During the 4th and 5th flash the CPU will logically try and flip all the ports of the PIA and then read back the PIA to make sure they actually flipped. You may even notice the continuous solenoid pull in briefly at this time and the sound board may squawk. If solenoids lock on at the 4th or 5th flash the PIA is probably bad or the CPU cannot access it properly due to a bad IC socket, busted trace, two output ports shorted together.... etc.

U11 controls solenoids and is in the "corrosion zone". So it wold make sense to look around there. If you have a known good PIA you could pull U11, give a good visual inspection for problems like snapped ic socket contacts. Then set the known good PIA at u11 and boot test again.

U11 is a bit discolored in my middle where all the business gets done like it may be toasted. Might be one of the chips where they sanded down the imprint to put Bally's custom number on a motorola part. U11's socket might just be wasted too from the previous battery leak.

If you rotate U10 and U11 and if the flash count changes one of those PIAs is bad. Having a known good PIA to try would be nice. I'd try replacing U11 first.

in theory only one decoded solenoid should be able to lock on at once. even if the cpu is running wild. The continous solenoids could still get held on tho from a locked CPU but if more than one locked on decoded solenoid indicates there is for sure something wrong on the SDB.

Since the flash pattern did not change with changing the PIA chips and assuming you counted the flashes right it probably means the MPU socket is bad. No 4th flash = U10. No 3rd flash = U8. Make sure you are not counting the "flicker" right at when the power toggle flips.

You can isolate the MPU from everything but power input to make sure nothing external is making the PIA test fail. On MPU only J4 plugged in and on SDB remove J4 and all the larger size 0.156" connectors if solenoids are still locking on.

sounds like you are in the home stretch.

All the solenoid signals come from mpu u11, leave J4, go to driver j4, to driver 74154 / Ca3081, to tip120. Usually j4 on both the MPU and driver need female repinned and male headers resoldered / replaced. Specially since your MPU corroded.

I was just mentioning the order or flow of how the CPU fires a solenoid.

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Do you hear that solenoid lock on when you power up or does the fuse just blow?

If the fuse just blows measure resistance across the coil lugs. If you get a reading of 0 resistance cut the diode off of the coil and check again. If it is still shorted the coil is bad.

If the coil kicks and locks on right at power up something is still damaged in that driver circuit. If you put a new TIP102 in and it still locks on right at power up then the CA3081 (a chip with seven transistors inside of it) at U1 is likely bad.

If you want to do more testing to be sure... With a logic probe, or even the DMM checking voltage, you can confirm the CA3081 is the problem. With the fuse out or blown, put the game in solenoid test. For Q4 solenoid driver circuit, probe at U2 (the 74154 chip) pin 7. During solenoid test that pin should remain high (5v) and pulse down to low (0v) when the computer wants to fire Q4. If the signal is working there, then you pretty much identified it has to be U1 since you already replaced the main TIP102.