A special solenoid is fine at power-up but locks on as soon as a game starts or diagnostics are entered.

Special coil locks on only when a game starts or diagnostics begin (pre-Rev 3)

Cause: This points to the special solenoid switch-sense circuit, not the drivers. A shorted 1N5234 zener among D1-D6 by connector CN18 was the cause in the author's case.

Checks

• Diode-test D1-D6: good ones read about 0.4-0.6 V (about 1.8 V with the leads reversed); a shorted one reads 0 V.

Fix

• Replace the shorted zener.

Systems

• Data East CPU Version 1/2

Related tips

• Special solenoids: switch-fired on CPU Rev 1/2, CPU-fired on Rev 3
• System 11 CPU board part numbers and spotting features
• A coil or flasher comes on and stays on at power-up
• Check every coil's resistance before first power-up
• Testing the legacy special-solenoid circuit at CN18
• Constant power coil drivers Q8-Q13 and Q23-Q30
• A coil has power but never fires
• Pop bumper or slingshot machine-guns on pre-Back to the Future games

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