Coils fire wrongly or switches misbehave, often on games with three- or six-target drop banks.
Strange coil and switch faults from a drop target bank short
Cause: The 43 V wiring that runs coil to coil on the drop bank wears through on the metal frame, putting coil voltage on the frame and into the switch matrix through the target switches (page 24).
Checks
- Unplug the drop bank's harness connector and reboot; if the fault clears, the bank wiring is the problem.
Fix
- Find and insulate or replace the chafed wire, and check the switch matrix parts for damage.
Systems
Related tips
- Bank shot lane scores by itself now and then: replace the switch diode
- Closing one switch kills the whole column: slam tilt switch touching the coin door
- Lower playfield center target fires the kickout and ends play: switch wired backwards
- Odd switch problems from corrosion at the DIP switches
- Random game faults when a switch without a diode sticks
- Switch matrix confused by a bad MPU DIP switch diode
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)