Known good U6, U9, U11, good sockets and correct jumpers, yet no flicker.

Single reset parts that hold the LED on

Cause: A failed VR1 zener, Q1 or Q5 keeps the LED on; CR5 can too. A bad Q2 can leave the LED lit whatever the CPU is doing.

Checks

• Test CR5 on the meter diode range: about 0.3-0.6 one way, no reading reversed.

Fix

• Replace Q1, Q2 (2N3904), Q5 (2N4403), CR5 (1N4148 or 1N914) and VR1 (8.2 V zener) as a set; they are cheap.

Systems

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

Related tips

• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• MPU boots only some of the time
• Quick test of reset transistor Q1
• Game reboots at random: R11 heating Q5
• Bench-testing a -133 MPU: swap the R113 diode for a resistor
• MPU VR1 is an 8.2 V zener; the manual's "1N9598" is a misprint
• Converting a -133 MPU to a -35

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