Pre-show testing found the game running but without sound. Checking the power supply found the +12V low at +6.98V and the +5V running a bit high at +5.50V, both measured at the supply. I removed both the game PCB and power supply to investigate on the bench.
I was expecting that the cause of the lack of sound was due to the low +12V but in fact the sound wasn't working even with correct power supplies. Looking at the sound Z80 signals on the scope revealed RD & WR were active, suggesting the Z80 was running. When the game was coined, that should have generated a sound, the RD/WR pattern on the scope didn't change. Poking around with a scope lead on the pre-amp and power amplifier inputs generated noise, suggesting the audio output stage was OK. The schematics showed that the Z80 INT line is triggered by the main CPU on sound commands but I didn't see the INT line active low when the game was coined. Checking clock input of IC F16 (LS74) pin 11 found it pulsing when a sound should have been generated. Shorting IC F16 pins 8 & 9 triggered a sound to be output and I suspected IC F16 (LS74) was bad. Replacing IC F16 (LS74) brought back all the audio and fixed the game.
Testing the IGR Electronica power supply on the bench was consistent with the cabinet - low +12V and high +5V.
Having worked on these power supplies before, I'd only encountered bad TL494 switching regulators as a problem. Since a I'd already bought a quantity of them I started out simply replacing the +12V TL494 and that brought back a good +12V and also brought down the +5V.
With both game PCB and power supply reinstalled in the cabinet the game was working properly again with no sound issues.