A digit stays lit or glows brighter than the others.

One digit always on or brighter than the rest

Cause: A shorted level shifter (Q1-Q6) or digit driver (Q7-Q12).

Checks

• Very carefully jumper display TP2 (190 V) to the base of that digit's driver Q7-Q12. If the digit turns off, replace its level shifter Q1-Q6; if not, replace the driver.

Fix

• Replace the indicated transistor.

This test puts 190 V on a jumper; a slip can destroy the MPU 6800 and 6821 chips and sound board chips as well as giving a shock. Only attempt it if you are confident.

Systems

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

Related tips

• One segment always lit on a display
• All 7-digit displays show a dim first digit and comma
• One digit missing on a score display
• Seven-digit and six-digit displays
• Score display digit parts by position
• One or two score displays completely blank
• Display test counts out of order (0,1,0,1,4,5...)
• Making Bally and Stern 7-digit displays swap between makes

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