LG 50PT350 sound but no picture

[ultimatt]

First, I'm new at the whole TV thing. I have some experience with vintage audio but this will be only my 2nd attempt at at TV (I was able to fix a 2008 Panasonic Plasma).

When I power the unit up I get a relay click and sound but no picture at all, the screen does not light up at all. The unit accepts commands from the remote and the volume works. The problem started about a month ago, I would turn the set on and get no picture. If I turned it off then back on the picture would come on. The problem got worse and I might have to turn it off then back on 5-6 times before the screen would work. I would like to try and fix this set.

So, the Va and Vs voltages checked out as dead on. I have no idea where the test points are for any of the other voltages. I checked every fuse I could find and they all checked out.

Most of the info I can find says that the sound but no picture problem is likely a YSUS problem with either the board, the buffers or both. However I would like to confirm the problem before I start throwing parts at the unit.

Any advice or guidance would be appreciated.

Thanks

[capkid]

Re: LG 50PT350 sound but no picture

Can you post a pic of the label that lists the panel voltages? I've attached pics with the test point locations on the YSus and ZSus.

[ultimatt]

Re: LG 50PT350 sound but no picture

OK...sticker reads:

5v/Va:55v/Vs:205v
300v/-193V/NA/90v

took measurements and got following results -

Va=55v
Vs=205.3v

YSUS -
+VSC = -35v
+VY = 14.4v

ZSUS -
VZB=89.8v

so, I assume the 300v and -193v on the sticker are the YSUS voltages and I am way out of spec.

If I need a new YSUS board is there any way to test the 2 Y Buffers to see if I need them as well? Is there a particular resistor or transistor that fails on the YSUS that can be replaced and upgraded? I do not see any burst or swollen caps on the board.

Thanks.

[Hamie]

Re: LG 50PT350 sound but no picture

Before you buy a new ysus, unplug the buffer boards and check the voltages again.
You might find when the buffers are unplugged the ysus voltages are ok and it's the buffers dragging them down.

[ultimatt]

Re: LG 50PT350 sound but no picture

Ok...unplugged power ribbon from lower Y Buffer and re-measured

YSUS -
+VSC = -35v
+VY = -178v

So now +VY has now gone from +14v to -178v?

[Hamie]

Re: LG 50PT350 sound but no picture

Ok ysus is bad. Probably not that hard to repair.
Check for shorts on the heatsink igbt's and diodes.

You may also have shorted buffer ICs, you'll need to check these.

There are a few videos on YouTube how to check buffers.

[capkid]

Re: LG 50PT350 sound but no picture

I'd totally disconnect the buffers from the YSus then retest. Also, I'm not sure what the 300V on the panel label represents, but VSC is normally somewhere around the 130-150V range for most LG panels.

[ultimatt]

Re: LG 50PT350 sound but no picture

OK...ordered a YSUS board.

One question, when checking the IC's on the Y buffers I should be checking between the ground(s) on the power to the buffer board and all the pins on the IC ribbon connectors to see if I have a short to ground...correct?

Also, If I miss one could a bad IC kill the YSUS board?

Thanks

[capkid]

Re: LG 50PT350 sound but no picture

OK...ordered a YSUS board.

One question, when checking the IC's on the Y buffers I should be checking between the ground(s) on the power to the buffer board and all the pins on the IC ribbon connectors to see if I have a short to ground...correct?

Also, If I miss one could a bad IC kill the YSUS board?

Thanks

I believe you can check that buffer as you described - not sure, though. LGs buffer failures are normally more forgiving than Samsungs, so a bad buffer shouldn't kill a YSus.

[ultimatt]

Re: LG 50PT350 sound but no picture

Well...luckily I was able to cancel my order for the Y Sus board.

I did some more digging and found a thread talking about problems with the logic board that sounded like my problems. The very tiny LED on the logic board was not lighting up when the set was turned on. No real way to test the board, but took and chance a bought another one for $25.

Installed new logic board and set fired right up.