A pop bumper or slingshot locks on at power-up. New TIP122, 2N4401, coil diode, 7402 (12A/12B) and even the 6821 PIA (8H/9B) do not cure it.

Special coil stays on even after replacing its driver chain (Rev 3)

Cause: A part in the unused legacy special-solenoid circuit has shorted: most often one of the 0.1 uF capacitors C40-C45, sometimes a 6.2 V zener D1-D6 or the 4.7k x 8 resistor network RA24/R24.

Checks

• Check C40-C45 for a short.
• Diode-test D1-D6.
• Check RA24 for shorted elements.

Fix

• Replace the shorted capacitor, zener or resistor network.

Systems

• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Replace C33 on the speech and sound board
• System 11 CPU board part numbers and spotting features
• Always check the coil diode when a driver transistor has failed
• Constant power coil drivers Q8-Q13 and Q23-Q30
• A low resistance coil means checking its driver chain too
• Check every coil's resistance before first power-up
• Finding a coil's driver transistor without a manual
• Firing a coil by grounding its TIP122/TIP102 tab

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Two, Clay Harrell (pinrepair.com)