All of the voltages check good? Start at the power supply and go from there. Check all the connectors. That is one of the biggest issues on any Sys 1 - 80b. I can not stress that enough.
I had a gold wings that was locking up and it was the daughterboard. I reflowed the joints and it fixed it. Also my sound board led went out and it was the connector at the power supply. Turn the game on a tap on the daughterboard on the known working CPU. They are so fragile it might not be working now. Mine got messed up just from the ride home. Changing the game prom could have stressed the joints on the bottom.

The main thing was the 5 volt but you have confirmed it. Was that at the power supply or at the cpu board. There should be a test point on the cpu somewhere I would think. If I had 80b schematics in front of me I could be more help. It still to me sounds like the daughterboard on the cpu. Also the I know 5 volt comes off of J2 on the power supply and feeds the sound board. My soundboard quit working and it was a burnt connector there.

Also I remember when I had daughterboard issues one row of lights flashed on the playfield and the game didn't boot.

Yeah those ground boards at the transformer suck! I cut them off and went ring terminals per pinwiki.