After the display ribbon cable was unplugged and refitted, the lower display shows random decimal points and digits that aren't crisp.

Display shows random periods and shaky digits after its ribbon was unplugged

Cause: The ribbon connector was not seated properly (a damaged pin or cracked header joint can do the same).

Checks

• Make sure the ribbon's red stripe goes to pin 1 at both ends and that the connector is fully seated.
• Look for a bent pin and check the header joints on both boards.

Fix

• Reseat the ribbon connector; replace the ribbon or reflow the header if the fault returns.

Systems

• WPC Alpha-Numeric

Games

• The Machine: Bride of Pinbot

Related tips

• Display shows only a few broken left digits after board work: ribbon cable reversed
• Alphanumeric WPC games: weak or dead score displays
• Alphanumeric display segments or digits missing

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