A score display is missing one or two digits.

One digit missing on a score display

Cause: Usually an open 100K digit resistor (one of R1, R3, R5, R7, R9 or R11, plus R56 for the millions digit); the original 1/4 W parts overheat and drift. Otherwise the digit's MPS-A42 level shifter or 2N5401 driver, or a missing enable signal on display J1 pins 4-9.

Checks

• Game off: measure all the 100K resistors.
• Game on in display test (two presses of the test button): clip a lead to display TP3 (ground) and ground the collector of that digit's level shifter (Q1 ones to Q6 hundred-thousands; at the Q/R junction, for example Q6 with R11).
• If the digit lights, replace the MPS-A42 level shifter; if not, replace the 2N5401 digit driver (Q7 ones to Q12).

Fix

• If one 100K is bad, replace all of them with 1/2 W resistors mounted slightly off the board, and brush the soot off the display.
• Replace the transistor indicated by the test.

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
• Score display J1 to MPU J1 connections
• Seven-digit and six-digit displays
• Same digit missing or stuck on every display
• One segment always lit on a display
• Score displays dim

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