LED stays on, but briefly shorting U9 pins 39 and 40 starts the flash sequence.

MPU boots only when U9 pins 39 and 40 are shorted

Cause: Reset section fault, or +5 V taking longer than the roughly 50 ms reset window to settle (often a weak C23 on the solenoid driver board).

Checks

• Rebuild or test the reset components first.
• Check C23 on the solenoid driver board.

Fix

• To lengthen the reset time, solder a 470 uF electrolytic on the solder side: positive to the collector (top leg) of Q1, negative to Q1's emitter (ground).
• If the board now boots on its own, the 5 V supply was settling too slowly; fix the supply as well.
• Expect the opening flicker to become a longer flash with the added capacitor.

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
• What each MPU LED flash proves
• Good MPU stays LED-on with a PC power supply
• MPU boots only some of the time
• Game reboots during play from slam switch noise
• MPU LED locked on with no flicker: likely causes
• MPU stops before the seventh flash: check the 43 V at R113
• MPU reboots right after the seventh flash

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