVs, I'm going to guess that you have failing connections on the processor on the main board. It's going to need to be professionally re-balled in order to remedy the problem.
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Old 01-22-2019, 01:56 PM   #4
Wilderone
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Default Re: LG 50PX990 Plasma Powering itself off

Check for cold solder on back of power supply. Common on these power supply boards & with these symptoms.

Last edited by Wilderone; 01-22-2019 at 01:59 PM..
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Old 01-23-2019, 09:43 AM   #5
Daveatfernie
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Default Re: LG 50PX990 Plasma Powering itself off

Thanks for the replies,

I'll check the back of the PSU board. I could look to get the processor reballed as we have pretty good BGA reflow capability at work. Certainly a couple of things I can check to be going on with.


It may well be though that if I find nothing wrong with the PSU board I'll see if there are any second user boards I can swap in. I'll have a chat to the lads and see if we have any suitable stencils.

Thanks again
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Old 01-23-2019, 09:49 AM   #6
Daveatfernie
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Default Re: LG 50PX990 Plasma Powering itself off

Apologies, Is it the board with the FPGA I'd replace or the board with the XD processing chip on it (the BGA with the two memory IC's next to it)
One appears to be the control board (50R1X_CTRL) the other (the one with the FPGA) is I assume is for OSD and test signal generation?
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Old 01-23-2019, 02:55 PM   #7
Wilderone
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Default Re: LG 50PX990 Plasma Powering itself off

I would be having a look at the back of the power supply first, only take you a few minutes to unscrew it & have a look for cold solder joints, then hit them with a soldering iron.
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Old 01-23-2019, 02:58 PM   #8
Wilderone
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Default Re: LG 50PX990 Plasma Powering itself off

Quote:
Originally Posted by Daveatfernie View Post
Apologies, Is it the board with the FPGA I'd replace or the board with the XD processing chip on it (the BGA with the two memory IC's next to it)
One appears to be the control board (50R1X_CTRL) the other (the one with the FPGA) is I assume is for OSD and test signal generation?
The Mainboard is the one with the large black heatsink, Inputs, HDMI etc.
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Old 01-28-2019, 10:12 AM   #9
Daveatfernie
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Default Re: LG 50PX990 Plasma Powering itself off

Only just managed to do some investigation today. Surely enough on the back of the PSU board there's a few cold joints around the transformers. I'll refit the board tonight and test it. If this ends up being the culprit I'll post pics so others can see where the problem was. Definitely cracking around the transformer pin so likely as not to cause a power loss under thermal stress.
I was quite surprised to see how the board has discoloured around the transformer. Do the boards discolour a lot on their own or are the components really running that hot?
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Old 01-28-2019, 03:10 PM   #10
Wilderone
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Default Re: LG 50PX990 Plasma Powering itself off

Quote:
Originally Posted by Daveatfernie View Post
Only just managed to do some investigation today. Surely enough on the back of the PSU board there's a few cold joints around the transformers. I'll refit the board tonight and test it. If this ends up being the culprit I'll post pics so others can see where the problem was. Definitely cracking around the transformer pin so likely as not to cause a power loss under thermal stress.
I was quite surprised to see how the board has discoloured around the transformer. Do the boards discolour a lot on their own or are the components really running that hot?
Just heat stress mate, you can imagine how long & many times that board has heated up & cooled over it's lifetime.
Have done a few of these now, nice simple fix.
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Old 02-04-2019, 07:00 AM   #11
Daveatfernie
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Default Re: LG 50PX990 Plasma Powering itself off

Seemingly sorted. Wish all repairs were as straight forward as this.

I've attached a couple of photos in case people don't know what they're looking for with a dry joint, cold joint, stressed joint.

These pins were the main culprits for me although I reflowed the whole set of pins on the board.

Thanks again for the help
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Old 02-04-2019, 05:06 PM   #12
Wilderone
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Default Re: LG 50PX990 Plasma Powering itself off

Quote:
Originally Posted by Daveatfernie View Post
Seemingly sorted. Wish all repairs were as straight forward as this.

I've attached a couple of photos in case people don't know what they're looking for with a dry joint, cold joint, stressed joint.

These pins were the main culprits for me although I reflowed the whole set of pins on the board.

Thanks again for the help
Well done, your welcome.
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