Troubleshooting LG Plasma

[DanFen]

Hi everyone

I bought a non-working 50" PG Plasma for real cheap, just for some fun and hopefully some learning. My background is technical IT, and in fact it's the first time troubleshooting a TV.

Problem: Power LED turns on, then off after a second or two.

So I've started with identifying and familiarising myself with the boards. Tested PSU OK. Visual inspection for burnt down components, or bulged caps... all looks fine. TV is however very dirty as it was in a restaurant, so it's difficult to identify burnt components.

Since LED turns off after a second, I thought there must be a short circuit somewhere, so I've started disconnecting boards. Power stayed on as soon as I've eliminated the YSUS (EAX64282201).

I couldn't see any fuse on this board so I started testing the mosfets for short to ground. Since I never did it I am not sure if I'm actually testing them correctly.

First of all I only have a DMM. So my first question for you would be: do I have to remove the mosfets out of the circuit to test properly, or can then be tested in circuit?

Thanks

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[tvtimmy]

Re: Troubleshooting LG Plasma

You will have to remove the board and flip it over to check the igbt's marked with a "Q".
Check for shorts across any of the 3 legs. If you don't find any, check the buffer boards for shorts. Make sure you have all the screws back in place when you turn it back on.

[DanFen]

Re: Troubleshooting LG Plasma

There's no "Q" markings anywhere on the board at the connections of the IGBTs or Mosfets.

Is it accurate to test for short circuit while in circuit or do I have to take them out?

[tvtimmy]

Re: Troubleshooting LG Plasma

You can test then in circuit. There should not be any continuity between the legs of the igbt's. Can you see what the part number on the board is of this igbt?

[DanFen]

Re: Troubleshooting LG Plasma

30F126 is the part number of that particular IGBT.

I'll test IGBT for any continuity. Does the same test apply to mosfets too?

[tvtimmy]

Re: Troubleshooting LG Plasma

You should test them both like that and then in diode mode.

[DanFen]

Re: Troubleshooting LG Plasma

Tested all of them, 15 in total.

I've used continuity mode on the DMM and checked all 3 legs of each. 9 of them had some sort of continuity between 2 or even 3 legs. Does this make sense?

[tvtimmy]

Re: Troubleshooting LG Plasma

Yes. All but the diodes are probably bad. Remove one and test it in continuity and diode mode.

[DanFen]

Re: Troubleshooting LG Plasma

So I've removed a KF7N60 and tested as following based on a video I've found:

Continuity between Drain & Source: no continuity (and I am guessing this is good?).

Then I've tested the On/Off state by charging the capacitor. I did this with the multi-meter by:

1. Holding the negative on the Source, and touching the Gate with the positive lead.
2. Then I moved the positive lead onto the Drain to test and got no connection.

So I guess this is bad. From what I understand there should be a connection.

... and then to test the Off state, you'd touch the Gate and Drain with the finger, and should get no connection. But there was no point to do that at this stage since there was no connection when it was supposed to be in On state.

Hope this method is correct.

Yes. All but the diodes are probably bad. Remove one and test it in continuity and diode mode.

[tvtimmy]

Re: Troubleshooting LG Plasma

If you find an igbt that tests short in circuit, remove it and test it.

[DanFen]

Re: Troubleshooting LG Plasma

Removed a RJP63K2 (that was a nightmare! ahaha). This is exactly what I did (MM in diode mode):

Negative lead to the Emitter & Positive lead to the Collector: got 0.540
(should I even get a reading like this?)

[tvtimmy]

Re: Troubleshooting LG Plasma

That one reads good.

[DanFen]

Re: Troubleshooting LG Plasma

So I wonder if I'm testing for short in circuit using the wrong method?

I've put the MM into continuity test and put the leads onto each pin combination. If I got a beep then I considered that as being shorted.

[ljintexas]

Re: Troubleshooting LG Plasma

Loose grounds/board mounting screws can cause havoc with larger plasmas. Sounds like for starters, you have IGBT issues on the Y sustain board. A problem on one board can cause problems on another. Buffer boards can damage sustain boards.

[DanFen]

Re: Troubleshooting LG Plasma

The screws were all tight for sure, as I've checked those.

The problem I seem to be having is to find which IGBTs are faulty. Seems like continuity testing in circuit detected shorted IGBTs, but then when removed they're fine (or seem to be fine if I am testing them correctly)

[DanFen]

Re: Troubleshooting LG Plasma

I wonder if I should just buy a replacement board or try to fix

Is it accurate to test IGBTs / FETs in circuit? or is it always recommended to remove for accurate readings?

[tvtimmy]

Re: Troubleshooting LG Plasma

Sometimes the igbt's don't all go bad. You will have to remove them one at a time, noting which ones go where, until the shorts all go away.

[DanFen]

Re: Troubleshooting LG Plasma

Can do that, for the sake of learning

What I'm not sure about is, if I was testing them correctly while in circuit. I was setting the meter to continuity mode (beep mode) and alternating between all 3 pins. So S with G, S with D, and D with G, and see if it beeps. If it beeps I was noting it as short. Is this the right way or am I messing it up?

Thanks.

Sometimes the igbt's don't all go bad. You will have to remove them one at a time, noting which ones go where, until the shorts all go away.

[tvtimmy]

Re: Troubleshooting LG Plasma

Yes, that's the right way.

[DanFen]

Re: Troubleshooting LG Plasma

OK, so at least I'm on the right track. But this doesn't necessarily mean that the IGBT/FET is damaged right? Am I right to say that other components might be causing the short to ground on it? And if the answer to this is positive, that means that the IGBT/FET testing out of circuit might have good results... meaning, the IGBT/FET can be good?

Yes, that's the right way.