LG42PC1D power board swap

[tom66]

Re: LG42PC1D power board swap

PFC circuit puts 400V across those large 450V caps on PSU primary.

[flocko]

Re: LG42PC1D power board swap

did have the same thing with the other power board had to drop the Vs to 170v to get a cleaner video that is why i thought it was not giving out enough wattage, and why i rebuilt the old board, would a bad control board do this if you lift the Vs it looks like an LCD with a bad T-con

[flocko]

Re: LG42PC1D power board swap

Have started from scratch with the set up again, have set
Vs 186v
Va 60v
-Vy -200v this was where i went wrong yesterday if you reverse the meter leads it reads 193v or -200v when across the resistor
Vsc 120v read 120v across the cap either way
Vz bias 100v all voltage taken with white screen and no av input
with the screen showing menu or the poor signal jumping around looks good, as soon as i put some video input in to it thats when it goes bad,you can still see the menu in the background, could this be a main board problem.

[tom66]

Re: LG42PC1D power board swap

You have severe image retention (IR); this is usually -Vy, Vsc or the SETUP/SETDN/RAMPUP pots.

[flocko]

Re: LG42PC1D power board swap

Thanks tom I think your right every time i check -Vy its changed and the little stand off board when you flex it the voltage comes and goes, i will look into this and let you know what i find

[flocko]

Re: LG42PC1D power board swap

just checked -Vy and set it again to -200v on white screen when you turn this off the -Vy jump to 239v does that sound right,in ism method i have been trying to trim in setup/ set down pots by jumping from normal/ white/ orbiter and inversion,i seem to get one looking ok then the next looks bad, do not seem to be able to get a good setting to cover them all
In ISM the readings for =Vy
Normal 239V
White 200v
Orbiter 239v
Inversion 200v

[tom66]

Re: LG42PC1D power board swap

I would try to replace the 10u400V and 22u400V Samwha caps (RD/RG usually) on the -Vy and Vsc module, as bad capacitors can cause this issue and I have often encountered bad Samwha.

[flocko]

Re: LG42PC1D power board swap

The ramp up in voltage from white to normal does this show a problem,and the polarity with the meter leads across the resistor why do the voltage readings change when inverted, any way am going to pull the board out and see what caps i have and change at lest the ones on the modules thanks again for your help Tom Flocko

[tom66]

Re: LG42PC1D power board swap

Panel draws more power on white, so seeing a drop or increase in Vy with brightness changes indicates an issue with those capacitors or some other part of the Vy module.

[flocko]

Re: LG42PC1D power board swap

had hoped that these modules were plugins would have made life easier to change the caps, do you normally take them off the main board or do them were they stand.If i take out the two caps in between i may be able to just get in there,have to order caps think i will replace all of them.

[tom66]

Re: LG42PC1D power board swap

Never done them myself. I'd imagine desoldering the caps from the board, with it still on the Ysus, may be possible but it will be difficult.

[flocko]

Re: LG42PC1D power board swap

Hi all back to this one today, have changed the caps on the stand off boards and that made it a little better, but i was still seeing the voltage jump up and down on the -Vysame as before, and the only cap i had not changed was the 150Uf 100v samwha WB, i have had a problem trying to find a replacement, i found a 100ZLH150MEFC and put that in but now i cannot get the -Vy above - 165v when i am looking for -200v can anyone put me right with this

[tom66]

Re: LG42PC1D power board swap

ZLH may be too low ESR for that circuit. i.e. It's TOO good.

[flocko]

Re: LG42PC1D power board swap

Hi Tom It was about all i could find. but with it in i dont seem to be able to adjust -Vy just sits at 165/7v in full white screen but when i come out of menu back to normal -Vy jumps to 235v.