LG 50PC5D Half Picture Mystery - Help!

How did you confirm both backlights were functioning properly?

1. Replaced lower buffer board (with new sealed part). No change.
2. Replaced main board (with used part). Same area of picture missing but remaining picture now shrunk horizontally - later discovered I had ordered a board for a 42" set instead. I initially assumed the board was defective.
3. Replaced logic board (with new part). No change.
4. Replaced Y-SUS board (with used part). No change.
5. Ordered yet another mainboard, also used. No picture at all this time... it was only now that I discovered that these boards have different firmware depending on the set - this one was actually meant for a 26" LCD believe it or not.

At this point the set has been sitting in my shop for over 2 months and I'm now out over $200. I'm really at a loss to know what to try next, I don't want to waste any more time or money but at the same time I'd like to get it working and at least cut my losses. I'd like to try replacing the upper buffer board next but thought I would get some advice first. Hardly worth the trouble to fix these anymore.... between long delays waiting for boards to arrive, defective boards (happens frequently), wrong boards ordered or shipped, and of course low resale value, it just isn't worth it. Thanks for any advice you can offer.

Are the backlights both working?

Guys ; That is a plasma TV, there is no backlight.

To the OP, do you have a scope ?

Also, given the symptom and the fact you already changed the low Y buffer, I would suggest inspecting the Z connectors on the other side. There are us ually three, but they are actually all one big wire once they get to the Z board. Sometimes it feeds smalller boards which in turn connect to the PDP.

If you don't find any problem there, then it would be nice to have a scope to proceed.

Another thing is many times there is a little connector betwee the upper an d lower buffer boards. It might concievably be missing if someone else was in it.

In addition, any partial loss of backlight would shutdown the whole array in a global dimming unit, and even a locally dimmed backlight wouldn't have a sharp line of demarcation as the OP describes.

I wonder if he shouldn't carefully inspect the ribbon cables.

, I would suggest inspecting the Z connectors on the other side. There are usually three, but they are actually all one big wire once they get to the Z board. Sometimes it feeds smalller boards which in turn connect to the PD P.

e to proceed.

and lower buffer boards. It might concievably be missing if someone else wa s in it.

The set was never opened before, all cables including the buffer connector are intact. Yes I do have a scope.

OK. Have any ideaa how thsae pricks work ? they got wires going across, ver tically and ohrizontally, two sets hori, wait a mini=ut, well who cares.

What these boards on the side do is to charge the individual cells, or pixe ls. the brightness is always the same, the PERCIVD brightness is controlld by the duration of the arc, or dicharge. The sustain boards sustain the arc until a certain turnoff point. that long board on the bottom (DO NOT TAKE THAT SHIT APART)is addressing the vertical columns of pixels. It has to tri gger them just right, and really that whole picture you see is not all in s unc with itself. It has to be faster to do that nad that is why you see les s 3D sets becaus they ned the higher refresh rate as well as more accurate timing.

There is a certain waveform it needs to do that, and when the video require s it, the logic board changes those waveforms that charge the pixels. Somet imes the Ysus and Zsus waveforms will stay the same, somethinges they chang e, but they have certain characteristics.

Wiht a live power supply and everything else, but no pix, I scope the Zsus, and then I scope a few of the Ysus lines going into the buffer. Stay away from those ouputs form the buffer right now. You only get into those when y ou are missing a few lines or something, rare. (unless it is a connector)

Now, going odwn the line on the Ysus putputs TO the buffer boards you will see a progressive change in those waveforms. Very similar but wiht a but di ffernet timing. FYI these are further divided down by the buffer booardsa n d those little ICs switch a hellacious amount of current.

Why ? Because of capacitance. The panel does present quite the capacitive l oad. Makes a CRT look like a resistor. So when you read the output at the P DP, you have to disconnect the Zsus to find out what you are reading, becau se you could see a waveform there from a Zsus not putting out that is comin g from the Ysus and buffers.

There is also a possibility that the connection integrity on the Z side of the PDP has been compromised by dropping or something environmental. You pe rsonally connected that lower Y buffer, so you know it is connected. Hopefu lly you have already checked to see if there is a lil eentsy pluge between upper and lower, and I am pretty sure alot of them had them back them. Barr ing all that. Look for waveforms on the Y buffers and Ysus, make suere they 're there, disconnect the whole Z side of it. (not wiht it running of cours e) If the Y side still hase drive on the lower end where the picture is mis sing, you probably have a bad PDP. don't take that to the bank, but we need to know those waveforms. Actually when it has one. Check it both way. oyu see this on the Y side with the Z diconnected, you see this on the Z side w ith the Y disconnected.

In fact, with the Zsus disconnected you should see the Y wafevorms on the p ins from the PDP. If any of those lines from the PDP are not capacitively c oupling the ?Y signals (yes, hook them up for this test) we get into the ra lm of rally condemning the PDP.

But not yet. Get that info and then we get back to this.

And about bad boards, is it possible you just got a bad lower buffer in the first place ? Seriously, a change of symptoms...

Changing the lower buffer got you no change in symptoms, right ?

You might have just gone on a garden path if you got a bad lower buffer boa rd.

Sealed how ?

Enouugh for now. Go with this and get sone waveforms. you really may have j ust gotten a bad "new" board. Where did you buy it, Shopjimmy ? They do not say their borads are good, they just say they will give you most of your m oney back if they ain't.

Can you send a picture of the lower buffer board ? Dropbox it or something. I want to see it because I think I have worked on a coule but donot have t he odels memorized like the old days. It should have like six chips on it a nd maybe six power transistors. Changed a bunch of those transistors.

Keep us posted. I do not need the waveforms absolutely, we just need to kno w each side is putting out and making it to the PDP, it is at the top, at t he bottom either side could be at fault. Stay away from the board at the bo ttom for now, it is very unlikely with this symptom. The logic board and th e main board are very unlikely, and I have no idea how so many differnet bo ards even worked in there. Sometimes you can't get one three serial numbers awa THSAT OU KNWO WORKS to fire up in the other set which has absolurtely no noticable differences, especially screen size.

Sorry, try to deal with the typos. If you figure out what I mean you will figure this out. Unplug certain things and asee if BOTH ends still have a waveform.

And you had a dead board and you got a dead board. If you hadn't changed the Ysus I would say a shorted lower buffer damaged it which is common.

But not likely in this case. "We" think.

OK I will do some more testing and get back to you. BTW the replacement lower buffer and logic boards were sealed in the original factory boxes and wrapping.

Couldn't find the scope probe, so I did some quick testing with my DMM in stead, measuring a few of the outputs of the Y buffer boards. With the set tuned to snow, the readings were about 91 VAC all the way down. With a blac k screen (A/V input), about 116 VAC. Unless there's something special that I need a scope to see, it appears that the outputs from the buffer boards a re good - if not, I would expect some kind of difference between the output s to the live and dead areas of the screen, even if just measuring AC volta ge. At this point would I be wrong to assume the panel is bad?

Not quite.

Completely unplug the Z side of the PDP and see if the AC voltage readings are the same. Should be three plugs on the other side, just disconnect all of them no matter how many there are. Remeasure what you just measured.

Those PDPs have a shitload of capacitive load, that's why the Ysus side uses 60 amp MOSFETs.

And while the Zsus is disconnected, look for some AC there too. See what you got.

When one side stops putting out, the output of the other side is capacitively tranmitted to the other side. That might not really be the right word but it's close enough for now.

Just for th hell of it I hav to ask - that picture is CUT OFF, not SHRUNK, correct ?

Sometimes you have to ask, really. Somtimes peole iss shit like this. It is unlikely that it is shrunk but I have seen some really strange things in my carreer.

On Thursday, September 11, 2014 10:35:36 PM UTC-4, snipped-for-privacy@gmail.com wrote :

stead, measuring a few of the outputs of the Y buffer boards. With the set tuned to snow, the readings were about 91 VAC all the way down. With a blac k screen (A/V input), about 116 VAC. Unless there's something special that I need a scope to see, it appears that the outputs from the buffer boards a re good - if not, I would expect some kind of difference between the output s to the live and dead areas of the screen, even if just measuring AC volta ge. At this point would I be wrong to assume the panel is bad?

Not necessarily. The change in voltage on the outputs may not be the resul t of the scan ICs conducting or not, but may be a result of the global outp ut being throttled back at the logic/control board. Plasma TVs are designe d to reduce the brightness of a scene if it carries too much white to preve nt over stressing the buffers, sustains, and smps.

If you can't find your scope probe, you can buy cheap ones on ebay for less than $10 delivered. They work OK although they tend to round out high freq uency traces.

Finally got the scope rigged up and looked at some waveforms. Here is what I typically saw coming out of the buffer boards:

This was measured both with and without the Z board outputs connected, and at the good and bad (dead) half of the screen. It was the same under all conditions, although it changed slightly depending on the scene (menu brought up, etc.) BTW here is what the TV picture itself looks like:

A faint, red image can still be seen with the Z board disconnected, and it too cuts off just above halfway up the screen. I can't think of anything other than a bad PDP at this point, but I welcome any advice...

If, with the Z board connected, you got the same waveform on all the Z conn ectors to the PDP, I think you are screwed. Connect it all up and just chec k resistance. If you got continuity, at this point it is almost surely the PDP.

Off it. Maybe sell off the boards. The only PDPs I saw going bad in any sig nificant numbers were Samsung, and they are different. Only thing else is t hat you might luck out and find a comptible PDP in something else but I do not know what fits what. Plus there are zillions of different connector typ es even to the same PDPs.

The chances of you cobbling sonmething together are slim. Very slim. Youy m ight have to chance connctors on a board, or use th buffer and Z boards tha t go with the PDP which could be bad and that's why it isn't sitting in the guy's livingroom right now. They purposely do that to sell more TVs. Every one you fix is a sale lost to them, so, I owuld say just part it out, ocnf ident that the boards are good. You could even have doubts about the lower buffer, but I doubt the doubts. I think it is the panel.

nnectors to the PDP, I think you are screwed. Connect it all up and just ch eck resistance. If you got continuity, at this point it is almost surely th e PDP.

ignificant numbers were Samsung, and they are different. Only thing else is that you might luck out and find a comptible PDP in something else but I d o not know what fits what. Plus there are zillions of different connector t ypes even to the same PDPs.

might have to chance connctors on a board, or use th buffer and Z boards t hat go with the PDP which could be bad and that's why it isn't sitting in t he guy's livingroom right now. They purposely do that to sell more TVs. Eve ry one you fix is a sale lost to them, so, I owuld say just part it out, oc nfident that the boards are good. You could even have doubts about the lowe r buffer, but I doubt the doubts. I think it is the panel.

First bad PDP I've encountered, at least now I know what to look for so I d on't waste time and money like this again. I'm finding that these flat scre ens aren't worth trying to fix, despite their resale value - too much risk of defective boards, wrong boards ordered, other boards failing afterward e tc. I found a couple of online TV parts suppliers who say they buy used boards, so maybe I can recoup at least some of my losses that way.

Actually I've never seen one fail that way. They usually just don't work, b low the buffers, or sit there and hum.

This one sounds like it lost half of the connections on the Z side. Not tha t here's anything you can do about it.

Could be the boards at the bottom, but I only give that about a 5 % chance, if that. The cost and trouble of replacing them is simply not worth it, be cause of the unlikelyhood of them being the problem.

If you have room for it maybe stash it and look for a junk unit. Bad PDPs a re rare enough for that.

If you put it in your boneyard and don't steal the parts, you might be abe to find out what models have compatible PDPs on Shopjimmy. If you look up t h Zsus and Ybuffer boards and there are other models they both fit, it is l ikely th set uses the same PDP, even if it is a different brand. Like if yo u had a Samsumg there are a couple of Zenith models that would work. Howeve r I have no idea about an LG.

And that method of finding compatibility is going to miss alot that may wel l be compatible. Fact is, they aren't all that different except the connect ors. there are a few different sizes and two resolutions, 720 and 1080. At the bottom there are COFs that may differ, but they hav no reason to change them for th sake of change when all they have to do is make the commectors on the sides different. I've considered trying to deal with that and said "Naw, I don't think so" and I am the King of the riggers. Regular TVs it wa s getting to the point I almost needed no parts, I could find a suitable re placement in my boneyard. Didn't care whatthe part number was, just found o ne with a similar circuit and th same application and bam, it worked.

Them days aree over :-(