After a CPU board swap and repinned switch connectors, one entire column of switches suddenly stops registering.

A whole switch column stops working: replace the ULN2803 column driver at U20

Cause: The ULN2803 column driver (U20) on the CPU board had failed for that column. The board showed signs of old corrosion and the chip had been replaced before.

Checks

• Use the switch matrix chart to confirm every switch in one column is dead, then trace that column to its CPU pin in the manual.
• With power on, probe the outputs (pins 11 to 18) of U20 with a logic probe and compare each column, and check the input side too.
• Check the traces and through-holes from U20 to the switch connector for corrosion damage.

Fix

• Replace U20 (ULN2803), preferably in a socket.

Systems

• WPC DCS

Games

• Demolition Man

Related tips

• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test
• Why CPU U20 fails and how to replace it
• Switch matrix chips on the CPU board
• One switch does not work
• Whole switch column dead after a diode went in backwards: column driver chip
• Row or column "ground short" errors
• Switch matrix dead after a screwdriver slip under the playfield

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