Three of the six digit positions (here digits 2, 3 and 5) are dark on every score display. Another MPU drives all the digits fine, so the fault is on the MPU even though its PIAs test good.

Bally -35: same digit positions dark on every display: open digit drive resistors

Cause: Failed series resistors in the digit drive lines on the MPU. The digit drives come straight from PIA U11 (port A), through a small series resistor and capacitor for each digit.

Checks

• Confirm the fault stays with the MPU by trying another MPU or a test fixture.
• Repin the display connector anyway if it hasn't been done; it is a common cause of digit faults on these games.
• Follow each dead digit's line from U11 and measure its series resistor.

Fix

• Replace the open resistors on the dead digit lines (R92, R94 and R95 on the aftermarket surface-mount MPU in this case).

Systems

• Bally MPU AS-2518-35

Related tips

• A display not working from MPU or sound board faults
• One score display dark: poor contact at MPU J1 pin 20
• Score display J1 to MPU J1 connections
• First checks before repairing Bally score displays
• Only even-numbered score displays show numbers
• Same digit missing or stuck on every display
• All score displays blank with the MPU fully booting
• Flickering score displays

Source: Pinball Repair help group!! (Facebook group)