[MadMan]

Alright I experimented a bit today. First thing I tried was to change R5 to 10k. It made a big improvement in stability. yay! In fact, about 99% of the problems could be adjusted out, and did not come back with changing images. Now, I made a set of test images and use them in a slideshow for testing. I have a regular test pattern, all-white, all-black, then 4 sets of half black on top, bottom, right and left. They don't really approximate rapid scene and lighting changes, but they work well enough for basics.

First thing I noticed was that on the all-black and top-half-is-black* images, the black level starts out at about dark gray, then fades to medium gray over the course of 1-2 seconds. However this happens in reverse when going from all-black to side-half-is-black, where the black area starts (darker than it is with all-black) dark gray and darkens down to a good black. Which, I'm guessing, is what you intended to happen. Why it happens in reverse on all black is beyond me.

*top-half-is-black improved with later modifications.

Then I tried lowering C3. This brought back some of the stability problems and generally made it worse. I then raised it to .2uf and it made things better. Better stability and better evenness of black areas, but not really better black levels. All-black was still the same medium gray. It also seemed to help a bit with retrace lines. I added .1uf more to it, which improved it only a tiny bit more, it seems it's reached the point of diminishing returns at .3uf total.

I then tried doubling C5, which didn't do much besides raise the brightness momentarily while it charged up.

I played with the value of R6, but that didn't seem to do much besides alter the overall brightness.

Also, I just noticed when I turn the set off a spot remains on the screen, which it never did before. But only for maybe 5 seconds, and it's about the size of a quarter, so I don't think it'll hurt anything.

So then I put it back to factory spec, and examined the black levels. Interestingly with the test images I have, they all exhibit perfect black levels until you get to all-black, which is so light gray that it's nearly white. So at least the all-black has some improvement with the new circuit. However when watching actual widescreen video with black bars, it is noticeably worse than it is with the new circuit, that is to say - the changes in black level are more prominent. And also, because of that fade in time with the new circuit, it sort of takes the edge off of rapid light-to-dark and dark-to-light scene changes, by allowing it to slowly fade down or up, so it's not as jarring. So that's good. Which capacitor manages that, though? I changed the values of both of them and it didn't seem to make much of a difference in the fade in time.

Also I measured the crt cathode voltage to ground:
Sam's says: 115
Factory spec: 150
Current circuit: 145
idk, make of that what you will. However I do not have a VTVM, I'm just using a cheap digital multimeter.

Below is attached a schematic of how I have it currently. I understand that it'll never be perfect, but I feel like it could still be a bit better. If you have any more suggestions, I'm listening. And thanks again for all the help!

Oh also, if I should hook up a scope, would I connect to the cathode? And what would I be looking for?