After years of wanting an Atari 2600 I finally decided to go ahead and buy one recently. My budget was pretty low, but I was able to find an untested lot with three CX-2600A systems, six joysticks, two sets of paddles, and around 30 games. I figured worst case scenario I could probably piece together a complete working system, but as it turns out all three work, and all but two joysticks work (those two have problems with the cables, the circuit boards still work well).
After trying out all of the games, though, I discovered a weird graphical problem with the "best" system (the one with the cleanest circuit board and case). It seems to be having trouble rendering sprites, on some games like Asteroids and River Raid about half of the sprites will glitch out and stretch across the screen, but they flicker in and out, sometimes appearing normally. It only happens on a select few games (the vast majority of games I tried showed no apparent issues), and the games play normally apart from this graphics issue (for example, collision detection still works on the stretched sprites). Also, these games work fine in the other two systems, so I know it isn't a problem with the game cartridges themselves.
That's probably going to be pretty hard to picture, so here are a couple of gifs of the problem:
First is Asteroids, note the orange and purple rocks. Both always display the same glitches (orange stretched way out, purple only slightly glitchy) and the other two are always normal.
Second is River Raid, this one is much more noticeable. About half of the sprites remain normal while the others occasionally stretch across the screen. Also, as you can see collision detection is still working on the problem sprites, making this one almost unplayable.
I'm wondering if anyone knows what the problem is and how to fix it. I'm able to solder and desolder components, but I don't have access to electrical equipment to do any kind of diagnostics apart from just turning on the Atari and seeing if it works. I have the other two working units if I need to actually replace any parts, but if possible I'd like to fix rather than replace so that all three work well. Also, just to be clear, this is the CX-2600A model with every component together on one small circuit board.