LG42" plasma

[Cornboats]

LG42PQ3000. Standby red to blue. Screen channel identification- full screen for a few milliseconds then Ysus and z sus voltages drop. Sound is there but no picture.

Ysus- -vy from -175v down to 3v & Vsc from 145v down to 15V

z sus 80v down to 50v

VS, Va and 5v remain.

When I disconnect supply from y sus to z sus Ysus voltages remain up.

When I disconnect Y drive Ysus and z sus voltages remain up.

I thought it could be the Y drive board but I've had 2 boards from different suppliers

with no difference to the outcome. Does anyone know the best way to check the Y drive board in case I've been sold duff boards?

I'm loosing my patience with this one so any help or advice please.

[downunder]

Maybe you're on the wrong tack. You say that when you isolate the Z-SUS, the voltages stay up. Z-SUS board is the main problem on this chassis. And you need to monitor the heatsinks on the replacement Z-SUS. If they get very hot very quickly, you'll need to change the Control board as well

[Cornboats]

Thanks for taking the trouble to reply.

I've tried a replacement z sus and control board with the same result- but of coarse like all other replacement parts theres no gaurantee that you swopping for a working board.

Would you expect the Y sus voltages to dive aswell if the z sus was faulty?

The Ysus VSC drops from 145V to 15V.

I disconnected the LVDS form the control board and the voltages stay as label. To me this means that its the screen load thats pulling down the boards which points me back to the Y drive board ,which has just been supplied.

Would you know how to test these drive boards. I've done a diode test across from input to output connections but now I'm stumped.

[downunder]

Perhaps I'm on the wrong tack. Is this correct?.....PSU supplies Vs and Va to the Y-SUS, thence from the Y-SUS they are fed to Z-SUS. If unplugging the Z-SUS from the Y-SUS, with everything else connected, restores Vs and Va, then I'd expect the Z-SUS to be dragging them down. Re the Y-Drive, apart from the diode test, if it uses 2 drive boards (upper and lower), then unplugging one at a time from the Y-SUS may help. Usually if you unplug top one, say, and it's faulty, you'll get a picture in the lower half of screen and vice versa.

[Cornboats]

Hi
Looking at the connector going from the y-sus to the z -sus it reads;
+Vs
+Vs
NC
Gnd
Gnd
NC
ER.com
ER.com

When the Ydrive board is disconnected ( theres only the 1 on this model) Y-sus and z sus voltages stay up.
This also happens if the control board LVDS cable is disconnected.

The VS and Va voltages coming from the smps board remain as per label always.

There must be a way to test these Y drive boards other than by substitution.
 
 

[downunder]

Ok, my bad, must have misread your thoughts. Some other forum member may have further thoughts on how to test Y-Drive boards, or there are a few youtube videos relating to this, although I find such videos tend to skip over finer details.

As a matter of interest, there should be a cable between the Z-SUS and the buffers along the bottom of the display panel. Try unplugging that and see if Vs stays high. Couldn't download the service manual last night,
but I've had the occasional faulty panel dragging things down until I unplugged that lead. Just an idea.

Impressed by your diction, btw. Should gladden the cockles of Turnip's heart.

[Cornboats]

Hi. I disconnected bottom buffers as 1 of the first checks. Y sus and z sus stay on which made me think these may be the culprit so I changed for another set of known working. Same symptons I'm afraid.
 

[downunder]

That's a worry, then. I'd be inclined to suspect the display panel itself. You appear to have run out of PCBs
to replace.

[Cornboats]

I think you might be right.
No one else has come back to me regarding how to test y drive buffer boards. Pity!

Thanks for you're input anyway.