LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

[pulper111]

Hello - I have been working on fixing two LG50PA6500s. Neither of them had any picture and both broke with a loud pop noise. One is my sisters and the other is mine.

One of them had a bad y buffer (bottom) that was discovered by finding that there was a short. I replaced the y-buffer (bottom) and now there is picture, and sound, but the picture is very noisy with the top 1/2 picture being very muted with weird colors and the bottom picture having weird colors. I've attached two links to pics below. Any idea what I'm dealing with here? I'm glad that I got this far but it would suck if this is as far as I can go. Thank you.

https://imagebin.ca/v/3XszgAEnHx0g

https://imagebin.ca/v/3XszpqJweqw2

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Strange fault. Not 100% sure what's going on.

I'd check -Vy and Vscan just in case something is wrong there.

Make sure connectors are all seated well, including the ribbon cable that goes from the main board to the control board, and the Y-sustain to the Y-buffers.

Make sure all screws are installed and tightened. These late-generation plasmas rely on the screws to create the ground for the circuits.

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Thanks Tom for the reply! i didn't have all the screws in but put them in and made sure they were tightened. no difference.

another item i noticed - the fuzziness/weird colors on the top 1/2 of the screen is there even between channel changes (ie when the screen should be black).

getting ready to check the -Vy and Vscan values. however, before i do (and potentially do it wrong and get a loud spark), i'm just confirming based on my reading that voltage should be set to DC and i'm checking across the two vertically placed resistors on the y-sustain board right above the white 20 ohm resistor that read Vy and Vsc.

I do believe that it might have something to do with the tightness of the connections with the buffer boards to the panel (the ribbons) or the Y-sustain to the buffer boards. From a visual perspective they all look good. However, is there any way that i can check to make sure that they are tight? some continuity check perhaps? it a pain in the butt removing both types of connections.

thank you very much for your help!

[chrismurphy]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

check the 470 ohm resisters on the buffer board

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Sorry - I don't see any resistors I can check on the buffer board. Can you provide some guidance as to where I would find it? Thank you.

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Please supply clear photos of all of the boards in your TV, the overall layout, and the panel sticker (has a model number like PDP50K9###### printed on it).

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Hello – here are the pics you mentioned. I hope they are clear enough and (of course) hope they help!

Any black marks on the caps etc are just to remind me that they have been tested and are good, according to the readouts on the MESR-100.

Overall:
https://imagebin.ca/v/3Xzy11IZalKN

Y-Sustain:
https://imagebin.ca/v/3XzyNBgN3L2J

Power Board:
https://imagebin.ca/v/3XzypHrKFpMp

Controller Board:
https://imagebin.ca/v/3Xzz69AIZaNI

Z Sustain:
https://imagebin.ca/v/3XzzEtAx2Jv2

Board with HDMI, etc (sorry – don’t know name of this board):
https://imagebin.ca/v/3XzzTsKvXotQ

Z Buffer:
https://imagebin.ca/v/3Xzzgfzjw8xo

Y Lower Buffer:
https://imagebin.ca/v/3XzzrbVYVtUK

Y Upper Buffer:
https://imagebin.ca/v/3Y009Ot1jyjV

Y Sustain with both Y Buffers:
https://imagebin.ca/v/3Y00Ilc0oAyT

Sticker:
https://imagebin.ca/v/3Y00XRPp0sXN

Let me know what you need next to help. Also if you get a chance please let me know about my question above regarding the testing of Vy and Vscan so I can provide those to you, as well as any way of testing the ribbons and other connectors on the Y-Buffer and YSustain board (to make sure that I accidentally didn't ruin them!).

Thank you guys!

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

-Vy and Vscan are tested across the resistors on the board marked -Vy and Vscan. In your case they are to the right of the bottom half of the upper Y-buffer board.

I can't see any scan resistors on your Y-buffer boards, as LG seems to have replaced them with the single large resistor on the Y-sustain board. (This is also what Panasonic does.) A failure in this resistor would cause problems across the whole set.

You can try and check for Vscan voltage on both boards. I will need a close picture of each of the connectors that go between the Y-buffer and Y-sustain boards so I can see where the traces go.

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

-Vy was 190V and Vsc was 151V (across the resistors on the Y-Sustain board). Not sure where the other Vscan should be checked. Kindly let me know if you get a chance.

I can take more pictures in a few hours. Thank you for your continued help with this.

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

So Vsc and -Vy are generated correctly (150V and -190V are about right), the Vsc is passed to the buffer boards as a floating power rail (well everything to the left of the Y-sustain is "floating", it's a long story). If there's a cold joint or a bad series component then Vsc to one buffer board might be bad. Failing that, a connection might be damaged, or it could just be a bad board.

So once you get those pics of the buffer connectors I will figure out where the Vsc is supplied to each buffer board. (Some manufacturers of plasma TVs call this VFG_H). The job of the scan boards - or Y-buffer boards - is to apply VFG_H to each line of the panel in succession to allow data to be written to that row. VFG_L, the main sustain current output, is passed to the plasma panel to all rows simultaneously to sustain the panel and make pretty pictures appear.

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Here are some close up pictures of the area you asked for. However, I'm not sure if they are good enough or if you need the back of the boards too. Let me know.

Just out of curiosity (hopefully) at this point, If i did screw up and one (or more) of the ribbon connectors from the y buffers to the panel are loose (at the panel), is the TV toast then? Would I be having the symptoms that I am having now if this is the issue?

Here are the pictures. Thank you!

https://imagebin.ca/v/3Y6OyUj0dsSY

https://imagebin.ca/v/3Y6PBbjzKugE

https://imagebin.ca/v/3Y6PLuXTJKpe

https://imagebin.ca/v/3Y6Ph8qlrdEY

https://imagebin.ca/v/3Y6Pr8I5XI6c

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

It wouldn't be likely. Usually you'll just get black lines or bars with disconnected or incorrectly installed cables.

I'm not certain where the Vscan voltage is for each buffer unfortunately, so you won't be able to do this test. I've had a good look and I can't see the isolated pins - it's on one of those connectors somewhere!

One thing to check is make sure that thin ribbon cable going between both buffers is well seated and isn't damaged.

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Tom - I am taking a look for any poor solder joints and found two unsoldered joints, one in the connections between the Y sustain and the Y-buffer (upper) and one in the connection between the Y-sustain and the Y-buffer (lower). I've attached links to pictures. It seems weird to me that these would be completely unsoldered by accident, especially considering that they are also not-soldered on the other Y-sustain board that I have from the other non-functioning TV. However, I figured it's worthwhile to ask.

I've double checked all ribbon connections to the y-buffer boards, including the connection between the upper and lower boards. I will continue to look for any other poor connections, but am wondering if I'll be able to find the issue. :-( If there are any suggestions, please let me know. Considering I do have picture and sound, I'm assuming that it's not a bad capacitor or bad resistor. Let me know if you disagree. Thank you for your help!

https://imagebin.ca/v/3Y7pZ2CBfQS1

https://imagebin.ca/v/3Y7rRnuKhk30

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

That's an isolation pin. The thought is that the gap between the traces on the circuit board isn't sufficient, so they add an unused pin between high voltage components.

In fact, that's quite useful, because the only high voltage - relative to other pins - that goes to the Y-buffers is the Vscan supply. So you know that these pins are the Vscan pins that go to the buffers.

Please see the image. The red points are Vscan. Once the boards are installed in the set, you can test from these pins on the top. You will want to test to the floating ground pins. That can be any pin on the connector on the rightmost of the image for both buffers.

Another thing to verify is that the control board ribbon that goes to the Y-sustain is properly seated and no pins are damaged.

Finally there is a small possibility that the lower buffer is damaged, because it sends the signals to the upper buffer through a buffer IC. It would be difficult to determine if this is the fault without an oscilloscope and high voltage isolation kit, so unfortunately it will probably be a crap-shoot at this point.

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Thanks for the picture. Regarding your question on the pic, that is the big white resistor in the middle.
I'm a little confused with your directions regarding checking the Vscan.
1. Do I check both boards for Vscan and provide the numbers?
2. Any of the circled pins on the picture are fine for the positive lead?
3. I'm not quite clear regarding the floating ground pins. You indicate on the right most of the image, but I'm assuming you mean the right most of the image for the right connector, and the left most for the left connector. If i choose one of the last two pins on the appropriate side, will they be the ground?

The control board ribbon is properly seated, as far as I can tell.

The lower buffer was damaged previously on this TV. It had a short in it (checked by continuity testing between the pins attaching the Y-buffer to the Panel and the pins attaching the Y-buffer to the Y-sustain), so i replaced it with the lower buffer board from the other TV which did not have a short. Interesting the board numbers on the Y-buffer lower are the same between the two TVs (the EBR#) but not the KP number on the sticker. When I've researched purchasing another Y-buffer board, I have not seen reference to the KP number so I assumed that is isn't important. Nevertheless, the number on the ribbon that connects the two buffer boards was different (LS1208 vs LS1238). Just mentioning this in case it matters. I did replace the upper one with the upper from the other TV but that did not make any difference to my symptoms.

Thanks Tom!

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

OK so I'll briefly explain "floating ground" as it's a term few people understand. Basically the buffer ICs are high speed "shift registers" which could be thought of as an array of switches, which get toggled one at a time. Think about a load of light switches but you can only ever turn one on at a time and you can only go forwards. So turn light 1 on, turn light 1 off and light 2 on, and so on. They are kind of like that.

Because they are high speed logic circuits they need a good ground and so they make the Y-sustain output, which is definitely *not* ground (it swings +/-200V!) the ground for the buffer/scan chips, and isolate the logic signals feeding them using isolator devices which can basically send information over a barrier of either light or magnetic energy. You can see an isolator in this pic on the bottom right, it's the white chip that crosses the gap in the two copper planes, right at the bottom.
https://imagebin.ca/v/3Y6PLuXTJKpe

Anyway, the Vscan output is referenced to this "floating" ground. The "light switches" inside the scan/buffer ICs can switch between the floating ground and the Vscan voltage. If they are switching at the wrong time, not often enough or too often, you might see what you are getting on the panel right now.

So when you test the Vscan you test to the floating ground output.

The floating ground carries the main power that the plasma panel uses during sustaining. So there are a lot of pins in parallel. While the continuous current is only around 1 amp, the peak current is upwards of 200 amps, several thousand times a second!

Since you are getting light on the panel - and one half is "almost" okay - I'm inclined to suggest, at least not in the immediate from what I can see, that it is not a Y-sustain issue, because the Y-sustain supplies both boards with the same floating ground output.

Anyway, that was a bit of a digression...

This connector in the bottom of the picture is a floating ground connector. Use this for the upper buffer. You can use any pin on this connector.
https://ibin.co/3Y6Pr8I5XI6c.jpg

Then test from this using the black probe on this floating ground, to the Vscan on the upper buffer that I identified previously. You can use any pin in the Vscan group.

The floating ground for the bottom buffer is the middle one of these. Same rule applies, any pin in this connector, black on FG, red on Vscan part
https://imagebin.ca/v/3Y6PLuXTJKpe

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Hi Tom:
Thank you for the information. I did understand a little bit of it! I appreciate your time.
The measurements for both the upper and lower buffer were approx 151V (would vary from about 150 to 151.6 or so).

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

In which case, all I can suggest is that you replace perhaps the bottom buffer. I'm not certain though.

[pulper111]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Thanks Tom for your help here. I appreciate your time. I have ordered a bottom board off of ebay so will see if that works. I guess the values I just gave you were "good" values?

Just to confirm, based on your judgment and previous emails, the following are unlikely to be the culprit due to the symptoms I am having:

1. The power board
2. The x-sustain
3. The y-sustain
4. The controller board

Let me know if you disagree since this still will help decrease the time spent with further troubleshooting. The culprit sounds like, based on your emails, to be either the y-buffer (lower) board, a ribbon cable not properly attached or broken, another connector not properly attached or broken, or a solder joint that is not done properly.

One more thing that I now remember when this TV went bad was a spark at the top connection (where the Vscan pins are) between the Y buffer (lower) board and the Y-sustain board.

If I have further questions along the way I'll write in (or if i'm able to fix it). Thanks again!!!

[tom66]

Re: LG50PA6500 after replace y-buffer (bottom) picture is very noisy and cut in half

Definitely not power board or X/Z board

Small chance (5%) that a data signal from buffers on Y-sustain is bad.

Small chance that there is also an issue with the control board.

I don't know what you mean by a spark, but that's not normal at all.

Good luck!