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

[pulper111]

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

I've re-soldered a few connections on the Y-sustain board that were a little suspect but it has not made any difference. I'm wondering though if you wouldn't mind taking a look at the following pictures and letting me know if it provides any additional insight. The pictures were taken when no input (antenna, HDMI, etc) was provided to the TV. It provides I think a clearer picture (no pun intended) of what is going on. The top ½ of the screen has a purple/pink blotchiness to it over the entire ½ of the screen. The bottom ½ is black (what it should be). When the LG “no signal” shows up on the top ½ of the screen, it causes the “blotchiness” to bleed down, vertically, into the lower ½ of the screen (see the first link).

I should get the lower buffer board tomorrow but in the meantime, I'm hoping this might help shed some light on the cause of this issue. Thank you!

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


https://imagebin.ca/v/3YLEyXuDyQ21


https://imagebin.ca/v/3YLFO9Dql7dV

[tom66]

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

All you are seeing is that the upper half of the display is overaddressed for one reason or another. I'm not sure why, though. It could be anything like a faulty control board, signalling issue from control to Y-sustain or Y-sustain to Y-buffers, or lower to upper, it's a mystery.

One thing that makes me suspect it's the lower Y-buffer is that this set chains all the data from the lower into the upper. Remember my lightswitch analogy. Well this set has 1080 lightswitches for a 1080p panel. Each switch is toggled from bottom to top in order, it's not like each board does it in pairs, it's one after another. And for one reason or another that's not happening correctly on the upper board. So my thought here is the signals that control that board are bad for one reason or another but not certain to be completely honest.

[pulper111]

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

Thanks Tom for your reply. Really helps clarify. Will write back in once I try the new lower buffer board.

[pulper111]

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

I've replaced the lower buffer board. Now, I have sound (i had sound before too) but absolutely no picture. While this sucks, I'm hoping that this might give further insight into the cause. Let me know if you have further questions/suggestions. Thank you.

[tom66]

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

The board you received must be defective or not connected right. Try the old board and see if the original symptoms return.

[pulper111]

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

Thanks Tom for the quick reply. You're right - the board has a short when tested with continuity and the resistance between the Vscan pins and the floating ground pins is only 8.6 ohms. On the other board it is in the millions of ohms. I'll replace with the original and if the original problem doesn't resume, i'll check back in. Off to find another board.

[pulper111]

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

I've rec'd a new buffer board. Surprisingly, the company sent me both the upper and lower buffer boards. I decided to attach them both and fire up the set. Well, I got a blank screen with sound. I then tried their lower buffer with my upper buffer, and received pretty much the same result as with my lower buffer board, except with a 2" black and white horizontal line across the center. I've attached a link to the picture. Because of this result, I believe (correct me if I'm wrong) that the Y-buffer lower board is not the culprit.

https://imagebin.ca/v/3YgxoVjlZF8E

Turns out that their upper buffer board is bad with a short.

Another question that has come up during my troubleshooting:

When I test the resistance on the lower buffer board between the Vscan pins and the floating ground pins, I get a significant difference depending on whether the board is attached to the Y-sustain or not. For example, when I test with the board not attached, I get in the Millions of ohms or OL. However, when I test with the board attached to the Y-sustain board (without power), I get a reading that initially jumps up to Mega ohms, and then settles down to approximately 68000 ohms.

Is this expected? Thank you.

[tom66]

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

Your resistance result on the Y-sustain seems about right.

The lines you are seeing are probably due to the ribbon going to the panel not being perfectly connected. I had a similar symptom on my Panasonic plasma when I had to do a repair. See attached. You just need to remove the ribbons and reseat them. Do not attempt to reseat the cables while the panel is on, or the buffers may be damaged.

I'm really not sure what's causing this fault unfortunately, it sounds like you've been able to swap the lower buffer successfully and had no solution, but the upper buffer still hasn't been swapped? In which case, there's a chance that could be bad - but it's still up in the air at this point, unfortunately.

[pulper111]

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

Well, I purchased a Y-sustain board off of ebay. replaced the one i had in there, and, lo and behold, it worked! (except for the black line across the center, which I fixed, as suggested by Tom, by simply making sure the ribbons were in properly). This is great news! Now I have to fix my sister's TV next time I'm visiting her, which I know needs a new Y-sustain board (which is why I didn't mind buying this one to test on my TV first).

Now, I'd love to use the Y-sustain board that I had. Now knowing that the Y-sustain board was the cause of my TV problem, is there anything specific that I can look for or test on the old Y-sustain board to see what the problem is? I did previously try to fix any (minimally) cracked or poorly soldered joints with no benefit. Should I look harder for those, or is there something else? Thank you for your help!

[tom66]

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

Glad to hear you fixed the issue.

Since it's the Y-sustain that's bad, it's almost certainly down to one of the small surface mount buffer chips that carries the data across to the buffer boards.

Remember what I said about the floating ground output - well all the Y-sustain board does for the buffer board signals (the clock, data, latch, and output enable signals) - at least on modern sets like these - is buffer the data that the control board sends it directly. It doesn't do anything else with them. It simply uses isolator chips to send the data across the isolation barrier.

If you feel relatively comfortable with surface mount work I could probably highlight what parts you need to swap. You would swap parts from the Y-sustain that is completely dead in your sister's TV to the one that shows the distorted image.

[pulper111]

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

Thanks Tom! my soldering skills are not the best but i'm willing to give it a try. the nice thing is that I've got nothing to lose b/c both boards are not good. I'm hoping though that there will be a way to make sure that the part that I'm taking from my sister's board can be checked to see if it is still good before soldering it into the other board?

I just discovered how much easier soldering is with using flux first. why i didn't do that before is beyond me. hopefully with this improvement, i'll be able to make this work. If you would kindly let me know what area/parts I should be working on, that would be greatly appreciated.

Thanks again!

[tom66]

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

It will be the back of the Y-sustain board. Upload me a clear picture of it and I'll show you where you'll be working.

[pulper111]

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

Hi Tom - here's a picture. let me know if you need anything else. thank you!

https://imagebin.ca/v/3Z9stg62FcSe

[tom66]

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

Can you get me a clearer picture of this part of the board, maybe in two halves?

[pulper111]

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

Hopefully this is better! The marker markings you'll see on the board are where I resoldered. Thanks!

https://imagebin.ca/v/3ZErmowRPx4e

https://imagebin.ca/v/3ZEs8MbuGzp6

[tom66]

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

It'll be these chips.

If you don't feel comfortable removing them and reusing them, you might be able to order the parts from DigiKey.

I've highlighted the relatively "likeliness" that a part is causing the fault by seeing the general direction of the signals, so if you feel not confident swapping all three parts then you could try swapping them in this order.