Sega Hang-On Arcade PCB fix

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.

Image below:

I had a good pulsing clock signal passing through the 74112 Flip-Flop, moving along the signal chain pin 4 of the 7404 Inverter was showing no signs of life, from this point on in the circuit the clock signal is divided up and sent to different areas of PCB, with the CPU being an essential component ro receive the clock signal to do anything like run the game.

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...