In the upper left drop target bank, the top target drops but doesn't score, and knocking down the other two makes the game act as if all three are down.

Drop target never scores: switch wire moved to the wrong side of the diode

Cause: A previous repair had put a jumper on that target's switch with the wire on the wrong side of the diode; there was also a broken wire on another switch in the bank.

Checks

• Compare the wiring on the bad target switch with the working ones: the wire and diode should be arranged the same on each.

Fix

• Move the wire back to the same lug as on the other switches (replace the diode if it still doesn't work), and repair any broken wires.

Systems

• Williams System 7

Games

• Black Knight

Related tips

• One switch closing lights up its whole row in switch test: bad column driver chip
• Switch matrix chip and PIA keep failing: coil voltage leaking into the matrix
• Column 1 switches and switch wire colours

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