Flicker and two flashes, then the sequence stops.

Two flashes only: U8 5101 RAM

Cause: The 5101 RAM at U8, its socket, or broken traces to it (U8 sits in the battery corrosion zone). If those test good, U19 (4011) is a frequent cause.

Checks

• Ring out each U8 pin to the matching pins on U7, U6 and U11 (see the U8 connection list).

Fix

• Replace U8 with a fast enough 5101 and its socket.
• If U8 and its lines are good and still only two flashes, replace U19 (4011).

Systems

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

Related tips

• Keep spare 6800, 6821 and 5101 chips
• Tracing the VMA signal through U19, U14 and U15
• 5101 RAM at U8 fails often; speed matters
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Three flashes only: U10 PIA
• Five flashes only: U12 timer or open R113
• Game keeps resetting: replace the 5101 RAM (an NVRAM works)
• Odd high scores usually mean a bad 5101 RAM

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