A coil energizes as soon as the game is switched on, heats up and may blow a fuse.

Coil locks on at power-up and gets hot

Cause: Its driver transistor on the solenoid driver board is shorted.

Checks

• Switch off at once. Measure the coil; if it is under 2.5 ohms its windings are now shorted too.

Fix

• Replace the shorted driver transistor.
• Replace the coil if its resistance has dropped, or the new transistor will fail on the first firing.

Systems

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

Related tips

• Finding the coil that blows F4 (and stops the last flash)
• Coil locks on at power-up or stays on
• Game locks up after the seventh flash
• Solenoid driver board: how the MPU selects a coil
• MPU locks with LED on because of C23 or Q20 on the solenoid driver board
• MPU shuts down or resets: broken C23 ground return wire
• Saucer fires at power-on and blows the playfield fuse: wrong coil killed its transistor
• Drop target coil fuse keeps blowing: shorted diode on the coil

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