How Bally coils are switched
Every coil has 43 V DC on it whenever the game is on; the power wire runs from coil to coil under the playfield. The solenoid driver board fires a coil by grounding its other lug through a TIP102 (originally SE9302) driver fed by a pre-driver in a CA3081 array, so in attract mode both lugs of a healthy coil read about 43 V to ground.
Systems
Related tips
- Test the coils behind the solenoid expander
- Coil locks on at power-up or stays on
- No coils work at all
- Solenoids fire out of order in solenoid test
- Find a shorted coil circuit by plugging SDB connectors one at a time
- Flippers and some coils work, others are dead
- Wrong coil fires or a coil never fires
- Solenoid select lines: MPU PIA U11 to SDB 74154 pin map
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)