A switch fires the wrong coil (for example a slingshot resets a drop bank), one coil never fires, or in solenoid test one coil fires for two numbers.

Wrong coil fires or a coil never fires

Cause: One of the encoded select lines PB0-PB3 or the bank select CB2 is lost between MPU PIA U11 and the SDB 74154: bad U11 or its socket (in the battery corrosion area), cracked solder or worn pins on MPU J4 or SDB J4, swapped switch connector wires, the wrong game ROMs, or rarely a bad 74154.

Checks

• Confirm the ROMs at U2/U6 are for this game.
• Swap PIAs U10 and U11 and see if the fault changes.
• With the connectors plugged in, check continuity from MPU resistors R97-R100 (and R106) through MPU J4 and SDB J4 to the 74154 at SDB U2.
• Unplug the drop target bank harness and reboot: a chafed 43 V wire touching the target frame can feed the switch matrix (page 24).

Fix

• Replace the PIA or its socket, reflow or repin the J4 headers and connectors, or replace the 74154 as testing shows.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• No coils work at all
• Solenoids fire out of order in solenoid test
• Test the coils behind the solenoid expander
• How Bally coils are switched
• Baby Pac-Man solenoid board select lines
• Solenoid selection: four MPU lines decoded by a 74154
• Find a shorted coil circuit by plugging SDB connectors one at a time
• Flippers and some coils work, others are dead

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)