This fix was so simple that it barely counts as a “repair”, but the symptoms look scary, so it’s worth writing up anyway for The Public Good.
I purchased a Salamander 2 sub-board for a Konami GX main board that I’d picked up a few months previously.
When powering it on for the first time, the self-diagnostic failed with component 22D on the Main Board. The game won’t boot up – it just loops through the self-diagnostic repeatedly.
Location 22D happens to hold a 93C46N EPROM that is used to hold soft DIP settings and high scores.
It turns out that whenever you change game sub-boards on the Konami GX, they don’t re-initialize the EPROM. So the game reads in the EPROM contents from the previous game, doesn’t recognize them, and flags the EPROM as BAD.
Powering on the board with the Test button in the bottom-middle held down forces the game to re-initialize the EPROM. Then the game boots up and operates normally.
I wish every “repair” was this straight-forward, but shame on Konami’s programmers for not reinitializing the EPROM automatically when unrecognized data is read from it!