The MPU LED is always off.

Dark MPU LED: follow +12 V through R29 and Q2

Cause: +12 V not reaching the LED circuit, or a failed LED.

Checks

• TP2 should read +12 V.
• Both ends of R29 (470 ohm 1/2 W) should read +12 V.
• The Q2 leg nearest U11 should read about +11 V.

Fix

• If the voltages are there, short the two adjacent legs of Q2; if the LED still does not light, replace it.

Systems

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

Related tips

• Twilight Zone: "clock is broken" error
• MPU stops before the 7th LED flash (no 43 V)
• Intermittent or dead MPU from old closed-frame sockets
• Testing an MPU in the game with only J4 connected
• MPU board test points
• MPU board test point voltages
• Why the MPU has ROM jumpers
• Telling a -133 MPU from a -35 or -17

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