Without documentation, it's really hard to say. In order to find out for sure you'd need to analyze the game ROM(s).

IIRC, there weren't marks on the boards - was Danny or another person able to help direct you to the manufacturer? Each mfg of something like that, unless they all used the same set of code for their ROMs, would have different error codes.

That sure sounds like garbage on the display, though, and not necessarily a specific error message. Is there a memory clear button? That would rule out something like that... maybe. Does voltage to each chip read appropriately after adding the new power supply. You can find specs for most chips online. Testing is more difficult than it might seem, but doable with time.

Again, it would be best if documentation was available.

Sorry I don't have better info for you. Disassembling the ROM(s) would be just about the only way I could think that you'd be able to figure it out.