Drop target switches 17 to 21 never register, while the reset coils work and nearby switches 23 and 24 are fine.

Five drop target switches stop registering together: no ground at the opto board

Cause: These target switches are optos on one board behind the targets, and that board had lost its ground connection.

Checks

• If other switches on the same driver line work, the driver transistor is fine; look at the opto board.
• Measure power and ground at the opto board connector.
• Look for cracked solder on the board's header pins.

Fix

• Restore the ground to the opto board and repair any cracked joints; clean the optos while you are there.

Systems

• Williams System 11B

Games

• Swords of Fury

Related tips

• Drop target not sensed when down
• Opto switches dead all at once
• System 11B opto board fails with good optos

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