On the bench fixture the MPU gives 6 of its 7 flashes (5 of 6 on Baby Pac-Man and Granny and the Gators) and never reaches attract mode.

MPU on the bench always stops one flash short

Cause: The last flash checks for the +43 V solenoid supply through the zero-crossing circuit; a PC supply does not provide it.

Checks

• Confirm the earlier flashes all appear.

Fix

• Clip a jumper from the top leg of R23 (the end nearer U12) to the top leg of R17 (the end nearer TP3); this links U12 pin 3 to U14 pin 14 and makes the board think 43 V is there.
• Or feed about 25 V DC from a small transformer and bridge into TP3 (see the 25 V supply procedure).

Systems

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

Related tips

• Baby Pac-Man and Granny MPUs look for 6 V AC, not 43 V
• MPU stops before the seventh flash: check the 43 V at R113
• MPU stops after 7 flashes unless the top saucer coil is unplugged: bad driver chip
• No seventh MPU flash with a good solenoid fuse
• MPU boots only when U9 pins 39 and 40 are shorted
• Reading the MPU flash code
• What each MPU LED flash proves
• What a good MPU does straight after the last flash

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