Wierd plasma screen behaviour as tv warms up. Anyone identify?


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Wierd plasma screen behaviour as tv warms up. Anyone identify?

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pnb
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Jun 20, 2013, 5:37 AM

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Wierd plasma screen behaviour as tv warms up. Anyone identify?	Can't Post

Hi all,

Gave up on repairing my old Samsung lcd after spending too much money and bought a 'new' 2nd hand tv: An LG 60PV250 60" plasma which is 2 years old (out of warranty of course).

A few weeks in and the TV is starting to behave strangely as can be seen in the following time sequenced photos after turning it on from cold (i did a factory reset just prior to these photos)

Initially picture is fine, then some slight blackness appears in the lower 2/5ths of screen after about 4 minutes. As the set 'warms up' this gets worse until it is almost completely black but occasionally flashes through the picture if the frame is bright. The image is still there, just hard to see or overlain with a ghost image.

That is two Korean TV's dead in a short time for me and I am really starting to rue the day I threw out my perfectly good 13 year old sharp CRT!!!
It used to be that owning a yacht was the way to tear up money, but this new 'improved' technology seems to be a money pit!

Photo's below.. progress from no problem to almost complete blackout after 15 minutes.

At some times a ghost the image from the top of the screen appears in the bottom section as can be seen...

Can anyone identify the most likely problem where I should start looking?

74f100
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Jun 21, 2013, 12:44 AM

Post #2 of 9 (2584 views)
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Re: [pnb] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

An odd sequence, but should be contained in the lower buffer. Try completely disconnecting and removing it, then see what happens. If the problem is in the lower buffer, you shouldn't have the screen completely white after running a few minutes.
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pnb
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Jun 21, 2013, 1:37 AM

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Re: [74f100] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

Thanks for the tip 74f100.

That screen being completely white is not part of the error, sorry I was not clear on that. At the end I set the screen to 'white background' in the menu to show the aberration area clearly and how it is not so much of an issue where the screen is very bright.

I think that is the lower Y-drive board you suggest to disconnect?
Is this still valid considering the white screen is not part of the issue?

According to the service manual, the TV should not be run with that board disconnected unless both are removed, but I guess if I remove both there will be no picture so impossible to tell if that is the problem?

I will see if I can check some voltages on that lower board however the manual says that not all voltages can be read as they are covered in silicon..

I am prepared to replace just that lower board "Y-drive board" but would like to be certain that is the problem first.

It is bizarre that it gradually gets worse. Almost as if a component is worn out and varies from specification once hot. Have to have a look and see if there are any valves in there!

pnb
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Jun 21, 2013, 1:41 AM

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Re: [74f100] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

What I find curious is the area of the aberration does not appear to correspond directly to the lower board control area. It is only around 2/5ths of the screen that has the problem, and in addition there is that weird feedback from data that should be going to the upper board (top part of screen) appearing on the bottom of the screen.

74f100
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Jun 21, 2013, 8:44 PM

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Re: [pnb] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

With just a bad lower (or upper), the whole screen can display the defect. Any thing that effects the ramp up pulse.

COMPLETELY remove the lower, and see how the upper half of the video looks. I've run them for a few minutes as part of troubleshooting. They put that in there because techs were removing the ribbon cable between the buffers, then leaving the lower connected to the panel and would blow the Y sustain. I told them about that in the training classes, but LG didn't want to change it (I proved it to the engineer).

But again, don't run it for more than 5 minutes or so. It does load the Y incorrectly. And when reconnecting the panel ribbon cables, make absolutely sure they are back in straight. If not you WILL blow the buffer again.
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(This post was edited by 74f100 on Jun 21, 2013, 8:55 PM)

pnb
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Jun 23, 2013, 10:05 AM

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Re: [74f100] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

Hi,

I made the test you suggested.

First off I checked and fine tuned all voltages ( VS, VA, -Vy, VSC, VZB) which had minor deviations of a volt or two.
I dont have an oscilloscope so cannot check waveform.
I completely removed the lower board Y-drive board (physically) including all ribbon and TCP cables and ran the set for a minute.

This is the result: multicoloured unmoving blotch. (On top half of screen and obviously exactly half of the screen, whereas the issue I am having is for an area less than half of the screen.
I only ran it for a minute. Not sure a 5 minute test would have made a difference as the problem normally only starts after the set has been on for 5 minutes.

Not sure what that means?

While I had that lower board out I took some photos. No visible evidence of damage but the main chips are covered by a heatsink.

Underneath the board there is some curious solder run under a few of the pins on 3 of the chips, but no short circuit.

Checked the heat of the Ysus board as the issue seems to be heat related.

One of these heatsinks gets astonishingly hot (see photo). But maybe that is normal?

Checked the back of the board, no evidence of bad solder joints due to heat, touched up the transistor legs to be sure. Inspected all other components with magnifying glass, no obvious damage.

At the bottom of the rightmost heatsink is a mosfet? which I suspect is the source of most of the heat on that heatsink as the board is much browner around it's legs than the other transistors along the line which are of a different type.

The chip is stamped as K8A60DA 1:8

I only have a 30w soldering iron, so I know it would be nigh impossible to get that line of chips on the heatsink out for testing.
There is a chance I could get it off the heatsink in-situ and then de solder it, but will only attempt this if this sort of chip has a high probability of causing the effect I described.
Apart from that the only observation I can make is a buzzing from the ysus board that corresponds to the lower half flickering but I guess that is only to be expected as signal frequency cuts in and out?

Put is all back together and it is back to it's old behaviour - after a couple of minutes the bottom 2/5ths start flickering and it gets darker/worse with occasional brief flickers of clarity.

(This post was edited by pnb on Jun 23, 2013, 12:43 PM)

pnb
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Jun 24, 2013, 2:30 AM

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Re: [pnb] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

Today I tried running a fan blowing cool air over the various components/boards to see if there was any effect of cooling on the symptom.

To my surprise cooling components with air did not appear to have any effect so now I am starting to doubt whether it is heat related even though the problem only starts after 5 minutes or so.

Starts off with an increasing darker section on the bottom of the screen, eventually it goes alomst completely black, then after a few minutes is lighteens up again but then there is this ghosting/duplication problem as demonstrated in this video.

i still have no idea which board is the culprit.

[URL=http://tinypic.com/r/5kmtcy/5]View My Video

Original Video - More videos at TinyPic

http://tinypic.com/r/5kmtcy/5

(This post was edited by pnb on Jun 24, 2013, 2:32 AM)

74f100
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Jun 24, 2013, 9:30 PM

Post #8 of 9 (2529 views)
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Re: [pnb] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

The fan won't make that much of a difference. I would have expected it to extend the time it takes to fail a little bit, but not correct it.

I still suspect the lower buffer, and the Y sustain second.
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pnb
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Jul 9, 2013, 12:05 AM

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Re: [74f100] Wierd plasma screen behaviour as tv warms up. Anyone identify? [In reply to]	Can't Post

Thanks for your help with this 74f100.

You were correct it was the lower buffer card. Tom66 from another forum gave me a good idea to verify the relevant card prior to ordering a replacement - by heating up the cards individually prior to power-on to force the problem to occur from the get go. This method confirmed 100% to me that the lower buffer card was the problem so I ordered a replacement with no fear of wasting money.

TV is working a treat again. Thanks again. I can see this can be an interesting hobby so I am on the lookout for another TV to fix.

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