LED stays on; the CPU never starts.

Testing the MPU reset circuit at U9 pin 40

Cause: The reset circuit holds the CPU reset pin (U9 pin 40) low until +5 V is stable, then lets it go high. If it never goes high the CPU never runs.

Checks

• Put a logic probe or meter on U9 pin 40 and switch on: it should go from low to high (about 4.75-5.5 V) almost at once.
• U9 pin 2 should also go high.
• With pin 40 high, ground the junction of R1 and R3 (right end of R1) to TP4: pin 40 should drop to 0-0.5 V while grounded.

Fix

• If pin 40 never rises, or will not drop when the R1/R3 junction is grounded, replace Q1 and Q2 (2N3904), Q5 (2N4403), CR5 (1N4148) and VR1, then look for corroded traces.

Systems

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

Related tips

• MPU boots only some of the time
• MPU LED locked on with no flicker: likely causes
• Single reset parts that hold the LED on
• Quick test of reset transistor Q1
• Game reboots at random: R11 heating Q5
• Converting a -133 MPU to a -35
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Bench-testing a -133 MPU: swap the R113 diode for a resistor

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