A coil pulls in and holds, often as soon as the game is switched on before the seven MPU flashes finish.

Coil locks on at power-up or stays on

Cause: Usually a shorted driver transistor on the solenoid driver board; also possible are its 1N4004 board diode, 330 ohm resistor or the CA3081 pre-driver.

Checks

• Switch off immediately.
• On the solenoid driver schematic, follow the coil's connector line back to its Q transistor, and note the CR diode, the driving U chip and its pins.
• Test that transistor with the game off (see the transistor test entry).

Fix

• Replace the shorted transistor with a TIP102, and also the coil diode, the board CR diode and the 330 ohm resistor (page 21).

A locked-on coil can overheat and burn within moments; switch the game off as soon as you see one.

Systems

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

Related tips

• How Bally coils are switched
• Test the coils behind the solenoid expander
• No coils work at all
• Solenoid selection: four MPU lines decoded by a 74154
• Flippers and some coils work, others are dead
• Solenoid select lines: MPU PIA U11 to SDB 74154 pin map
• Wrong coil fires or a coil never fires
• All score displays blank with the MPU fully booting

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