View Full Version : LG RU-42PZ90H screen issue!


holmesuser01
08-13-2017, 03:39 PM
Hi gents.

Working on my first plasma flatscreen. Picture is good, color is good. Near the center of the screen is a 2" or so wide black stripe from the top to the bottom of the screen. Sometimes, it will light up this space and look fine, but most of the time this bar is present.

I've got the back off and have tapped around the circuit boards, and it made no changes. This bar is there on all of the different function settings.

I'm an old NTSC repair guy in real life.

Any ideas? All will be appreciated!

Bruce

Findm-Keepm
08-13-2017, 10:20 PM
Dunno if this will help, but these sets had a reliability upgrade:

http://knobsandpots.com/images/KK%20Model%20SVC%20Bulletin(Reliability)_0507.ppt

Caps, again.

CoogarXR
08-14-2017, 08:51 AM
Plasma TVs have buffer boards that run along the top or bottom of the panel (early large plasma TVs have them on both, but not so much anymore). Anyhow, each buffer board has a ribbon cable that connects to the panel, and controls a vertical "stripe" of the TV.

That being said, you have one of 3 problems:
1) The ribbon cable just needs reseated (open the ribbon cable connector, pull out the ribbon, wipe it off, push it back in and lock it down)
2) The buffer board is bad (they're cheap on ebay usually)
3) The ribbon cable itself is bad, or the bond between the ribbon cable and the panel is failing (not fixable by the average Joe)

holmesuser01
08-14-2017, 10:47 AM
I found a warm capacitor on the module labeled 6871qyh027a (listed as main board on ePay) It's labeled C57, and is a 680uf 250v cap. C58 right beside it is cool.

Could it be this easy?

I moved all the ribbon cables around the chassis and nothing caused the stripe to come and go.

Occasionally, it will turn on from cold with a full good picture for a few seconds, and then the stripe re-appears.

Think I should just try replacing these two?

CoogarXR
08-14-2017, 01:37 PM
You sure you are moving the right ribbon cables? The ones I am referring to are probably under a heat sink along the bottom of the screen. The board number you mention is the YSUS, which is a common issue on those TVs, but won't cause the issue you are experiencing.

holmesuser01
08-14-2017, 04:41 PM
I saw ribbon cables on each side of the screen. I'll go back and look around the bottom of the screen and get back. Thanks for all the suggestions. I'll worry about the warm caps later.

Edit: Found the ribbon cables you mention. Had to disassemble the brackets holding the various inputs and outputs. Now I need to support it differently so I can power it up and move the cables at the same time.

Disassembled a bit further. Moved all the ribbon cables with a plastic straw. No difference. Disconnected the short cable from its holder one by one, cleaned each ribbon, put it back. Did them all. No difference. I put it back together. Someone before me has been in here. The screw heads were all butchered. Back to square one.

CoogarXR
08-14-2017, 09:29 PM
If the ribbon connections made no difference, you are down to a bad buffer board or a bad panel.

I wouldn't spend too much time or money on it. Those models can randomly eat Ysus boards, and even just rebuilding them can be costly, let alone replacing them with an already rebuilt board.

dr.ido
08-15-2017, 08:15 AM
I don't know what panel is in your set, but if it is V5 or V6 (and possibly others, it's been a while) panel there are SMD resistors either side of the COF IC on the ribbons that you unscrewed.

Dry joints on these resistors will cause the black vertical bar for that part of the panel.

I've a had few where the resistors have gone open. If this was caused by a shorted driver IC then replacing the resistors won't help, but I have has a few where the resistor itself somehow failed and replacing it did fix the problem.

It's worth tacking in a couple of resistors, but I wouldn't buy any parts for it. As others have already said they will take out buffer and sustain boards.

They seem to develop an intermittant fault in the panel itself. I've had few flicker and kill the buffer and/or Ysus that I had just installed.

holmesuser01
08-15-2017, 08:55 AM
I agree. Not buying boards, etc. for it.

Am I OK disconnecting these ribbons one at a time until I find the one that is running the dead screen section? I'm kind of leery running it with disconnected ribbon cables.

CoogarXR
08-15-2017, 10:37 AM
It will be the ribbon directly below the dead section. There's no reason to run it with them disconnected. And for anyone reading this in the future, I wouldn't disconnect ribbons with the set running, the contacts are too fine and it's easy to make contact with the wrong pins.

holmesuser01
08-15-2017, 12:59 PM
I'll check those resistors.

holmesuser01
08-15-2017, 02:49 PM
I FOUND a break at the smd resistor on the ribbon, and soldered it.

I was carefully re-seating the clip that holds the ribbon to the pcb and the clip broke off. It's finished. Unrepaired. Going to put it on the street with a FREE sign on it. I've wasted too much time on it already. Thank you so much for all the suggestions.

I've got a Christie branded plasma screen that is the next project.

holmesuser01
08-15-2017, 02:51 PM
It will be the ribbon directly below the dead section. There's no reason to run it with them disconnected. And for anyone reading this in the future, I wouldn't disconnect ribbons with the set running, the contacts are too fine and it's easy to make contact with the wrong pins.

I would never connect or disconnect anything on a set that is running, ever. Thanks again for your encouragement.

CoogarXR
08-15-2017, 03:20 PM
I FOUND a break at the smd resistor on the ribbon, and soldered it.

I was carefully re-seating the clip that holds the ribbon to the pcb and the clip broke off. It's finished. Unrepaired. Going to put it on the street with a FREE sign on it. I've wasted too much time on it already. Thank you so much for all the suggestions.

I've got a Christie branded plasma screen that is the next project.

Are you sure it actually broke? They have little nubs that sit in sockets, and if you flip them open too hard they'll just pop out. You might be able to snap it back in.

If you are going to toss the set, pull the Y and Z boards and hold on to them. Like I said before, that failure-prone Ysus is used in a lot of TVs, and a working one is handy to have around if you are going to work on more plasma TVs. You could even sell it on ebay (if you are into ebay selling).

holmesuser01
08-15-2017, 03:28 PM
The little nubs are broken. Like I said before, I'm not the first on in this part of this set. The screws, including the ones holding the heatsink, were mauled out by monkey hands.

CoogarXR
08-15-2017, 03:42 PM
LOL

Well, it was worth a shot ;)

holmesuser01
08-15-2017, 04:52 PM
It was all fun until the little plastic piece snapped. Again, thank you for the valuable assistance.