A segment stays on in every digit of one display.

One segment always lit on a display

Cause: A shorted MPS-A42 segment driver, or the 4543 decoder.

Checks

• Display test. Logic probe on the driver base: if it pulses, replace the transistor.
• If not, measure the base with a meter: it should be 5 V DC. Much higher (for example about 110 V) means replace both the transistor and the 4543; exactly 5 V means replace the 4543.

Fix

• Replace the indicated parts.

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 always on or brighter than the rest
• One digit missing on a score display
• Seven-digit and six-digit displays
• Score display digit parts by position
• Display test counts out of order (0,1,0,1,4,5...)
• Making Bally and Stern 7-digit displays swap between makes
• Display glass dim, digits missing or slow to light
• Same digit missing or stuck on every display

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