The board will not come out of reset.

Quick test of reset transistor Q1

Cause: A failed Q1 (2N3904).

Checks

• Ground the R1/R3 junction while watching U9 pin 40: it should dip low for a moment and then return high.
• While the junction is grounded, Q1's collector should read about 4 V DC.

Fix

• If pin 40 does not respond, or the collector does not reach about 4 V, replace Q1.

Systems

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

Related tips

• Testing the MPU reset circuit at U9 pin 40
• Single reset parts that hold the LED on
• MPU LED locked on with no flicker: likely causes
• MPU boots only some of the time
• Lamp and switch matrix transistor positions on the CPU board
• Flipper button resets the game or a slingshot tilts it
• Whole switch column dead and board fails the jumper test: column transistor
• Monday Night Football: switch column transistor Q54 keeps failing

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