Easy job: the game soon after POST coined up immediately.

Ran the I/O test and the coin button didn’t work (but it showed a 0 instead of 1).

Testing pin 16 of Jamma connector (coin input) showed that it was stuck low all the time.

Tracing back with the multimeter I found out that the responsible for coin inputs was a 74ls257 TTL @19C which I replaced.

That fixed the problem.