In switch test, closing one switch makes other switches in its column register too (for example 51 also shows 01, 21, 31, 61, 71), although the playfield diodes test good.

Closing one switch shows several in the same column

Cause: A fault in the lamp matrix (a shorted lamp socket or a shorted lamp transistor) feeding back through the shared strobes.

Checks

• Run the single-lamp test and watch for two lamps lighting together, or a dim lamp.
• Unplugging A3P4 (lamp returns) doesn't isolate it, and removing A3P3 kills both matrices, so trace through the lamp test.

Fix

• Repair the lamp socket or replace the lamp driver MosFET.

Systems

• Gottlieb System 3

Related tips

• Pressing a flipper also triggers the opto switches: bad diode and botched ground repair
• Switch diodes are on the A17 board, not the switches
• One switch row works in test but not in play: overheated strobe MOSFET Q36
• A whole row of eight switches dead

Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)