It's been a while since I did an actual board repair, but this was a cool one. So I was testing a midway space invaders deluxe and noticed that there were whole lines in which there were no pixel data for all sprites. There would actually be other graphics glitches where lines would just appear randomly.
Since the game actually booted, I went ahead and burned myself a copy of the space invaders deluxe V1.3 test rom. This is where having some 28C16 EEPROMs around is super useful, I don't like using them for longevity, but they are excellent for testing like this. I figured this would have been a RAM issue, but the RAM tested good according to the test ROM, so it was elsewhere. Instead it failed on the shifter test. Unfortunately, the numbers on it didn't really give me that much insight.
Stumped, I immediately reached out to some people who were more familiar with these boards, they suggested that it may actually be the custom chip on the sound board. Fortunately, I had access to another boardset that was known good, so I was able to swap the chips, unfortunately I had gotten the same thing, so it was one of the supporting chips.
My solution for this was to pull all the chips directly involved with the custom, so the 74153s and the 7442. I typically don't like the idea of pulling this many chips out of circuit, but since I had everything on hand, I felt this would be the easiest way to test it. The idea here was that I can swap them out with known good chips, finding out which ones were good and which were bad.
Fortunately, the chip causing the error ended up being one of the 74153s, I believe it was at A3 on the sound board, although since I pulled them all out at the same time, there's a chance I made a mistake there. But I was able to replicate the graphics glitches when I put it back into A3.