First checks before repairing Bally score displays

Steps

• Measure solenoid driver TP2 (155-190 V, set to about 170 V) and TP4 (input) against GND; equal readings mean the HV regulator has failed.
• Inspect MPU connector J1 (upper left), which feeds the displays and sits near the battery. As a quick test, lightly sand the header pin faces with 400-600 grit paper; if faults improve, replace the J1 header and housing pins.
• Reflow the header pin joints on every display board, removing the old solder first and avoiding bridges (page 52).
• Check the six (seven on 7-digit boards) 100K resistors on each display board and upgrade them (page 54).

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

• Bally and Stern score display boards
• Flickering score displays
• Replace a score display glass
• No score displays light at all
• How a Bally display board drives its glass
• Score display J1 to MPU J1 connections
• Score display digit parts by position
• Score displays dim

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