repeat spin= 3.7.1

On the problem deck, check every pin inside the Molex connector housing where it connects to the other harness that runs back to the backplane board.
If just one ground pin is pushed in and not touching the female pin, you will not have any lights.

I'm assuming you have the 20-pin, 2-row Molex plug on the problem deck...or something else?

If I were you, I'd post some pictures of your button deck connections.
Maybe someone will spot something amiss or we'll be here for 3 weeks and 100+ Replies guessing what you have.
If you have a digital multimeter, check for continuity with every single wire on the problem deck.
There's only 20 wires....shouldn't take too long to find the bad one.
The harness with the mostly white wires, is the one you want to check...it's for the button lamps.
The other smaller harness with mostly orange wires, is for the microswitches.

I know you've said you've tried other harnesses...but there's no other answer right now.
How come the other deck worked and not this one?
Was the problem narrowed down to the deck buttons or one of the harnesses?
Swap again with the good parts, ONE by ONE, until you find out exactly what's NOT working.

bump

 

When there's an error code 12 you have to powerup the machine while holding the CPU Board "ALL CLEAR" button for about 2-3 seconds. The coin deflector chute solenoid (just below the comparator) should pull in indicating the CPU has loaded the system program and is running. The fact that ERROR 12 is returning means there's a problem with the CPU Board battery circuit for the HM6116L RAM chip. Could be the battery voltage is below 2.8 volts, the battery switching diode D5 is bad or there's no 5vdc to the RAM chip...switching diode D4 could be bad.

Error code 12 could also be telling us that there's a RAM ERROR. Could be a bad ground somewhere in the cabinet, the 5vdc is shorted to ground or there's a defective component on the main board.

My ERROR CODE 12 experience has always been a bad battery or a defective "charging" circuit and in most cases clearing the error at startup didn't prevent the machine from running. At this point I'd do a thorough check on the circuit checking for an open trace or bad component and assuring that we have 5vdc to pin 24 of the 6116 RAM chip from the battery (power off) and VCC (+5vdc - power on).

I'll keep looking for a logical reason why you can't get a run permissive.