The backbox mystery wheel stops in the wrong places or acts erratically. In the wheel diagnostics the position count climbs above 200.

Mystery wheel loses its place: wheel motor control board diodes or opto

Cause: The wheel motor control board has lost track of where the wheel is. Usually a failed diode or the opto on that board; less often the CPU switch matrix, poor solder on the interconnect board, or the supply to the wheel board.

Checks

• Watch the position count in the wheel diagnostics; it should never go above 200.
• Rule out the CPU switch matrix with the jumper-wire test.
• Reflow the header pins on the interconnect board, which is known for cracked joints.
• Check the wheel opto registers when blocked, then test the diodes on the motor control board out of circuit.

Fix

• Replace the bad diodes on the wheel motor control board, or fit a replacement board.

Systems

• Williams System 11B

Games

• Cyclone

Related tips

• Mystery wheel won't turn and its opto never registers: reflow the power supply headers
• Lamps under the slingshots out: wire in the wrong position in interconnect plug J12
• System 11 backbox boards and supplies
• One topper dome flasher pair dead while its shared knocker works: broken wire
• Flash lamp dead on an early System 11 game
• The interconnect board (Banzai Run and later)
• Interconnect board and flipper power board fuses
• GI out on Banzai Run and later: burnt interconnect connector 2J6

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