One drop target (and, it turns out, the spinner on the same switch row) never registers, even after the drop target switch wiring and diode were corrected and the MPU connector J2 was repinned.

Drop target and spinner on one switch row dead: wire hanging off the slam tilt switch

Cause: The row wire on the slam tilt switch, which sits in the same row of the switch matrix, was barely attached; the row is daisy-chained through it, so switches further along lost their connection.

Checks

• Use a switch matrix chart to find every switch on the dead row, including cabinet and coin door switches.
• Follow the row wire from a working switch to the first dead one and check each joint, gently tugging on the wires.

Fix

• Resolder the loose wire on the slam tilt switch.

Systems

• Stern MPU-200

Games

• Galaxy

Related tips

• Credit button does nothing in switch test: daisy-chained coin door wire come loose
• One drop target only registers if it is the first one down: reseat and clean the PIAs
• Spinner hits garble the displays and trigger other switches: post screw touching GI
• MPU switch connector pins J2 and J3
• Switch reads closed even with switch connectors unplugged: shorted resistor on the MPU
• Test a playfield switch diode
• What bad switch diodes do to the matrix

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