Every digit of every display reads 2 or 7.

All digits on all displays show 2s or 7s

Cause: A bad U10 PIA port A output, a failed U20, a faulty 5101 RAM, or burned resistors in the top right of the MPU board.

Checks

• Swap U10 and U11.
• Look for burned resistors near the top right of the MPU.

Fix

• Replace U10, U20 or the RAM as indicated.

Systems

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

Related tips

• Same digit missing or stuck on every display
• All 7-digit displays show a dim first digit and comma
• Display test counts out of order (0,1,0,1,4,5...)
• Only even-numbered score displays show numbers
• Flickering score displays
• Score display J1 to MPU J1 connections
• All displays strobe or flash rapidly
• One score display dark: poor contact at MPU J1 pin 20

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