After fitting a new replacement PPB board, the game fires coils at random when it should fire flashers, the classic sign of a bad A/C select relay, though the relay, connectors, CPU transistor and drive voltage all check out.

Random coils fire instead of flashers with a new driver board: relay has the wrong coil

Cause: The new board's relay was marked as a 24 V part but had a 48 V coil inside (a factory error), so it only pulled in now and then.

Checks

• Measure the relay coil's resistance and compare it with the correct 24 V part.
• Watch whether the relay actually pulls in when the flashers should fire.

Fix

• Replace the relay with a genuine 24 V coil relay.

Systems

• Data East CPU Version 3

Games

• The Who's Tommy Pinball Wizard

Related tips

• Solenoid driver transistor positions on the CPU and PPB boards
• 50 V coils use PPB TIP36c transistors, pre-driven from the CPU board
• How a coil or flasher is switched: PIA to TTL to pre-driver to driver
• Three-stage drive for 50 V coils
• What the PPB/MRB board carries
• MRB and PPB playfield power boards
• Replace the pre-driver along with a failed driver
• Check PPB solder joints: the board runs hot

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