LG 42PX4RV-TA, Black screen but sound ok

[reality]

Hello everyone, there is a Lg LCD 42PX4RV-TA i am working on, it powers up and produces sound when connected to cable but the screen is black, no picture or even backlight. Can anyone please help?

[newtothis]

Re: LG 42PX4RV-TA, Black screen but sound ok

Seems like your post got overlooked.

Pics of boards... one of each board, one of entire back of set so we can see all boards together... and pics of any silk print next to any connectors on the PSU

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Thanks a lot Newtothis for noticing my thread, i was wondering.
Please, here are the pics;

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Please, help!!!

[newtothis]

Re: LG 42PX4RV-TA, Black screen but sound ok

Ok do you have a flashlight?

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Yes! I have one.

[johnboy1313]

Re: LG 42PX4RV-TA, Black screen but sound ok

The boards to the left and right of the power supply are your sustain boards (the ones with the large heatsinks on them). Chances are, the one on the left, the Y-sustain, has gone out. Do you have a digital multi meter? If so, check the fuses on that board. It wouldn't hurt to check the fuses on the board on the right as well.

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Thank you! I have checked all the fuses in both the Y-sustain and Z-sustain and they are good.

[tom66]

Re: LG 42PX4RV-TA, Black screen but sound ok

Do you have good Vs, Va voltages going to your Z- and Y-sustain boards? Compare to the plasma panel label.

[newtothis]

Re: LG 42PX4RV-TA, Black screen but sound ok

Ignore me... Didn't realise it was a plasma lol

[freakaftr8]

Re: LG 42PX4RV-TA, Black screen but sound ok

Also monitor the heat coming off the heat syncs on the far right and left boards. One of them warm and one cool? Both should be warm after 10 minutes or so of operation. A bad buffer board can blank out the picture. But yes check voltages first.

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Plasma panel board board indicates Va=65v, Vs=188 while my meter reads Va=67v, Vs=190v. All heat sinks in both the Z-sustain and Y-sustain boards are warm few minutes of operation.

[tom66]

Re: LG 42PX4RV-TA, Black screen but sound ok

Those voltages are good. Now turn out all the lights in the room and close curtains. Can you see a glow emitted from the screen?

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Thanks Tom, i have done as you instructed. There was no glow at all.

[tom66]

Re: LG 42PX4RV-TA, Black screen but sound ok

OK, check the -Vy, Vsc and Vzbias (or Vzb.)

This manual:


should cover the PDP42V7, which I believe is the panel in your TV (confirm by Vs/Va sticker)

It indicates where the tests are made, and how to read the panel's voltage label. For now, the Vset-up does not need to be tested.

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Thank you very much T om for the provision of the service manual. It was really needful and helpful. Below are the readings i got;
Meter Readings: -vsc= -58v, -vy= -59v
Label Indication: vsc= 115v, -vy= -75v

[tom66]

Re: LG 42PX4RV-TA, Black screen but sound ok

Looks like you have a shorted Y-buffer board, this connects to Y-sustain. Unplug the Y-buffer from the Y-sustain (you may leave it connected to the plasma panel ribbons) and see if the voltages rise. Note: be very careful with the orange flex ribbons on the plasma panel, they cannot be replaced if broken!

Since TV uses two buffers it may be possible to run it temporarily with only one buffer to confirm half the plasma panel works. Sometimes you will get half a picture, other times you get random picture noise. You can at least see if the voltages rise with one board removed.

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Please I am very sorry Tom, i did not read the voltages properly, placed the leads on the wrong points.
Here are the right readings;
Meter Readings: vsc= -114.5v, -vy=-69.5v
Label Indication: vsc= 115v, -vy= -75v

[tom66]

Re: LG 42PX4RV-TA, Black screen but sound ok

We all make mistakes, no need for apologies!

Are you sure the Vsc is negative? Is that with buffer boards in or out?

Next verify Vzbias. (See manual pages as linked previously. Voltage on the Z-sustain board, opposite the Y-sustain.)

If you do not have any panel glow then the fault may be on the control board, which is underneath the main video board (which has the A/V inputs and TV tuner.)

Any access to an oscilloscope?

[reality]

Re: LG 42PX4RV-TA, Black screen but sound ok

Yes, the vsc is negative with buffer in. As for the Vzbias, my meter reads 165v exactly and that is what is indicated in the Label. I do not have an oscilloscope presently.