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

I see some of the repair kits on ebay include these little guys, are they mini resistors ? any chance I would have fried them too ?

[tom66]

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

Yes it's possible, check them to see if they measure 47 ohms.

[goldbird78]

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

ok.....I removed all the 45F122 transistors and there were two shorted between all pins. I am not sure of the pin designation so I will just go left to right as 1(left), 2 (middle) and 3 (right) when the transistor is facing you with the writing. The all the other chips had open readings between pins 1 & 2 and between pins 1 & 3. I did get readings between pins 2 & 3 of 1.9-2.5M ohms depending on the transister. some of the transistors had a stable reading and some of them had readings that would fluctuate up or down. I'm using a Flue 112 true rms meter.

On a second note I did not remove the 20n20 yet because there are only two and when on the board I am getting a high resistance between pins and they don't appear to be shorted. Should I still remove these to test ?

Lastly I had to cut some of the silicone goop binding the capacitors together to be able to get the screws out of the transistors, is this critical to put back ?? I have to ask because I thought not all the screws were needed for testing and look where tht got me

[goldbird78]

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

Can someone help me with below. Thanks

ok.....I removed all the 45F122 transistors and there were two shorted between all pins. I am not sure of the pin designation so I will just go left to right as 1(left), 2 (middle) and 3 (right) when the transistor is facing you with the writing. The all the other chips had open readings between pins 1 & 2 and between pins 1 & 3. I did get readings between pins 2 & 3 of 1.9-2.5M ohms depending on the transister. some of the transistors had a stable reading and some of them had readings that would fluctuate up or down. I'm using a Flue 112 true rms meter.

On a second note I did not remove the 20n20 yet because there are only two and when on the board I am getting a high resistance between pins and they don't appear to be shorted. Should I still remove these to test ?

Lastly I had to cut some of the silicone goop binding the capacitors together to be able to get the screws out of the transistors, is this critical to put back ?? I have to ask because I thought not all the screws were needed for testing and look where tht got me

[tom66]

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

The 20N20s are probably OK. They are for the Vzbias circuit.

The silicon goop is not critical, it's primarily for shipping purposes so the solder joints on the caps don't fail in shipment.

Any reading in the MOhms is probably ok for the transistors. Shorts will be less than 50 ohms and the same in both directions.

[goldbird78]

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

Tom, Thanks for your help. another question, is it normal on these transistors to get high resistance ohm reading that fluctuate up and down ?

The 20N20s are probably OK. They are for the Vzbias circuit.

The silicon goop is not critical, it's primarily for shipping purposes so the solder joints on the caps don't fail in shipment.

Any reading in the MOhms is probably ok for the transistors. Shorts will be less than 50 ohms and the same in both directions.

[tom66]

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

Tom, Thanks for your help. another question, is it normal on these transistors to get high resistance ohm reading that fluctuate up and down ?

Yes. It is usually only bad if the resistance reading is low.

It is sometimes necessary to confirm with a transistor tester. You could consider buying one of these if you like:
https://www.ebay.com/itm/Mega328-LCR...AAAOSwTLxZgv5A

They can test almost any device and tell you if it's OK. Could save you if you miss a bad part. You would need to remove any part (that you want to test) from the circuit, though.

[goldbird78]

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

Hi Tom, I actually purchased one of these last week on E-Bay,
should get it in a couple weeks. Will this do transistors too ?

https://www.ebay.ca/itm/MESR-100-ESR...72.m2749.l2649

Yes. It is usually only bad if the resistance reading is low.

It is sometimes necessary to confirm with a transistor tester. You could consider buying one of these if you like:
https://www.ebay.com/itm/Mega328-LCR...AAAOSwTLxZgv5A

They can test almost any device and tell you if it's OK. Could save you if you miss a bad part. You would need to remove any part (that you want to test) from the circuit, though.

[goldbird78]

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

I think I am getting closer.... The good news is I replaced the six 45F122 transistors and the fuse doesn't blow anymore. The bad news is four of them shorted out again (Q11, Q12, Q13 & Q21) after the TV was on for about 10 seconds. I removed them again and verified the 4 are toast....I followed the traces on the back of the board where these transistors are and discovered that all the little resisters labeled 100 that appear to be in the transistor circuit read 5.2 ohms except A236 & A235 which was reading very high at 3.2K Ohms. Is this a problem and would this cause the above transistors to short out ?? Or is there something else I should look at. I have attached a few pictures. Thx

[goldbird78]

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

The white small "100" resistors are reading 5.2 ohns and the red is reading 3.2K ohms, these pair are the only ones that have such a high reading.

[will62]

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

The 100's are 10 ohm resistors, 2 in parallel will measure ~5 ohms. I can't read the other 2 (red), if they are also 100's, they are probably blown, and the 3.2k resistance you are measuring is something else in the circuit. Either pull them out of circuit, or lift one end.

[goldbird78]

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

Hi Will, Yes the red ones are 100's. With them blown will that cause the transistors to short out ? Thx

The 100's are 10 ohm resistors, 2 in parallel will measure ~5 ohms. I can't read the other 2 (red), if they are also 100's, they are probably blown, and the 3.2k resistance you are measuring is something else in the circuit. Either pull them out of circuit, or lift one end.

[will62]

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

They could have been taken out by the mosfet, but I believe those are gate resistors, so you need to go back to the driver IC as it would also be suspect
with blown gate resistors.

[goldbird78]

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

Hmmm, Driver IC, can you point me in the right direction there. I am a quick learning newbee. Thx

They could have been taken out by the mosfet, but I believe those are gate resistors, so you need to go back to the driver IC as it would also be suspect
with blown gate resistors.

[will62]

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

OK, had to read back a bit, you're dealing with igbt's, not fets. Either way, the gates need a signal in order to conduct. I'm not sure if you have a gate driver on that board, or if the signal is coming off-board. Can you read a part number on the top of IC306?

[goldbird78]

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

Where would i find IC306 on the board ?

[goldbird78]

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

Can you read a part number on the top of IC306?

If you are referring to the transistors they are 4F122. And i did remove the little 100 resisters and one is reading 513 ohms and the other is reading 1.22K ohms

[will62]

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

I don't want to side track you, looking for things that may not be on the board. The logic signals from your main board could be driving the igbts. IC306 is the 8 pin ic that is above the red circle in your pic. If you can give me a part number off the top, I'll just confirm what it is. If it's unrelated, you can go ahead and pull those resistors and replace them.

[goldbird78]

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

Don't worry about side tracking me this is a learning experience, I appreciate all the help. I have attached a picture.

[will62]

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

Yes, it's a gate driver. Datasheet attached. Page 9 has a pinout and page 8 is probably your best bet for checking it. Looks like a bunch of diode checks between pins.

Also, check resistance between pin 7 (HO) and the gate resistors on the igbts.