Switch test shows full rows closed even with the playfield unplugged, and the switch column driver U20 keeps failing.

Whole switch rows read closed and U20 keeps failing: replace U23 on the CPU

Cause: U23, which feeds U20, was getting good input signals but producing no output.

Checks

• With a logic probe or oscilloscope, compare the inputs and outputs of U23 and U20.
• Check the nearby resistors, diodes and capacitors and the voltages around them.
• After repairs, test the matrix by shorting rows to columns at J206 and J208 in the switch edges test before reconnecting the playfield.

Fix

• Fit a socket and a new U23, plus a new U20.

Systems

• WPC-95

Games

• Junk Yard

Related tips

• A whole switch row or column is dead
• Testing CPU switch rows and columns with a diode and clip lead
• Why CPU U20 fails and how to replace it
• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test
• Testing CPU switch rows with the switch levels test
• Telling a playfield switch short from a CPU board fault
• Row or column "ground short" errors

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