I can't get into audits or adjustments with the coin door buttons. If I press the Advance button the pin goes straight into MUSIC TEST and I can't advance out of it. I have to switch the machine off. The buttons were working fine before.
The ADVANCE and AUTO UP/MANUAL DOWN buttons connect to U14 which I swapped but no change. I also swapped U56 but misread the poor quality copy schematic and should have swapped U36 which is connected to the MEMORY PROTECT switch. Surprisingly swapping U56 didn't change anything
I have checked U36 (7404) pins 10 and 11 (MEMORY PROTECT) with the logic probe and it seems to be giving the correct output for a given input, which changes when the switch is pressed as expected.
I can start and play a game OK otherwise.
The only other IC in play is PIA U51. Any suggestions before I pull the trigger on this chip?