After an accidental short (which also killed the 12 V regulator on the driver board), switch columns 4 and 7 stopped working. Replacing U20 and then U14 brought column 7 back, but column 4 stayed dead.

WPC-89 CPU: one switch column still dead after a short: trace U14 to U20 to J206

Cause: The new socket fitted for U14 had a bad contact at the pin feeding column 4.

Checks

• For column 4, check continuity from U14 pin 2 to U20 pin 4, and from U20 pin 15 to J206 pin 4 (follow the same pattern for other columns using the schematic).
• Compare C14 and R70 with the matching parts on working columns.
• Meter from the IC leg itself, not just the socket, after fitting a new socket.

Fix

• Replace or repair the faulty socket contact (or solder the chip in) so the U14 output reaches U20.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics

Related tips

• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test
• Row or column "ground short" errors
• Why CPU U20 fails and how to replace it
• Switch matrix chips on the CPU board
• Switch matrix dead after a screwdriver slip under the playfield
• One switch does not work
• "Slam tilt stuck closed" at power-on

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