Prove the MPU switch inputs with the harness unplugged

Steps

• With the game off, set all MPU DIP switches OFF, then power on.
• Enter switch test: five presses of the test button on sound-board games (Lost World on), four on earlier chime games. With J3 unplugged, ground J3 pin 1 that many times instead.
• While in switch test, unplug MPU J2 and J3.
• Clip a jumper to J2 pin 1 and touch it to J2 pins 8-15 in turn, then to J3 pins 9-16; a switch number should appear for each.
• Repeat with the jumper on J2 pins 2, 3, 4 and 5.
• Power off, set all DIP switches ON, and repeat the whole test.
• A pin that does not register points to the J2/J3 header, the 1N4148 diodes, 390 pF capacitors and resistors on that line, a broken trace, or the PIA. Swap U10 and U11: if the fault moves, U10 is bad.

Systems

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

Related tips

• Switch matrix confused by a bad MPU DIP switch diode
• Some games add a sixth switch strobe ST5 (switches 41-48)
• Bally switch matrix: 5 strobes by 8 returns, numbered 1 to 40
• Rollover switch only registers with two diodes fitted: failing PIA at U10
• Odd switch problems from corrosion at the DIP switches
• Coin door switch shorted to ground causes lamp and other odd faults
• Self test will not start from the coin door button
• Player 1 display shows credits and ball in play

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