Several switches that share one switch return line at the MPU never register, though continuity from the MPU connector to each switch is good and the pin has been repinned.

Four switches on one return line dead with good wiring: failed 6821 PIA on the MPU

Cause: One of the 6821 PIA chips on the (aftermarket) MPU had a failed input.

Checks

• Check continuity from the connector pin to every switch on that line, and between the switches.
• Try another MPU: if the switches work, the fault is on the board.
• On a socketed board, swap the two 6821 PIAs to see whether the fault moves.

Fix

• Replace the faulty 6821 PIA on the MPU.

Systems

• Bally MPU AS-2518-35

Games

• Harlem Globetrotters On Tour

Related tips

• Only three players allowed on an aftermarket MPU: reseat J1, clear memory, reset DIPs
• Bally MPU loops through its 7 flashes and resets: check R134 and the interrupt traces
• Only GI lights and MPU LED never flashes: loose wiring at J3 on the solenoid driver board
• Controlled lamps stuck or not flashing: reflow MPU and lamp driver header pins
• Display digits flicker then vanish after the game warms up: poor board connector contact
• Background sound works in test but not in games: check DIPs 29-30, then reseat connectors
• MPU stops before the 7th flash until sound board power is unplugged: replace CR3
• No background music, only target sounds: set feature adjustment 18 to 03

Source: Pinball Repair help group!! (Facebook group)