Midway Ms Pac-Man arcade pcb repair
Board 100% dead – volt-meter on various TTL chips showed only 1V instead of 5V. Edge connector was very dirty/corroded – after cleaning up with some 2000 grit sandpaper, all the TTL had a solid 4.98V but the board still remained totally dead. The crystal was found to be quite corroded – in fact the legs literally broke off when I tried to remove it. A replacement was fitted as was a replacement for the LS368 – bizarrely corroded despite other chips near it being fine. The socket was also completely trashed with rust.
With those fixes in place video output worked – the game showed random garbage but the CPU was obviously not running. A tip for Ms Pac is that you can remove the CPU daughterboard and put in a Z80 and rom set from a regular Pacman. After cleaning up the ROM sockets the Ms Pac board worked using the Z80 from a working Pac. This meant the next problem was localised to the daughterboard. Cleaning the sockets and ROM legs didn’t help – in fact the Z80 on the daughterboard itself was dead. With a fresh one installed the game worked.
However, audio only worked for about 10 seconds at a time then faded out to silence. I assumed this was just bad capacitors and recapped the entire audio section – which didn’t help at all… So I started at the speaker output in the schematics and worked backwards – all sounds go through the 4066 @ 1N and the LS273 @ 2M. The clock signal to the LS273 was dead so no sound data could get through. That clock is driven by the prom at 3M and it this clearly had some kind of internal failure. With a probe on the prom output pin I could see the voltage drain away as the sound died. Substituting 3M from a working Pacman confirmed the failure as everything worked 100%.