TP3 has about 21 V DC, but the board still stops at six flashes, or restarts the sequence over and over.

Still no seventh flash with F4 good

Cause: Parts in the battery corrosion area of the zero-crossing and IRQ circuits: R17 (150k, a frequent failure), R16 (2k), R18 (1.5M), diodes CR52 and CR49, the IRQ line (CPU U9 pin 4, linked to pins 37 and 38 of both PIAs) with its 4.7k pull-up R134, or U14.

Checks

• Check R17, R16, R18 and their solder joints.
• Check CR52 and CR49 (1N4148 or 1N914).
• Check continuity of the IRQ line and R134.

Fix

• Replace the failed part; replace U14 last.

Systems

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

Related tips

• Bench-testing a -133 MPU: swap the R113 diode for a resistor
• Converting a -133 MPU to a -35
• Convert a -35 MPU to a -133 for Baby Pac-Man and similar games
• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• Single reset parts that hold the LED on
• MPU boots only some of the time
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks

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