All score displays are dark.

No score displays light at all

Cause: Usually fuse F2 (3/4 A slow-blow) on the power supply regulator board is blown, typically because one of the four 1N4004 diodes there has failed open or shorted.

Checks

• Check F2 out of its holder.
• Game off: test the four 1N4004 diodes on diode range, about 0.5 V one way and open the other.
• If F2 and the diodes are good, measure TP2 on the solenoid driver board; 230-250 V means the HV section has failed.

Fix

• Replace the bad diodes and F2; rebuild the HV section if TP2 is wrong.

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

• Replace SE9302 drivers with TIP102, and renew the parts around them
• High-voltage input fuse FU2 blows
• System 80B alphanumeric displays dark: check the display board supply
• One controlled lamp never lights, bulb is good
• Game locks up or behaves strangely, worst in multiball
• First checks before repairing Bally score displays
• All score displays blank with the MPU fully booting
• Lamp lights twice or two lamps act as one

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