As I said in the finds topic, I recently found a few 80s systems. The 7800 looked to be in the best shape, but unfortunately, it hasn't been smooth sailing getting it to work. It turns on just fine, with the power light coming on, and it can play 2600 games without much issue, but when I popped a 7800 cart in, it was nothing but solid color screens, screens with various colored vertical bars, rolling, black and white scrambled screens, etc. The problem is somewhat similar as in this video:

Even though the old ones looked fine, my fiance replaced the capacitors for the heck of it, but that changed nothing. Unlike in the video, letting the system warm up didn't solve the problem, but it did gets us from no graphics to somewhat getting the logo screen. Also unlike the video, keeping the board away from the metal shielding makes no difference. There was one single time that a 7800 game actually started up, and I played for a while with no issue. I don't know what was unique about that one lucky time, but there were countless tries before that and countless times after. It didn't work again after that one time even when we touched absolutely nothing except pressing the power switch off and back on.

Does anyone have any idea what could be the problem and how it could be solved?