LG 50PS80 51" plasma - startup issue

[millermankjm]

I picked up an LG 51" that a seller said had a white screen. When we turned on, it had the normal unconnected TV noise. Took home and it showed a good picture on HDMI and composite inputs. After attaching the stand and flipping it 180 end on end to place on storage cabinet to view, it did not stay on for more than 5 seconds first time powered up, 2nd stayed on for 7 seconds and third it stayed on coming up with the very bright image levels (faces white) and little blacks. Key here was that when the tv shutdown the first two times, it showed the correct image as it was powering off thus leading me to believe that the screen is being overdriven while its stays on now. Today, I try to power up and it shuts off by itself within 10 seconds and continues to do that. Removed the Y-Sus and Z Sus cables and stayed on without image. Put the Z Sus cable back on and it stayed on without image. Put the Y Sus cable back in and it power off after about 10 seconds showing startup full white then fuzzy image then off in 10 seconds. The image was from my Roku XD displaying its main menu. When the relays click off after the 10 seconds, a very quick correct image of the Roku main menu comes into view during the display power down. This is the same quality as I saw earlier when I just got it. Hoping that since I have a good picture that its a simple issue with the power supply not being able drive the picture properly or the Y Sus board drawing too much power. Any help would be appreciated greatly!

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

These are the pics of the startup that I see..

[tom66]

Re: LG 50PS80 51" plasma - startup issue

Please post a sticker of the LG ELECTRONICS sticker on the rear metal plate comprising the plasma panel. (E.X. LG Electronics PDP50G7##### with various warnings and volt ratings.)

One of your voltages such as Vs or Vset may be too high. This will cause excessive power consumption leading to power supply shut down.

Have you tested with a "mostly dark" image? e.g. no signal display.

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

Tom, Thanks for your quick reply! I checked both Vs and Va and the were 195v and 65v according to the sticker at the top of the chassis. The picture just looks too bright and especially I can see some burning from the previous owner. Could the Y-Sus board be driving the display too hard or the signal from the control board causing the Y-Sus to drive hard?

[tom66]

Re: LG 50PS80 51" plasma - startup issue

There are more voltages than Vs and Va to check, in particular -Vy, Vset (depending on age of panel) and Vzbias. The Y-sus produces -Vy and Vset, and the Z-sus produces Vzbias. Error in these voltages can lead to your symptom. Post a picture of the sticker please.

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

Here they are...

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

I check -Vy at -181 and VSC at 141 according to the label on the Y-Sus board during the 10 seconds it was on.

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

I checked Vzbias at 193 which is definitely not 100! Pulled the cable from the PS to the Z-sus and powered it up. Came to life and stayed on with a decidedly dark screen which is so much better. The picture is yes dark but consistant.. like the brightness is way down. It does have a consistant red tint to it. Soooo, looking at the Z-sus board now for physical issues. What would be driving it to that level without me smelling something burning or toasting the display? I wont plug back in until I can understand how the voltage can get to the resistor test point. The doc I have I can send you but is similar to the 50PS60 2009 Training Slides.pdf from this website http://www.toms-service-manuals.com/...ith&modelsrch=

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

Latest pics after Z-sus cable disconnected from power supply...

[tom66]

Re: LG 50PS80 51" plasma - startup issue

Since Vzbias = Vs, and Vzbias is generated by linear regulation of Vs, I would look into a short on the regulation circuit, in particular the regulation transistor. Red tint with no Z-sus is completely expected.

I have never seen excessive Vzbias, very interesting to see that it causes similar issues to excessive Vset or Vs (brightness much too high.)

Can you post a picture of the Z-sustain?

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

I adjusted the pot for Vzbias and it had no effect on the output. I carefully reviewed where the TP for that measurement was several time according to the doc.. Its a leg of a raised transistor. Attached are pics. I dont have the schematic so where to go now?

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

Should have been resistor instead of transistor!

[tom66]

Re: LG 50PS80 51" plasma - startup issue

I can't see those images clearly. It is preferred to show pictures of your boards, because minor changes between boards and sometimes failed components are visible in the photo.

No schematic, but the circuit may be relatively easy to reverse engineer.

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

My Zsus pics..

[tom66]

Re: LG 50PS80 51" plasma - startup issue

Can you get close pictures of this area? Of both top and bottom. Ideally, I need to be able to see the individual circuit traces and the numbers on the transistors of those heatsinks. I can possibly figure out how Vzbias is regulated

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

These are the best I can do..















I've had the TV on without the Z-sus connected and the image has not changed, audio is the same so all else seems to be stable. I did try the pic adjustments to brighten but as expected, no results which was expected.

[tom66]

Re: LG 50PS80 51" plasma - startup issue

I highlighted an area in particular; I'm not in need of pics of the whole board. Just need to see how those three FETs are set up on the heatsink and how the Vzbias is generated.

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

Since I dont have it plugged in... I'll pop it out and take some closeups as best I can.

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

Here is the back...

[millermankjm]

Re: LG 50PS80 51" plasma - startup issue

And the front....