LG 52lg70 PSU problem, no standby

[Glitcher]

Got this 52lg70 with no standby light and no 5v voltage. Opened up, found blown main ac fuse, and two shorted W20NM60 FETS. Replaced everything, plugged it in, still nothing.

Rechecked everything after, Fuse tested good, new FETs tested good. Measuring across main PFC caps, there is low voltage, about 3V. I also noticed that I don't think the main voltage is reaching the bridge rectifier either (low voltage across it, both in dc and ac). the rectifiers seems to ohm out good though. Any thoughts on this??

[dcapper]

Re: LG 52lg70 PSU problem, no standby

I had a bridge rectifier that read good but I would plug the tv in and the house lights would dim and the fuse would explode. I replaced it and it was all good.

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

Fuse is still good after the initial plug in through. I think the rectifier is good but its the voltage is not getting to it.

[tom66]

Re: LG 52lg70 PSU problem, no standby

Since this PSU uses a relay, it might not engage until 5V standby present hence no volts across big caps...
But to do this it would need a small bride rectifier and another 400V cap, do you see one?

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

I only see the one rectifier near the main ac. I don't seem to see any other 400V cap though.

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

Does the relay have to switch on for the voltage to reach the rectifier?

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

Still trying to figure this out... I tried to draw out the schematic for the problem areas. Probably not perfect though.

[tom66]

Re: LG 52lg70 PSU problem, no standby

Is TH100 good, i.e. not open circuit? (Should be 10 ohms cold)
The common mode inductors on the input, are they OK?
Do you have ~120VAC at the AC terminals of the bridge?

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

Th100 is good,, and the inductors are good also. But I do not have the 120vac on the AC terminals of the bridge.

[tom66]

Re: LG 52lg70 PSU problem, no standby

R111 good?

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

R111 is labeled as 5W 6.8ohm J. Is that just 6.8 ohms? I'm getting an open circuit reading or its just out of range for my meter. Seems suspect I think?

[tom66]

Re: LG 52lg70 PSU problem, no standby

That would be 6.8 ohms, you need to figure out why it went bad, it could be random failure but also shorted MOSFET, or bridge rectifier.

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

Ah I see, I did have those two shorted FETS (W20NM60) when I first opened it up

[tom66]

Re: LG 52lg70 PSU problem, no standby

What you can try is removing those FETs and installing a new resistor, set should come out of standby and have sound. No backlight of course, but it will prove the rest is working.

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

Just put in another 5w 6.8 ohm resistor and removed the FETs. Standby light is back! And looks like its able to turn on normally just no backlight.

Just put the FETs back in, and looks like it seems to be working normally again.

[tom66]

Re: LG 52lg70 PSU problem, no standby

So, it's fixed?

[Glitcher]

Re: LG 52lg70 PSU problem, no standby

Yep I think so, it's been running without any issues so far

[jayg83]

Re: LG 52lg70 PSU problem, no standby

Got this 52lg70 with no standby light and no 5v voltage. Opened up, found blown main ac fuse, and two shorted W20NM60 FETS. Replaced everything, plugged it in, still nothing.

Rechecked everything after, Fuse tested good, new FETs tested good. Measuring across main PFC caps, there is low voltage, about 3V. I also noticed that I don't think the main voltage is reaching the bridge rectifier either (low voltage across it, both in dc and ac). the rectifiers seems to ohm out good though. Any thoughts on this??

when you mean "no standby" do you mean it doesn't turn off?. I have a LG 52lg70 that when you turn it off, clicks off and turns back on right away. Also has the LED red light constantly on.Where is the R111 located? thank you for your help.