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Old 08-25-2021, 04:28 AM   #1
Biruslapio
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Default LG 42PX4RV stuck in Stand-by

I'm now in possession of this very old 2005 plasma TV, the original failure described to me was that it only turned on after hundreds of attempts, eventually it just stopped working.

It has stable stand-by 5V and 4.4V AC-ON voltages, the main board red LED lights up, it has all of the secondary voltages generated by standby: 3.3V, 2.5V and 1.8V. Trying to power on via remote or power button does nothing, the signal is received, but it's stuck.

The power button was bad, replaced but that did not fix anything, replaced 10V 1000uF, 25V 470uF, 50V 47uF and 50V 10uF capacitors at the power supply, those are responsible for the 5V filtering and 5V switching controller, nothing changed.

Soldered a 100 OHM resistor between 5VSTB and RL-ON, 12V/19V/3.4V/5V worked fine.

I've measured mostly everything and no there's no shorts, no open inductors, no visible damage. There's two switches, one of them resets the main FLI8532-LF processor, the other one does nothing on stand-by.

Replacing the main board is a bit expensive costing about 1/3 of what the TV is worth, so I want to repair this one if possible, but I have no clue what to do next.

Main P/N: 6870VM0548E (5)

Service manual link:
https://www.manualslib.com/manual/74...px4rv-Rva.html
Attached Images
File Type: jpg 42PX4.jpg (1.34 MB, 12 views)
File Type: jpg 42PX4Main.jpg (1.76 MB, 11 views)
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Old 08-27-2021, 02:29 AM   #2
Gjackson
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Default Re: LG 42PX4RV stuck in Stand-by

PSU seems to be working fine. I had an lcd TV once that had the same issue, PSU was working fine but it wouldn't power on and it wouldn't respond to remote or buttons having only a red stby light. Managed to fix it by replacing the Main board. If you have a hot air gun you might want to try to reflow main board however in my case that didn't work. I'm definately not qualified to help you in component level repair of the main board as It's too complicated for me maybe someone with more experience can help you there.

Good luck!

Last edited by Gjackson; 08-27-2021 at 02:35 AM..
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Old 08-27-2021, 03:44 AM   #3
Biruslapio
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Default Re: LG 42PX4RV stuck in Stand-by

Thank you, reflow is my last resort if anything else fails, but at this point I'm considering two options:

Force the RL-ON pin with a jumper resistor WITH the main board connected, see what it does, maybe that would damage the FLI8532 main processor since it would receive signals that are not requested.

Reflow the FLI8532, it heats up very little during stand-by.
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Old 08-28-2021, 10:30 AM   #4
Biruslapio
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Default Re: LG 42PX4RV stuck in Stand-by

Update:
Tried the reflow first, then forcing RL-ON with main connected, nothing helps, all the LEDs light up red and the other chips heat up, but it's like the main processor is dead, or one of the ROMs is corrupted.

I'm not too keen on trying to power up Y/Z-SUS + logic board with the main board LVDS connected to see what happens, could it damage something if it sends garbage signal to the logic board?
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Old 09-03-2021, 05:42 PM   #5
Biruslapio
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Default Re: LG 42PX4RV stuck in Stand-by

Fixed, replaced main board, this TV is meh, but it was cheap enough, it's always fun to bring back a 2005 TV that was unopened and worked until last month.
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Old 09-27-2021, 02:33 PM   #6
Biruslapio
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Default Re: LG 42PX4RV stuck in Stand-by

I present to you, the first time I've seen a cracked plasma panel still work flawlessly, how this happened:

I booted up my old PS1 to this TV, was playing some games, Crash Bandicoot 3 and my controller were not cooperating, I raged and slapped the controller, it tore in half and pieces went flying everywhere, the rumble motor hit the screen and cracked it, I didn't even notice at first, but while cleaning the mess up the TV was on and I saw it, I had no proper reaction, turned the TV off immediately, but there was no point since it was working.

Only one pixel was affected, the thing is that it cracked the other side of the glass, the one that's glued very strongly to the metal, so as long as the crack does not expand and the gas does not espace, it's fine, very lucky to have this happen.

That was 3 weeks ago, the power supply was defective, it worked for two days after the main board replacement, then the standby voltage started jumping around and cutting off all the time, I bet that is what made the original board fail.

I took the power supply out and after hours of troubleshooting I found out that it was the primary optocouplers not being properly switched on by whatever IC there was, screw it, I decided to hot-wire the PSU since the screen is already damaged, I shorted the two optocouplers that were doing this, removed two transistors that enabled 5V standby and jumped 5V directly to the pin, it's all good now.

The anti-reflex coating is peeling off and half of the panel is already damaged by poor sustaining which happens on all LG PX plasmas from this era, a now with the crack this is unsellable, will keep it for playing PS1 and SNES.
Attached Images
File Type: jpeg 42PX1.jpeg (553.5 KB, 5 views)
File Type: jpeg 42PX3.jpeg (298.5 KB, 7 views)
File Type: jpeg 42PX2.jpeg (250.7 KB, 8 views)
File Type: jpg 42px4rv PSU.jpg (339.2 KB, 5 views)
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