LG 42PQ20D repeatedly tries to start

[dkneyle]

Great looking TV pulled out of e-waste dumpster at rubbish tip. Still has protective plastic all over it.

The blue standby LED flashes and goes steady but no signs of life. After about 30 seconds it resets and tries again, and just continuously cycles through. I'm thinking power supply? I've yet to start taking any measurements, but as always, thought I'd take the lazy approach and see if anyone had an immediate thought as to cause of failure?

I've posted some dodgy pictures of the overall layout of boards in the TV, and a close up of the power supply. Visually all looks OK to a quick glance. I've circled the cap that in other similar power supplies often fails and causes similar grief to that which I've described.

All thoughts and opinions appreciated!

[ReeceyBurger123]

Re: LG 42PQ20D repeatedly tries to start

What happens when you uplug the Y and Zsus boards does the set act differently ?

[dkneyle]

Re: LG 42PQ20D repeatedly tries to start

It looks like it's the Y-Sus board has failed. Power supply generates correct Va and Vs voltages. When the Y-Sus board is disconnected from the power supply, the power supply holds up and doesn't re-set. The Z-Sus power is routed through the Y-Sus board. Disconnecting the Z-Sus does not change the symptom so I conclude Y-Sus board is the failure. I've tended to be a board jockey with plasma repairs but would like to see if I could repair this one down to component level.
So what would be my next steps guys?

[tom66]

Re: LG 42PQ20D repeatedly tries to start

If Vs and Va collapse when the Y-sus turns on, or whenever it is plugged in, then it could be a short on the Y-sus. I'd check all IGBTs for short circuits. Rebuilding this would probably be quite easy, I've done a few sustain boards now and they're pretty simple. Having a scope is a massive help.

[dkneyle]

Re: LG 42PQ20D repeatedly tries to start

Yes Tom, looks like it's time to roll up my sleeves and pull the Y-Sus board from the set and start looking for shorts around those active components. There is a 15 volt test point on the Y-Sus board. At relay click on it jumps to 17 volts and collapses back to zero over a couple of seconds. There is an audible buzz comes from the Y-Sus board as it tries to start but that stops after about 1 second.

I'll keep posting as I work my way through.

[dkneyle]

Re: LG 42PQ20D repeatedly tries to start

I fear the set is uneconomical to repair. I pulled out the Y-Sus board and ran my meter's diode test function over the major semi-conductors on the Y-Sus board. About half show dead shorts between all junctions. I then moved onto the buffer board. The input sockets show dead short between all pins on all sockets with the exception of the bottom socket which only has about 50% on them shorted.

A Plasma expert can probably confirm for me that this has gone beyond the state of repairability. What would cause a major failure like this? No obvious signs of stress when looking at the boards.

[dkneyle]

Re: LG 42PQ20D repeatedly tries to start

By the way, the output of the buffers at the panel ribbon connectors all check out OK.

[ReeceyBurger123]

Re: LG 42PQ20D repeatedly tries to start

I would say the buffer is ok, your not testing it correctly thats why lol. Of course its repairable. How good are you at replacing fets like that ?

[dkneyle]

Re: LG 42PQ20D repeatedly tries to start

Thanks ReeceyBurger for the wise counsel about me not testing the Buffer correctly. Assuming it is OK then I will have a go at the Y-Sus board. I have some scrapped Y-Sus boards from old plasmas, so I'll use them for spare parts once I check compatibility of components I recover. Might be some time before I get to this task; got a few other projects on the go. So don't be concerned if you don't see me re-post for some time. You mentioned in an earlier post about using a scope. I invested in a PC-scope thinking some day I might need one. Is there a re-alignment needed after replacing blown FETs and other devices I might come across?