The game behaves erratically when a certain switch is stuck closed.
Random game faults when a switch without a diode sticks
Cause: Coin door switches, and some playfield switches on early games, were built without diodes; when one of them sticks it confuses the whole matrix.
Checks
- Look for switches with no diode and see if any are closed.
Fix
- Add a 1N4148, 1N914 or 1N4001 diode to each diode-less switch, band oriented as on the others.
Systems
Related tips
- Clear stuck switches using switch test
- Test a playfield switch diode
- Bank shot lane scores by itself now and then: replace the switch diode
- Switch matrix confused by a bad MPU DIP switch diode
- Switch reads stuck closed, or a pop bumper chatters
- Switches register with the wrong numbers
- Stand-up target always closed in test and ghosting another: failed switch capacitor
- Strange coil and switch faults from a drop target bank short
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)