Switch matrix faults return after repair: the matrix driver chip and the PIA on the CPU board blow again, often after the upper right three-bank resets or a pop bumper fires.

Switch matrix chip and PIA keep failing: coil voltage leaking into the matrix

Cause: Two faults together. A blob of solder on one of the three-bank target switches tied the metal target assembly into the switch matrix, and the worn wrapper on the bank's reset coil had let a coil wire rub bare against that metal. Vibration then shorted about 34 V DC into the matrix, killing the chips.

Checks

• Before fitting new chips, inspect the playfield for anything that could touch the switch wiring to coil voltage: bare coil wires, worn coil wrappers, and solder blobs bridging switch lugs to metal brackets.
• With the game off, check for continuity between switch matrix wiring and the metal target bank or coil brackets; there should be none.

Fix

• Remove the solder blob from the target switch, replace the damaged coil (and its wrapper), then replace the failed matrix chip and PIA.
• Check the coil has the correct coil stop and sleeve so the new wrapper doesn't rub through again.

Systems

• Williams System 7

Games

• Black Knight

Related tips

• One switch closing lights up its whole row in switch test: bad column driver chip
• Solenoid fuse F2 blows at power-up as every coil fires: CPU blanking circuit
• Damage after the black and white plugs were swapped
• Test the switch PIA IC11 with the Leon test EPROM
• Re-jumper a System 7 CPU from Defender/Hyperball/Star Light to a standard game
• Free play won't take on System 7: do a factory reset first
• Score displays dead but digits glow: bad wire in the CPU blanking plug (1J3)
• Drop target never scores: switch wire moved to the wrong side of the diode

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