LG 60PA6500 Burning Z sustain board

[Chevy2]

This LG keeps having the same issue with the Z board arcing over the two mounting points (or burning from behind). I first assumed that it was just an issue with the first board that killed the TV and after replacing it and having the TV work fine for 3 days, it happened again. I'm on my third board and it smells like it might be getting toasty again. I checked all the voltages and they are in spec, not sure what else to check at this point.

[tom66]

Re: LG 60PA6500 Burning Z sustain board

You must have ALL the screws installed in this board or failure will occur rapidly. I am surprised it lasted days. I have seen the case of a board that only managed three minutes!

This is because the frame of the plasma panel forms the ground plane for the PCB. No ground plane, or broken ground plane, bad things happen.

[ReeceyBurger123]

Re: LG 60PA6500 Burning Z sustain board

you must have all the screws installed in this board or failure will occur rapidly. I am surprised it lasted days. I have seen the case of a board that only managed three minutes!

This is because the frame of the plasma panel forms the ground plane for the pcb. No ground plane, or broken ground plane, bad things happen.

+100000000000000

[ReeceyBurger123]

Re: LG 60PA6500 Burning Z sustain board

What made you not put them back anyway ? Its just silly even for test purposes theres a few hundred volts passing through that board !

[tom66]

Re: LG 60PA6500 Burning Z sustain board

Failure occurs (at least on the 60PZ950 I'm looking at) because the bulk capacitor's negative lead and the IGBT pull-down bank are connected only by a screw and a thin trace. Only the positive side is connected via a fat trace, although the positive side is less critical, as fewer things are referenced to it (N-channel transistors referenced to a more-negative source pin.)

So by the laws of inductance, you get a voltage transient between two points where the difference should really be zero! This kills the transistors very quickly as you are likely to be massively exceeding the collector-emitter or drain-source breakdown causing transient breakdown effects and hot spots on the transistor die (same thing that ultimately kills Vestel's shit 17PW25-4 diodes: hotspots due to secondary breakdown.)

In this case, it looks like physical pressure, perhaps from the sticky pads below the board (thermal coupling pads) has been enough to make contact, but the contact hasn't been firm enough, so Mr Ohm got involved, where you have several amperes of peak sustain current and several ohms of resistance due to a small contact area, you get a lot of power dissipation, and so the small contact area begins to heat up. I'd imagine that eventually this heat causes a break in the contact, and then the above mentioned failure occurs killing the transistors.

[Chevy2]

Re: LG 60PA6500 Burning Z sustain board

You must have ALL the screws installed in this board or failure will occur rapidly. I am surprised it lasted days. I have seen the case of a board that only managed three minutes!

This is because the frame of the plasma panel forms the ground plane for the PCB. No ground plane, or broken ground plane, bad things happen.

Thats the first board and after I removed the screws that were arcing, so the TV would turn on. The current board (and the previous) does have all of the screws in place.

[Chevy2]

Re: LG 60PA6500 Burning Z sustain board

Failure occurs (at least on the 60PZ950 I'm looking at) because the bulk capacitor's negative lead and the IGBT pull-down bank are connected only by a screw and a thin trace. Only the positive side is connected via a fat trace, although the positive side is less critical, as fewer things are referenced to it (N-channel transistors referenced to a more-negative source pin.)

So by the laws of inductance, you get a voltage transient between two points where the difference should really be zero! This kills the transistors very quickly as you are likely to be massively exceeding the collector-emitter or drain-source breakdown causing transient breakdown effects and hot spots on the transistor die (same thing that ultimately kills Vestel's shit 17PW25-4 diodes: hotspots due to secondary breakdown.)

In this case, it looks like physical pressure, perhaps from the sticky pads below the board (thermal coupling pads) has been enough to make contact, but the contact hasn't been firm enough, so Mr Ohm got involved, where you have several amperes of peak sustain current and several ohms of resistance due to a small contact area, you get a lot of power dissipation, and so the small contact area begins to heat up. I'd imagine that eventually this heat causes a break in the contact, and then the above mentioned failure occurs killing the transistors.

The TV screen was replaced by LG before I got it. Did they maybe put the pad in the wrong place? Is there a picture of where it would go on this set, so I can confirm it's in the right place?

[tom66]

Re: LG 60PA6500 Burning Z sustain board

The pads aren't critical. They are insulation and dissipate some power but it should be fine without them.
If you have burn marks around the screw points then they may be contaminated, try claiming cleaning them with IPA or similar solvent as carbonisation can occur. Also, make sure that all screws are tightened and on both Y and Z boards as well as the sub boards.

[Chevy2]

Re: LG 60PA6500 Burning Z sustain board

The pads aren't critical. They are insulation and dissipate some power but it should be fine without them.
If you have burn marks around the screw points then they may be contaminated, try claiming cleaning them with IPA or similar solvent as carbonisation can occur. Also, make sure that all screws are tightened and on both Y and Z boards as well as the sub boards.

Ok, there was a lot of burning around the screw holes so maybe that's what's causing the charred smell now as it heats up. I'll try cleaning them up to see if it helps.

[Chevy2]

Re: LG 60PA6500 Burning Z sustain board

The pads aren't critical. They are insulation and dissipate some power but it should be fine without them.
If you have burn marks around the screw points then they may be contaminated, try claiming cleaning them with IPA or similar solvent as carbonisation can occur. Also, make sure that all screws are tightened and on both Y and Z boards as well as the sub boards.

Thanks tom66! Clean the standoffs and don't have the smell anymore over the last several days. Seems like this might have done the trick. Appreciate the help!

[tvtimmy]

Re: LG 60PA6500 Burning Z sustain board

I have the same problem. My 2 middle holes are toast. One had part of the metal ring still attached, the other one was gone, burnt to a crisp. The screws were still in.
I scraped off some of the green board covering and soldered some wire to it and put a ring one the other end. Worked but now I have some pinkish malcharge and what looks like solarization at times. I have to check the ysus board for cracked solder joints, the coils for sure.