The MPU LED stays dark from power-on.

MPU LED never lights at all

Cause: No +12 V at the board, or a dead LED (common).

Checks

• Measure MPU TP2 for about +12 V DC.
• If 12 V is there, bridge the two right-hand legs of Q2 with a small screwdriver; the LED should come on.

Fix

• If the LED stays dark with 12 V present and Q2 bridged, replace the LED (green, CR8).
• If TP2 has no 12 V, trace the supply back to the rectifier board.

Systems

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

Related tips

• MPU stops before the 7th LED flash (no 43 V)
• Using a PC power supply as an MPU bench supply: which wires
• Power and ground pins of the main MPU chips
• Intermittent or dead MPU from old closed-frame sockets
• Testing an MPU in the game with only J4 connected
• MPU board test points
• Seven flashes but no attract mode
• MPU board test point voltages

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