None of the eight drop target switches in column 7 register in test or play, and a switch error shows at power-up, while the column driver and wiring test good.

All eight drop targets never register: check the drop target board plugs

Cause: The drop target switch boards carry their own LM339 circuit and need 12 V and ground; a connector feeding them had been left unplugged.

Checks

• Before testing chips on the CPU board, confirm every drop target board has 12 V and ground at its connector.
• A steady level at the LM339 on a drop target board, with no change when a target drops, points to missing power.

Fix

• Plug in (or repair) the power connector to the drop target boards.

Systems

• Williams System 11C

Games

• Riverboat Gambler

Related tips

• Lane change not working on Taxi and later System 11 games
• One playfield switch does not register
• How wiring faults show up in switch test
• Strange behaviour across a whole row or column from a switch diode
• A switch does not register in the switch test
• Switch matrix dead after an under-playfield adjustment with power on
• Row or column error: CPU board or playfield?
• A whole switch row or column is dead

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