Check every coil's resistance before first power-up

Steps

• Set the multimeter to its lowest ohms range.
• Measure across each coil's two lugs; 2.5 ohms or more is expected.
• For any reading under 2.5 ohms, unsolder one wire from the coil and measure again.
• Still low: the coil or its diode is bad (and the driver transistor may be too). Cut one diode lead and remeasure the coil alone.
• Now normal: the coil is fine but its driver transistor is shorted; replace the transistor.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• Check for power at a coil and fire it by hand
• Drop target coil won't fire in test: replace its diode
• How Bally coils are switched
• Coil diode and wire orientation when replacing a coil
• No playfield coils work
• Use the solenoid test to find a coil that does not fire
• Coil locks on at power-up or stays on
• How the solenoid expander board doubles up coils

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