Samsung PN42B450 horizontal lines on black screen

[YamahaFreak]

This one is stumping me and Google doesn't seem to show anyone with a similar issue. The set powers up fine, has sound, responds to the various inputs and the remote control...but the picture is basically a black raster with various gradually morphing horizontal lines. They're more visible and brighter when a signal is being received, and go mostly darker and greenish when the signal is removed. After I cleaned it out and powered it up for the first time, I was able to see the full, good picture for fractions of a second at a time as flickers, once every couple seconds, in fits, but I haven't seen this for some time now. I've replaced the logic/control board and cleaned all ribbon cables to no avail. My gut says buffer problems, and the (however fleeting) glimpses of a full picture tell me the panel itself is okay, and that it's an image processing fault somewhere. Should I suspect the mainboard? Buffer issues usually present as horizontal lines across an otherwise normal picture, and vertical lines (which I don't have) are usually panel issues. But, I've never seen an all-black screen with horizontal lines. I went through a lot to get this set, so am hoping it can be restored to its former glory!

(Tried twice to use the site's image embed tool but the image just didn't show up at all)

https://imgur.com/a/EfN4TXw

[kf4rca]

I'd check power supply rails for similar waveforms.
For videokarma's image posting, I have to squeeze my images down to 640x480 before they will post.

[YamahaFreak]

Forgot to mention that when I first opened it up and gave it a once-over, I noticed one small electrolytic capacitor on the bottom right corner of the power supply was popped (CX824, 470uF 10V) and replaced it...nothing else looked visibly failed anywhere else in the set.

kf4rca, pardon my ignorance...what exactly would I be checking for on the power supply? Your mention of waveforms tells me I probably need a scope...I have one, but I'm not at all well-versed when it comes to using one...yet. I'll probably see if I can obtain another power supply board and swap them out, to see if that's indeed where the gremlins live.

[YamahaFreak]

Update: I replaced the power supply board with a known good one, and that did NOT fix the problem. No change. For good measure, I also checked and re-seated all panel ribbons between the panel and the buffer, as well as the connectors between the buffer and Y-Main. No change at all.

Now, the only things I haven't swapped are the mainboard and the buffer board...other than the cables and interconnects! Oh, and the X-main and related, since I wouldn't suspect those with a horizontal lines issue. Mainboard costs twice as much as the power supply...I might have to put this TV on the back burner until I can afford $100 for a mainboard. Should I try swapping the buffer out?

EDIT: Made some YouTube videos that show the problem in a bit more detail than a single still picture can.

https://youtu.be/PvZJqG2dAMw?si=7vky_3871suGganF

https://youtu.be/tKXAIKemB8w?si=14tYpfIuKPMDURjv

[YamahaFreak]

Replaced the main buffer board today and the picture came right back, big and beautiful! Really satisfied with this repair; the set was forgotten on the side of the road by the railroad tracks and now it's got a second life. 🎉

Photo here:

https://imgur.com/a/ttC726X

[Alex KL-1]

Excellent!

[YamahaFreak]

I can't figure out exactly how or why the buffer failed. There are eight main ICs on the buffer, and I figure if only one or even a few of those died, the picture wouldn't be totally gone...maybe it was the small 8-pin IC near the middle of the board. I've never seen a buffer fail in such a way before.