Some targets add players, tilt the game or end it, and scores come up wrong. In switch test, closing any switch in one column makes every switch on that switch's row show closed.

One switch closing lights up its whole row in switch test: bad column driver chip

Cause: The chip that drives that column of the switch matrix had failed. Here it was a 7406 on a newly installed replacement board.

Checks

• Use switch test to find the pattern: one column whose switches each pull in their whole row.
• Rule out wiring by unplugging the switch row and column plugs and jumpering one row pin to one column pin at a time; if a single jumper still shows several switches, the board is at fault.
• Check that no plug is shifted by one pin.

Fix

• Replace the faulty 7406 column driver on the board (or have the board's maker repair it).

Systems

• Williams System 7

Games

• Black Knight

Related tips

• Damage after the black and white plugs were swapped
• Switch matrix connector pins and their chips
• Switch matrix column test at 2J2 with Leon's code
• Switch matrix chip and PIA keep failing: coil voltage leaking into the matrix
• Logic chips worth stocking
• Lamp matrix row test at 2J7 with Leon's code
• Lamp rows stuck on: the 7406 at IC19
• Lamp row still dead after a new TIP122

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