(This is essentially a copy of something I posted in a Facebook arcade repair group not too long ago.)
I was replying to a post that had to do with a game that was showing garbage on screen but was otherwise not working. The type of garbage you see on screen can actually be a good diagnostic indicator. At least, it is a better diagnostic indicator than nothing on screen at all.
Static (here meaning not changing) garbage, without repeating or flickering horizontal or vertical lines, tells you that the video-generating hardware works and is outputting whatever it sees in RAM, and nothing is updating the RAM – a possible indicator of little or no CPU activity or of the CPU not being able to “get to” the video RAM. If showing characters along with the garbage, it can probably get to character ROMs as well.
Dynamic/changing garbage, sometimes along with other things like clicks from the speaker, flashing start LEDs, coin lockouts engaging, etc., tells you that the game is able to get at least some access to the video RAM and is *trying* to do something like starting to run but then failing and/or watchdogging. Especially if it exhibits a repeating pattern.
Flickering garbage or static (here meaning something similar to what you see when you tune to a non-existent TV station), or repeating data, especially if in rows or columns, can be an indicator of something like a bad buffer/transceiver or something else screwing with data or address lines, at least those connected to video RAM, or else indicate a failure of video (and possibly other) RAM.
You can get a lot of information from a screen full of garbage if you know what to look for.