LG 60PX950 Plasma with almost no picture, but good audio

[nadelman]

I have a 34 month-old 60" plasma that one day stopped displaying a picture. Since it was tuned to a live input I could hear the audio perfectly. Actually, the picture was dark except for the floating "No Signal" box that was extremely intermittent in flashing on the screen. Like it would flash on for maybe 2/10 of a second then nothing for 5 seconds then again, flash for 2/10ths. Also some white sparklies (?) in a line at the very top of the screen. Almost like a digital signal blinking in spots on/off.

Reading threads here led me to believe it was the YSUS and seeing a few posts about having a bad buffer board kill a good YSUS, I figured I would just buy all 3 and replace. So, I ordered and installed new YSUS and U/L buffer boards. Nothing. I was pissed! I thought I had it! Now the floating box and white lights don't appear at all.

I have yet to check voltages, and yes, have a DMM but no scope (and dont remember how to use a scope). Also, own 3 soldering irons and am decent with them.

Oh, the remote works perfectly. I can change the input (blindly) and cycle through the different inputs. Red light on lower edge of bezel lights like normal when turned off, and Blue light on lower edge of bezel lights up like normal (I think) when powered up.

I hope I've covered as much as I can remember to. Questions? Suggestions? Help, please.

Thanks guys.

[capkid]

Re: LG 60PX950 Plasma with almost no picture, but good audio

I'll wait for the voltage readings.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Oh, one more thing, I did check all the fuses (from Tom's troubleshooting guide) and they're all good. Will check voltages and report back...

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

OK, voltages checked. Seem ok?: Sticker says Va: 60 Vs: 203; tested at Va 59.6-59.7 Vs 201. 5v also at 5.1v. Oh, and for some reason I thought the hi voltages in a TV were AC, but DMM wouldn't register until I put it on DC. :0 Just shows how much I know! Oops.

CTRL board green led was blinking 3 short, 1 long, but then changed to 1 long/1 short twice then a 4 sec delay and repeated.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Oh, and all clicking of relays (?) all sounds normal, btw.

[capkid]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Ok, your PSU voltages look good. Also, you need to check the sustain voltages.

Y-Sus: -Vy and VSC. From what I can tell by a pic online, it appears that the -Vy and VSC test points are a couple resistors - see pic. Black and red arrows represent your meter's probes.

Z-Sus: Vzb. I don't have a pic of the Z-Sus, but you should see a test point somewhere on it.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

OK, here is a pic of the back. Will check other voltages next.

[capkid]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Unless you've already found the Vzb (Zbias) test point, post a closeup pic of the Z-Sus so that I can help you find the test point. On some of these boards, it's hidden under adhesive.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

OK, I think I found something. The VSUS voltages are bouncing around. Vy jumps from 0 to 27 to -13, etc... VSC does the same also from 0 (mostly) to 24 to -13, etc. Checked Z-sus Vzb, and it looks good at 103.9. Sticker says 105.

Now here's something else I dont understand. When I first found the screen failure, the "No Signal" bouncing box would flash (although briefly, but would flash). Since Ive take it apart, it doesn't do that at all. Nothing on the screen. Nada. Bopkis.

Also, given that I stupidly presumed that it was the YSUS board, I ordered and replaced it and BOTH buffer boards. So, not finding stable voltage on the YSUS board, I presumed TV Parts Guy (where I got the replacement boards) must have sent me a bad board so I put my original presumed faulty) YSUS board back in (left the new TV Parts Guy buffer boards in) and still the Vy and VSC voltages are still not stable, and with my old board back in the floating "no signal" box STILL doesn't appear. However I did notice that the blinking green LED on the control board is back to 3-short, 1 long.

[capkid]

Re: LG 60PX950 Plasma with almost no picture, but good audio

So, assuming your PSU, Z-Sus, Y-Sus, and Y-buffers are fine, the only boards left are the main board and the logic control board. I don't know of a self-test for the control board in this TV, but a self-test would normally tell you whether the main board or the control board is faulty.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Is it possible that any of the Y-Sus or buffer boards are in fact, bad even though I replaced all 3 from a vendor? I decided to disconnect the buffer boards and low and behold, voltage appeared on Vy and VSC close to the sticker values. Interesting, though, was that while I was measuring, one of the probes slipped and a humming that I had been hearing, stopped. Then the voltage disappeared. I turned it off/on and the humming was back and the voltage was back/stable.

I then looked at the front and saw lots of vertical lines, mostly white, but a few were interspersed with lots of color.

So, what does it mean when you remove the 2 cables from the YSUS board to the buffer boards and voltage that was unstable (on both AC and DC setting on DMM) was now stable and very close to sticker values?

As to the main/logic and control (is that different from the other 2 or is it the logic bd?) board tests, I thought I saw a post from Tom66 mentioning jumpers to test either the main or control boards. Any idea what Im talking about?

Thanks for all your help.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

What does it mean if the Vy and VSC only test ok when the Ysus buffer boards are disconnected?

[tom66]

Re: LG 60PX950 Plasma with almost no picture, but good audio

It means one or both buffers are shorted because the vendor hasn't tested them. Often cracked panels that they are salvaged from damage the buffer boards. I say put your old buffer board back in -- it's unlikely to be failed.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Thanks Tom. When I put the old Buffer boards in, should I use the vendor's YSUS or stick with my own (which is where I started)? I guess should I replace them one at a time? Can you suggest a protocol?

[tom66]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Stick with vendor's Y-sus, and replace both at once.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

OK, put the vendor's Y-sus and my original buffers (u/l). Got the floating "no signal" box back, but it only blinks on for 100 msecs or so and then off for a second or two. Changed the input to the tuner, and got faint snow only in a pixel-thick image along the two sides pof the panel. Tested initially, and Vy and VSC were bouncing across 0. Then checked again, and they were stable at/near sticker values. But still flashing the bouncing box on screen.

Then I think I messed up. As I was testing the Vy and VSC, my probe slipped and hit something else on the board, and I looked up to see a small puff of smoke, and then, of course, the Vy and VSC were no longer in spec. Vy went way up from 178 to 280 and VSC down from 149 to 18.

Did I fry the board? However, when it WAS good, there was still no picture. Seems like the original problem isn't on the Y-sus side because Ive now tried both my and the vendor's Y-sus and buffer boards. The only combo I haven't tried is my Y-sus with the vendor's buffers, or any combo of vendor/original buffer boards.

I'm wondering if that test that Tom suggested of disconnecting one buffer board to look for a stripe might tell me something.

[tom66]

Re: LG 60PX950 Plasma with almost no picture, but good audio

If the -Vsc and Vy were still varying, it's possible that your new Y-sustain was bad. It's hard to be sure though if it doesn't work well any more. Isolating the buffers can rule out any obvious shorts.

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

How do I isolate the buffers?

[tom66]

Re: LG 60PX950 Plasma with almost no picture, but good audio

Just undo all connectors, but leave them connected to the plasma panel (the orange ribbons.)

[nadelman]

Re: LG 60PX950 Plasma with almost no picture, but good audio

So I tried all combinations of mine/vendors Y-sus and buffer boards. The net result was that whenever I connected the buffer boards (and one test with just the upper board) the voltages were low/unstable, and whenever I disconnected the buffer boards from the Y-sus the voltages returned to normal/semi-normal (semi-normal meaning 144-148 instead of 148-149 and 173-178 instead of 178-179).

So does that mean BOTH my and the new(vendor) buffer boards are shorted?