If there is only scrambled video and no sound it could be a few things.
1. The linear power supply may have a voltage drop. Make sure to check with a multi-meter that the voltage meets the requirements for the game board. I don't know pinouts for this one but a basic JAMMA game runs on +5v and +12v DC....this probably is not JAMMA. Google it or look for a possible manual.
2. The game board is at fault. Possibly gouged traces. Faulty EPROMS or IC sockets. Dead components like that are a common case for older games because of the crappy construction and materials used to make the game boards.
3. I'd rule out the monitor considering the visual given and no audio, but don't assume that the monitor is in near perfect condition.....lol....these things are old.