One or two displays are dark while the others work.

One or two score displays completely blank

Cause: Most often an outgassed glass; less likely, failed transistors on that display board.

Checks

• Swap the dead display into a working position. If it stays dark, suspect the glass.
• Game off: test the display transistors on diode range, expect 0.5 to 0.7 V.
• With TP1 at +5 V and TP2 at 170-190 V and good transistors, hold a new glass (leads straightened, same orientation) against the board with the game on. If it lights even partly, the old glass is bad.

Fix

• Replace the glass or the failed transistors.

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

• Flickering score displays
• Replace a score display glass
• Bally and Stern score display boards
• One digit missing on a score display
• Score display digit parts by position
• Score displays dim
• All score displays blank with the MPU fully booting
• First checks before repairing Bally score displays

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