LG50 PC1D plasma

[Cornboats]

I have a fault with my LG 50 plasma. Sound but no picture. I've substituted ysus board and have full picture and sound so now I know its the Ysus. Checked voltages VSC =120v is ok, but -VY is only 25v instead of -200v.
Theres an add on upriser board with a potentiometer for adjusitng the VY so I've swopped this board but voltage still remains low. I've also replaced ipm's for known working ones.
I'd like to trouble shoot this to component level if I can. Does anyone know what the -vy does and how the voltage is derived?

Ysus pics uploaded -board number 6871 QYH039B

[Andrew F. Ali]

Re: LG50 PC1D plasma

How are you measuring -Vy? It should be across a resistor on the Y-sus Bd.

[Andrew F. Ali]

Re: LG50 PC1D plasma

It may be across R19 on the Y-Sus Bd. Though I'm seeing two -Vy test points. Not sure if it is referenced to chassis GND.

[Cornboats]

Re: LG50 PC1D plasma

Hi Andrew thanks for taking the trouble to reply.
There are 2 test points for -vy located just in front of ramp up circuit HS2. The manual shows using both meter probes across them.

Neither test points are referenced to the boards ground. I'm looking at the LG 2007 troubleshooting manual for this model and it says the ysus has a floating ground. Theres no continuity between either point and the boards ground, but this is the same on the working board aswell.

Theres a bank of power fets HS1 for making the waveform drive, but having checked these appear to be ok.

[Andrew F. Ali]

Re: LG50 PC1D plasma

OK. So to measure -Vy you go across the two Test Points. -Vy at 25Vdc. Is this with all the boards connected?

[Cornboats]

Re: LG50 PC1D plasma

Thats right, all boards are connected. The test points are just below PS101 -vy riser board in 1st photo. The working board allows + or - adjustment using the potentiometer on the -vy riser board, which I swopped, hoping it was that.
The label on the back of the set advises -vy to be -200, faulty board is reading -25v . No picture on the set completely black, just sound.