One line of the dot matrix display is dark because a metal leg (tab) on the older leg-style plasma display has broken off right at the glass, leaving nothing to solder to.

DMD line out from a broken pin at the glass: bridge it with conductive glue

Cause: The tab carrying that line has snapped where it leaves the glass, often from the display being knocked when the backbox insert is moved.

Checks

• Look along the top and bottom edges of the glass for a missing or cracked leg in line with the dark row or column.

Fix

• Apply a small amount of electrically conductive glue or paste to reconnect the broken leg to the stub of contact left in the glass, and let it cure fully before powering up.
• If there is no contact left showing, a careful repairer can grind a small notch with a rotary tool to expose it, then glue or solder a fine jumper from there to the broken pin.

The DMD runs on high voltage: unplug the game and let the display supply discharge before working on the display.

Systems

• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Display shows garbage or nothing though the glass is fine
• Missing rows or columns on a pin-style display
• DMD fully lit with no picture while the game plays: bad DMD panel
• Dot matrix display with dark gaps or dim areas: outgassed glass
• Blank display, garbage or diagonal lines: reseat ribbon cables
• Moving vertical bars across the DMD with known-good display parts: CPU board fault
• Dot matrix display supply voltages
• Which dot matrix displays fit WPC games

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