The MPU sometimes stops at the fourth flash and sometimes boots fully; when it boots, the score displays flicker and show only some digits.

Sometimes three flashes, sometimes seven, with flickering scores

Cause: A corroded, intermittently open trace to U10 on the component side near the battery.

Checks

• Clean and sand the corroded area; the open trace usually becomes visible.

Fix

• Neutralise the corrosion and bridge the broken trace.

Systems

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

Related tips

• Repair battery corrosion before chasing MPU flash codes
• Repairing battery corrosion on the MPU
• All score displays blank with the MPU fully booting
• Cut the original MPU battery off right away
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Memory cap or battery does not keep scores
• Corrosion under MPU J4 can destroy every chip
• Switch faults from corrosion under MPU J2 or J3

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