After years in storage, lamp test shows full rows lit (sometimes several at once), and lamp columns 6 and 7 never strobe but sit at a constant 12 V. Coils fire at random and the game throws balls out of the trough.

Two lamp columns stuck on and full rows lighting in lamp test: failed U20 and U23

Cause: The lamp column driver chips at U20 and U23 on the CPU board had failed (U20 first, probably after playfield wiring faults, then U23).

Checks

• Use lamp test to find which columns misbehave, and check those column outputs with a logic probe or oscilloscope.
• Lift a leg of nearby diodes before testing them; in-circuit readings mislead.
• Fix playfield wiring and lamp faults that may have damaged the drivers.

Fix

• Replace U20 and U23 on the CPU board.

Systems

• WPC-95

Games

• Junk Yard

Related tips

• None of the controlled lamps work, or they light in the wrong pattern
• A whole switch row or column is dead
• Row or column "ground short" errors
• After a new U20, work every flagged switch once
• Switch columns to U20 pins
• One switch also triggers another: corroded switch connector on the CPU board
• Testing CPU switch rows and columns with a diode and clip lead
• "Slam tilt stuck closed" at power-on

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