LG 42PT350-UD Audio no picture and no prime.

[Doc38343]

I read some of the other posts and while similar not the same though. I made a new one to ease the diagnostics. Fellow brought this one and he has already been online and trying to fix it himself. I am not sure what all he had done but the screws were out of the Y-board.
I have just started my diagnostics process so i put everything back.
Have red standby LED. When you press ON on the remote I hear the relays trip once and the red light blinks off then on and slowly fades out.
OK All voltages check right on power supply. INCLUDING VA and VS.
3.3v standby, 5.2 on 5v rail when turned on. The VA and VS stay on and are stable VS = 201.3v VA = 55.21v. It does not trip off stays on as long as i don't press the power on the remote or panel.
I did the test pattern test on the control board (from one of the other posts) and there is no test pattern nor change. It behaves the same.
Best I can tell the plasma has no prime. the last one i had like this had prime but no picture. This one has no screen activity at all not a flicker.
I am going to search for a service manual and i already found the LG_plasma panels guide from the other post. Hoping someone can shed some light for me, there seems to be a lot of experience with this model..
Plasma panel PDP42T3#### the attached guide does not appear to be for this model.
Thanks

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

Anybody got a service manual for this model please?

I am thinking it could be a fault on the output of the y board or even the buffer.
I just don't get why it is sending the all good to the control board... everything is on and stays on as far as VS VA and all. I have not tested the high voltage output from the y buffer connector... are there some test points and how safe is it to do that?
I could pull both boards and start going thru them out of circuit...
something is odd about this deal here...

[R_J]

Re: LG 42PT350-UD Audio no picture and no prime.

Where are you measuring the vs & va? on the power supply or the Y-sus board.
I can see from your picture the plug to the Y-sus is not connected properly. Look at the plug on the Z-sus and you can see it is inserted fully but the Y-sus is not

[Hamie]

Re: LG 42PT350-UD Audio no picture and no prime.

You need to check vsc -vy and z-bias.

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

Where are you measuring the vs & va? on the power supply or the Y-sus board.
I can see from your picture the plug to the Y-sus is not connected properly. Look at the plug on the Z-sus and you can see it is inserted fully but the Y-sus is not

Yeah the picture is from when i first took the back off... the screws were all missing too. He had been trying to replace the y board but he told me he had ordered the wrong one.
I get the 201v VS and 55v VA no matter where on the rail i test it.. z board, y board also. I am covered up busy so I will try what the fellow above said to test also.. if readings are good I am going to pull the board. Service manual is $10 I don't know if I want to spend that just yet... could be something as simple as an open resistor or shorted SMD cap... I can tell a lot from a good slo-mo bench going over. The fellow who owns it said SJ is where he got his info about it being a bad y board... I have learned to never assume anything so a testing we will go... Thank you. You right about that plug too.. it was half out and when I saw it I noticed the board was not even fastened. Screws were in a zip lock bag.

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

I went ahead and bought the service manual.. guy is calling me twice daily... HELP!!!

shared it for others in need .. Tom66 can add to his collection also

CRAPPPP!!! Great service manual, very detailed, nothing about the power supply, y sus, z sus or any other freaken boards, but hey if we wanna rebuild the A board it is there!

Back exactly where I started. "Life's Good"... for somebody i suppose

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

Need help on checking the VSC and VPP I found some pins marked on P211 bottom connector. VSC from pin to chassis ground was -55.6v
VY has a clearly marked test point and reads stable 200.1v (there is a resistor and test points are leg to leg and marked + on one - on the other.
I thought VSC was supposed to be way higher but then i have nothing to go by. The LG chassis document Tom66 shared is totally different from this model- it's T3 not T1
Could you guys tell me how to test the VSC and VPP correctly please?

I will get readings from the lower pins also.
I made closeup photos of P211 connector and markings.

VSC pin 1 and 2 is -52.91
VPP pin 3 and 4 is -52.91
SUS_ON pin6 is -195.3 (chassis ground)
STB pin8 is -195.3 (chassis ground)
SUS_ON pin12 is -195.3 (chassis ground)

I don't know but while i wait i'm going to pull the Y buffer and check for shorts

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

This thing is making me bat crap crazy... even when the buffer board is off. The Y board is totally disconnected. NOTHING CHANGES. Red standby LED... press power VS and VA activate on the PS and Z board and stay on. the red light (on front) goes out when turned on.. exactly the same as when all is connected. I am now going to disconnect the controller board from the Z board to see what happens.

Disconnected the Z-board from the controller.. same thing no change at all.
Now disconnecting the A main from the controller board.

Disconnected the A main from the controller..same thing no change at all.
Am i dealing with a bad main.... It turns on and stays on as if it was working, shouldn't SOMETHING different happen?
Shouldn't it trip the VS and VA back off?
It stays on as long as i let it.... over 15min

Last thing i just realized is that the control board has an LED on it. It has never once even flickered. No matter what is connected or not connected it does nothing either..... control board... main board.... something is not normal in any way.
I am out of my wit on this one.
Done for tonight, maybe one of you old school dudes have seen something like this....

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

Thanks for the helpful advise...

[tom66]

Re: LG 42PT350-UD Audio no picture and no prime.

If the LED doesn't light on your control board I'd be inclined to believe it's failed. I've seen it on similar models.

Vsc / Vscan is always tested across the resistor on LG sets. You should get around 150V, exact rating depends on panel specification.

With an oscilloscope you can check to see what the Y-sus and Z-sus outputs are doing, and see if the control board is working at all.

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

If the LED doesn't light on your control board I'd be inclined to believe it's failed. I've seen it on similar models.

Vsc / Vscan is always tested across the resistor on LG sets. You should get around 150V, exact rating depends on panel specification.

With an oscilloscope you can check to see what the Y-sus and Z-sus outputs are doing, and see if the control board is working at all.

Thanks tom66 .. Grab that service manual I bought for this model to add to your collection. It has a lot of good info for main board and programming setup.
I don't know which resistor and no documentation for this chassis. I have 200.1v across the VY resistor it is marked clearly.

I was also thinking the control board. since the fellow who this belongs to wants it bad I am going to put the ball in his court....
The buffer on this one is simply made and tests out perfectly on the bench.
I am still a little skittish around the high voltage when I don't have a schematic... blew a board out a couple of years ago because my hand shook at the wrong time and in the wrong place. Gave me a new respect for these PWM HV circuits. I now try to make the connections before I power....

Anyway thanks... I have a nice scope but that would be last resort. Guy keeps worrying me to death... if the control board is cheap I will probably do that.

[tom66]

Re: LG 42PT350-UD Audio no picture and no prime.

One thing I just noticed, make sure all the board screws are in place before you fire it up, if you do go get a control board. Failure to do so can damage the boards due to lack of ground. These sets, and most modern plasma TVs, use the chassis as part of their ground for the boards.

[tom66]

Re: LG 42PT350-UD Audio no picture and no prime.

One thing I just noticed, make sure all the board screws are in place before you fire it up, if you do go get a control board. Failure to do so can damage the boards due to lack of ground. These sets, and most modern plasma TVs, use the chassis as part of their ground for the boards.

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

One thing I just noticed, make sure all the board screws are in place before you fire it up, if you do go get a control board. Failure to do so can damage the boards due to lack of ground. These sets, and most modern plasma TVs, use the chassis as part of their ground for the boards.

I took the photos when first set it on the bench. Screws are all in now. Fellow it belongs to had them in a zip lock bag.....

Control boards were dirt cheap.... one is on the way, supposed to arrive next wed.. 30th

I will re-post result.

thanks again, I feel 80% confident the control board is the dog

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

Well I almost don't believe this myself... decided to kill some time while waiting on pc scans and put this tcon under the microscope to see what is what. I noticed (when in the TV and on) that there was no voltage at IC171. I pulled up the datasheet for it and found out it is a 1.2v regulator. So I followed the input pin... tested continuity to ground and bam dead short... a BAD CAP.... I haven't pulled it yet but I will and see what happens when i replace it..... gotta be kidding me..... that can't be it.....
Pic has cap in red IC171 in yellow pinout is left to right 1=Gd 2= output 1.2v 3= Input +Voltage that is shorted


The short is actually in the big chip IC201.... still glad to know ... back to waiting til wed

[tom66]

Re: LG 42PT350-UD Audio no picture and no prime.

Yeah, that was about what I expected. Bad control board.

[Doc38343]

Re: LG 42PT350-UD Audio no picture and no prime.

FINAL POST: Fixed
OK part came early. Not complaining about that.
Installed the control board and it works perfectly.
This is the second plasma to run around the Y-board loop only to be
a cheap simple control board fix.
Audio but no picture, test the control board as well as the y-board.

[tom66]

Re: LG 42PT350-UD Audio no picture and no prime.

Nice fix. These aren't terrible sets - the build quality is pretty decent. The boards that you'd expect to fail, the PSU, Y-sustain and Z-sustain, are all pretty rock solid.
I recall a PJ350 not to long ago on here with a control board failure too. May just be a ceramic cap under the CPU instead of the actual CPU.