LG 50PA6500 no picture, random pixels.

[Nathaniel17]

Hi everyone,

I have a LG plasma TV, model 50PA6500-UG that has recently stopped working. Would click when it was powered on then turn off again, had standby light working when off. Did some troubleshooting and determined that the Ysus board was no good so I replaced it.
Now all I get is random pixels going down the screen(Mostly red), menus function but do not come up on screen(can change output, change channels and adjust volume. Don’t know where the remote is, someone in the house lost it. So I am stuck with the controls on the front of the TV.
Both Ysus buffer boards appear good and ribbon cables have no damage.
Let me know where to start troubleshooting. Have a multimeter and soldering station. Can provide pictures if needed. Thank you.

[Nathaniel17]

Re: LG 50PA6500 no picture.

Here are the pictures. The pixels have gone away. TV does light up light grey for a second now before going black. Going to try hooking it back up to a video source in a bit.

[Biruslapio]

Re: LG 50PA6500 no picture, random pixels.

Are the buffers really fine? did you measure all the panel output pins? These renesas R2A2XXXFT ICs fail all the time, then the Y-SUS goes into protect mode and shuts down.
To exclude the no video source situation you need to unplug LVDS cable from logic board and short the two EXT_AUTO_GEN pads, this will make the TV display a test pattern.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Sorry still new to this, learning all the technical abbreviations and whatnot. Could you break that down a bit more so I can better understand it or direct me to a resource or thread that will help me understand it better? I would really appreciate the guidance.

[tvtimmy]

Re: LG 50PA6500 no picture, random pixels.

Here.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Thank you! I’ll give that a go.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

So... yeah I got no result. No change.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

This is what I got, in a dark room.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Going to try disconnecting the Y buffer boards one at a time from the YSUS board to see if that makes a difference.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Disconnected the top Y buffer board. Got this as result.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Reconnected the top board and disconnected the bottom board and the pixels came back. On the top half of course. Not the best picture but best I could do.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

I’m guessing bad buffer boards? Let me know what you all think.

Also reconnected the bottom board and no more pixels. The TV is definitely doing the test but not displaying anything. Can hear the different components on the X and Y boards make sounds at regular intervals.

[Biruslapio]

Re: LG 50PA6500 no picture, random pixels.

If the buffers are behaving weird, you should take them out and probe them with a multimeter.


https://www.youtube.com/watch?v=3Oq2MsU0XOQ

You put one probe on the big floating ground traces and the other one on the pins that go to the panel.

Anything measuring less than megaohms indicates a bad buffer board.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Both board pins all read around 860-865. No shorts.

Reconnected the boards, plugged in and turned on the tv and had it do the auto gem test again. Pixels were back but they were clearly corresponding with the test. Red, then green, then blue, then white and then different patterns.

Beginning to wonder if the new YSUS board has its own issues.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Also while I was at it I got a better picture of the IC from buffer boards.

[Biruslapio]

Re: LG 50PA6500 no picture, random pixels.

I've also happen to have two LG 60PA6500 top buffers with some of these exact IC shorted, they sometimes short like two pins, so you have to be careful when measuring, what happens when they go bad but not explode is that you get display, but VSCAN voltage will be floating up and down and the image will look horrible.
Z-SUS may be at fault too, but first, measure these voltages and report what they are measuring when all the buffers are connected, the panel has a label on the top center part, it contains the info about these voltages and what they should be, at your first photo of the whole TV's back it can be seen but not read because of the resolution.

VSUS
VA
VSCAN
-VY
ZBIAS

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

I am assuming that those values need to be read with the plugged in and powered on, right?

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

VSUS 203V
VA 55V
VSCAN all over the place
-VY 95-98V
ZBIAS 148V

VA doesn’t seem to go to YSUS board so grabbed it from the ZSUS board at where it comes in.

[Biruslapio]

Re: LG 50PA6500 no picture, random pixels.

According to the panel label:
VSCAN = 150V
-VY is supposed to be -190V
ZBIAS = 150V

Seems you have a problem with -VY, try running only Y-SUS board without buffers and see if -VY and VSCAN stabilize and are closer to what the panel specifies.

In my other post I also said that VSCAN floats when these buffers are bad, you don't even need to remove them from the TV to do the testing, just lifting the plastic clamp should reveal half of the pins that goes to the panel (the other half being covered by the panel connectors).

Positive probe in the floating ground and negative one on the pins, in diode mode you should read 0.75V or more for each one, you said 0.85V with your multimeter, when they are bad they will go straight to 0 or 0.1V. You did this but I asked again because sometimes only a few pins short and you can miss them if your multimeter is slow or if you go too fast.

If the buffers are normal and -VY and VSCAN become stable without them, then probing Y-SUS and Z-SUS boards is the next step.

[Nathaniel17]

Re: LG 50PA6500 no picture, random pixels.

Yeah I checked the buffers very carefully and slowly.

Checked the VSCAN and -VY with the buffer boards disconnected from the Y-SUS.
VSCAN =46-51
-VY= 47-51 but mostly 48-50
Checked them with DCV because of the inverters on the main power board (at least I think they are inverters).
If I check them with ACV I get 162V on the VSCAN and 64 on -VY but my multimeter does not like it. So I am assuming that it is not AC.

Included a picture of my multimeter so you can see what I am working with and to make it easier to let me know what settings to use on it. Don’t have a lot of experience with higher voltage stuff so I don’t want to make any mistakes.