OK, I'm back on this again...

I replaced the switch with a new one and I'm a little embarassed to say that I still can't get into diagnostics when I press the ADVANCE button.

disconnect the memory protect switch that the door pushes on when it is closed (if you have one fitted) if this is stuck closed it will enable the memory protect and the Advance button will not work.

Tried this, no change.

Short CPU board, 1J4, Pin 2 (GND) to Pin 3 (ADVANCE) that is the momentary switch: each time you short these 2 terminals it will simulate pressing the red button with the orange and green wire.

Yes, this will get the game into diagnostics, starting with the displays showing "11111111", then "22222222", etc. Is this what is supposed to happen?

try shorting the Green and Orange wire with a screw driver, this will simulate pressing the red ADVANCE button.

Tried this and nothing happens, but the weird thing is if I put the screwdriver across the tab for the green and orange wire and also touch the metal bracket on the coin door which holds the switch in place (ground?), then this will also get the machine into diagnostics. I'm pretty sure this is what happened before when I thought I was shorting the orange and green wire, i.e. I thought I was shorting only the orange and green wire but actually I was also acccidentally touching ground with the screwdriver. I should have removed the switch from the bracket when I did this test earlier. So I guess that the original switch was probably OK after all.

So, does this mean anything to anyone? I'm guessing there is some problem with the wiring between the switch and the connector on the board?