My Hang-On game failed recently while on, I returned to the game and the monitor was showing a black display and the attract sound appeared to be playing as normal. I re-powered the game and got the same black display with no sounds, test mode didn't work.
I check the PSU voltage and the 5V was steady across the four board stack.
I used my logic probe to quickly scan through some pins on the CPU, Clock and all data or address line were either high or low and not toggling as expected.
I opened the schematics and headed for the clock circuit on the CPU board.
I 'piggybacked' a new 7404 on top of the suspect one and the game started playing. I removed the IC and checked it in my tester. All 6 Inverter gates were faulty.
With a new IC soldered in place, the game is working fine again. The faulty IC was manufactured by Fujitsu with a date code of 8547, this era of IC's made by Fujitsu have a reputation for failing, Sega arcade PCB's do seem to have a fair amount of them installed...