After replacing a playfield switch, every switch on one column of the matrix stops registering. Another CPU board in the same game reads them fine.

Whole switch column dead after a diode went in backwards: column driver chip

Cause: The new switch was fitted with its diode reversed, and the column driver chip on the CPU board (a ULN2803) was damaged. A missing or reversed diode on its own normally just upsets the matrix, but shorting switch voltage onto a column can kill the driver.

Checks

• Fix the diode orientation first (match the band direction on the neighbouring switches) and retest.
• Swap in a known good CPU board to prove the fault is on the board and not in the wiring or connectors.
• On the suspect board, check the column driver and its matching resistor and capacitor for that column.

Fix

• Replace the ULN2803 column driver; socket it so it can be swapped easily. If a first replacement does not cure it, try another known good 2803 before chasing the LM339 or other chips.

Systems

• WPC DCS

Games

• Demolition Man

Related tips

• A whole switch column stops working: replace the ULN2803 column driver at U20
• Switch matrix chips on the CPU board
• Switch matrix dead after a screwdriver slip under the playfield
• Testing CPU switch columns with the switch levels test
• One switch does not work
• Switch columns to U20 pins
• Row or column "ground short" errors
• Why CPU U20 fails and how to replace it

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