Switch rows 5 and 6 never register. Replacing the LM339 row receiver (U19) and the 74LS240 (U13) on the CPU board made no difference; the column drives are fine.

Two switch matrix rows dead after replacing the chips: broken traces from U19

Cause: Traces from U19 were broken, even though there was no visible corrosion.

Checks

• With a logic probe and a board diagram, follow the row signals from the connector through U19 to U13 and find where they stop.

Fix

• Repair the broken traces with jumper wires.

Systems

• WPC DCS

Games

• Star Trek: The Next Generation

Related tips

• Dead CPU: the order to replace chips
• Bench-check the WPC CPU switch matrix circuit
• Dead CPU: no clock signal at the 6809
• Dead CPU: check the reset and IRQ lines
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• All playfield optos dead after replacing one opto: failed 16-opto board
• Ninth switch column on Twilight Zone, Indiana Jones and STNG
• Dead CPU: isolate it from the other boards

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