Reseat the MPU chips before anything else

Pull and refit every socketed chip, making sure all legs enter the socket and the notch faces the right way (a reversed chip is usually ruined). If this cures the fault, the socket that was at fault is bad and should be replaced.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Strip the MPU to three chips to get the first flicker
• Power and ground pins of the main MPU chips
• Double-check bench leads: a +12 V to +5 V short kills the MPU chips
• Testing an MPU in the game with only J4 connected
• MPU board test points
• MPU board test point voltages
• Why the MPU has ROM jumpers
• Telling a -133 MPU from a -35 or -17

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