Every lamp in two rows of the lamp matrix stays dark while the rest of the matrix works.

Whole lamp matrix rows dark: broken solid-core lamp wires

Cause: Broken wires. The lamp wiring under the playfield is solid-core and goes brittle with age, and wires also break at the insulation-displacement (IDC) connectors.

Checks

• Confirm every lamp in the row is out, including any in the backbox on that row.
• Meter each row wire from the driver board connector to the lamps; check the IDC connector closely.
• If the wiring is good, check the header solder joints and the row driver on the driver board.

Fix

• Repair or replace the broken wires (and the IDC connector if wires have broken inside it).

Systems

• WPC Alpha-Numeric

Games

• The Machine: Bride of Pinbot

Related tips

• One GI string dead after a restoration: unkeyed J115 plugged in upside down
• Lamp matrix wire colors (all WPC games)
• Single matrix lamp out with good bulb and wiring
• One controlled lamp out while the rest of the matrix works
• A whole lamp row or column is stuck very bright
• How the WPC lamp matrix works
• Testing lamp-row drivers with a test lamp
• None of the controlled lamps work, or they light in the wrong pattern

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