All displays lose the same digit, or all show 8 where they should show 0.

Same digit missing or stuck on every display

Cause: A missing digit-enable signal, usually a bad MPU J1 header or housing pin (pins 1-6 carry the enables); also a shorted display board, an open MPU resistor on J1 pins 2-20, PIA U11 or its socket, or MPU chip U20.

Checks

• Inspect MPU J1 and every display connector.
• Power off, wait 10 seconds for the HV capacitor, unplug one display, power up; repeat for each display to find one that is loading the line.
• Check the MPU resistors on J1 pins 2-20 for opens and bad joints.
• Swap U10 and U11 (U11 is the likely one) and inspect the U11 socket for corrosion.
• On the bench, a logic probe on the J1 pin in attract mode should show it changing; if not, trace back to U11, then replace U20.

Fix

• Repin or replace the connector, or replace the failed MPU part.

The solenoid driver / regulator board makes about 190 V DC for the score displays (around 230 V on some Stern boards); a shock from it hurts. Work with the game unplugged and let the filter capacitors discharge, and use one hand and insulated probes for live checks.

Systems

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

Related tips

• One digit missing on a score display
• All digits on all displays show 2s or 7s
• All displays show strange scores (one bad decoder)
• All 7-digit displays show a dim first digit and comma
• Seven-digit and six-digit displays
• Score display digit parts by position
• One segment always lit on a display
• Display test counts out of order (0,1,0,1,4,5...)

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