After work in the backbox (here a battery change), the display shows only some incomplete digits on the left.

Display shows only a few broken left digits after board work: ribbon cable reversed

Cause: The ribbon cable between the CPU board and the display board had been plugged in the wrong way round.

Checks

• Check the red stripe on the ribbon is at pin 1 on both the CPU and display boards.
• Look for bent pins on the headers.

Fix

• Plug the ribbon in the right way round.

Systems

• WPC Alpha-Numeric

Games

• The Machine: Bride of Pinbot

Related tips

• Display shows random periods and shaky digits after its ribbon was unplugged
• Alphanumeric WPC games: weak or dead score displays
• Flipper blows its fuse after a rebuild: diodes shorted by reversed power wires
• Alphanumeric display segments or digits missing

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