I think I fried a good ZSUS board on an LG plasma

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

This makes sense for the one.

From a T.I. paper on gate drivers:

"For completeness, the external series gate resistor and the MOSFET driver's output impedance must be mentioned as determining factors in high performance gate drive designs as they have a profound effect on switching speeds and consequently on switching losses."

Those igbts are in parallel. If resistance of 1 set of gate resistors goes high like yours did, they will switch at different speeds = disaster.

That doesn't account for the other 3 that are on a separate circuit.

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

And this makes sense too for the other 3. I have seen most of the kits for repairing Zsus boards come with 6 transistors, 12 gate resistors and a fuse. Last time i just replaced the transistors not knowing at the time the 2 resistors was blown. Anything else you can think of, just to recap this all started with a perfectly working board that i put in the exact same TV that went black screen with sound but I only put 2 screws in for testing and everything worked for 10-20 seconds and

I'm going to throw something else out there, and you tell me if it makes sense.
I wouldn't know without seeing a schematic, but let's suppose that one set of parallel igbts is responsible for generating the positive (high) side of the wave, and the other set of parallel igbts is responsible for generating the negative (low) side of the wave. If this is the case, then I know for a fact that the high and low igbts can not be on at the same time, without blowing them. There is usually some dead time between 1 set going off and the other coming on.

Now, if the 1 igbt with bad gate resistors (1 of 3 high), is not turning completely off before the other 3 igbts (low) turn on, then those 4 igbts would
blow. That is one way I could see those 4 igbts blowing together. In the attached pic, you can see just a bit of jitter at the positive to negative transition. That might be the dead time between switching that I mentioned above.

[will62]

Re: I think I fried a good ZSUS board on an LG plasma

When I saw your explosion emoticon, I thought you had already tried the board with bad results. That was from your previous try, right?

I did notice that the boards for sale on the web, have what looks like an insulation tape below all sections with transistors. You have that, right? And no chance that you cut the leads off a little long, possibly touching the chassis?

Other than those, can't think of anything else.

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

no I haven't tried it yet that was before and yes I put the insulating tape back on.

When I saw your explosion emoticon, I thought you had already tried the board with bad results. That was from your previous try, right?

I did notice that the boards for sale on the web, have what looks like an insulation tape below all sections with transistors. You have that, right? And no chance that you cut the leads off a little long, possibly touching the chassis?

Other than those, can't think of anything else.

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

I did notice something that may agree with your second theory mentioned of why the 3 transistors shorted with good gate resistors. If you look at the attached picture the 3 transistors on the left and 3 right are reversed. The red side is common to both but from different pins and feeds the panel ribbon connector, the blue on the left comes right from the fuse after going through a copper coil. Anyways I'm out of town tomorrow so Sunday afternoon will be the big test. If you or anyone have anymore thought it would be greatly appreciated. I will keep you posted when I get back. Thanks for all your help.

[tom66]

Re: I think I fried a good ZSUS board on an LG plasma

If your gate resistors are bad, your driver ICs are almost certainly bad. These would need to be replaced.

The driver IC provides the gate of the MOSFET or IGBT (most often IGBT), which is a small capacitor, with the many amps of pulse current needed to switch the transistor on and off.

The ICs cannot take much overstress and sometimes if the IGBT fails it will pulse the full Vs supply into the gate driver and resistor, destroying both.

It is not a matter of timing, more a matter that when the driver ICs go bad, then they can short to the +15V/+17V supply. This has the side effect of keeping the IGBT switched on all the time, which means if the other transistor in the set switches on at the same time, you're gonna have a bad time...

One thing you can do is a "bulb test". After the board is repaired - after you have replaced all of the transistors - cut both Vs (not Va) wires going to the z-sus board and put two 120V 100W incandescent (NOT ENERGY SAVING OR LED) bulbs in series with the Z-sus board.

If the bulbs light up bright and stay lit up there is a fault. It is possible some parts will fail, but the bulbs will limit the failure energy, so many fewer parts will be damaged, sometimes none. If the bulbs light up bright then dim, and the picture appears (It will be poor, and maldischarged, but there should be some image) then everything is ok. Then you can remove the bulbs, join the wires back together and enjoy your TV. Make sure to heatshrink the connection you had to break. You do not want it to arc against the chassis and cause damage.

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

Hi Tom, Thanks for the reply. If only two of the gate resistors in parallel controling one transistors failed and all the others are ok do you think the IC 's are bad or could it be just from the transistor shorting ?

I did ohm out the ic to ground and across the pins and all had open or high readings, would this be an indication they are good ? Mind you I could only find one will Will's help

Finding incondecent bulbs may be tough here most retailers haven't sold them for years. I will have to search some dollar stores.

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

One thing you can do is a "bulb test". After the board is repaired - after you have replaced all of the transistors - cut both Vs (not Va) wires going to the z-sus board and put two 120V 100W incandescent (NOT ENERGY SAVING OR LED) bulbs in series with the Z-sus board.

I'm assuming you put one bulb is series on each of the Vs lines ?

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

Board is in the TV ready to be tested. All shorted transistors have been replaced, the two blown gate resistors also have been replaced and I have made a bulb testing box. Just need to confirm that one bulb goes in series on each Vs wire.

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

Hey Will, do you think the other IC driver chip is IC307 ? I would like to compare ohm readings before testing the board.

[tom66]

Re: I think I fried a good ZSUS board on an LG plasma

I'm assuming you put one bulb is series on each of the Vs lines ?

Vs lines in parallel, cut BOTH, tie both wires together, then tie each end to bulb.
You basically want to interrupt the Vs from the power supply and make sure all current for Vs flows through the wires.

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

Vs lines in parallel, cut BOTH, tie both wires together, then tie each end to bulb.
You basically want to interrupt the Vs from the power supply and make sure all current for Vs flows through the wires.

Little confused here, which way should I wire the bulbs, see attached picture. Thx

[tom66]

Re: I think I fried a good ZSUS board on an LG plasma

#1

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

#1

Thank You

[goldbird78]

Re: I think I fried a good ZSUS board on an LG plasma

Success !!

Huge thank you to Tom66 & Will62 for there time and patience. If you were a little closer would be on me. Just to conclude it was the 4 shorted transistors and the two gate resisters in parallel feeding one of the shorted transistors. Thanks again

Btw, I will be starting a new thread for the original problem I have with my other 50PS60 before I started swapping boards and got myself into this mess.

[tom66]

Re: I think I fried a good ZSUS board on an LG plasma

Nice!!!

Repairing sustain boards is not easy so that's a pretty cool result.