Some games add a sixth switch strobe ST5 (switches 41-48)

Spectrum, Centaur, Medusa and Fathom use a sixth strobe for switches 41-48. Strobes ST0-ST4 come from MPU J2 pins 1-5 to PIA U10 PA0-PA4 (pins 2-6); ST5 enters on MPU J4 pin 5 to U11 pin 14 (PB4), with its return pulled to +5 V at J4 pin 16 through a 3.3K resistor on the playfield.

Systems

• Bally MPU AS-2518-35

Games

• Medusa
• Spectrum

Related tips

• Prove the MPU switch inputs with the harness unplugged
• Odd switch problems from corrosion at the DIP switches
• Switch matrix confused by a bad MPU DIP switch diode
• Coin door switch shorted to ground causes lamp and other odd faults
• Rollover switch only registers with two diodes fitted: failing PIA at U10
• Self test will not start from the coin door button
• Player 1 display shows credits and ball in play
• Pop bumper fires on its own when a flipper is used: replace its switch capacitor

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