The 5 V regulator section of the solenoid driver board makes 5 V with no load, but as soon as the 5 V is connected to the rest of the board (or the board is plugged in) all the low voltages sag to a volt or less.

Solenoid board 5 V collapses under load: open 12 V bridge on the rectifier board

Cause: The bridge rectifier that makes the 12 V on the rectifier board had an open section. It measured fine on a meter and showed 12 V unloaded, but could not supply any real current.

Checks

• Load-test the 12 V DC at the rectifier board with a 12 V automotive bulb from the 12 V test point to ground: a good supply lights it brightly.
• If the bulb lights on the transformer's 12 V AC winding but not on the 12 V DC, the bridge, its fuse, filter capacitor C23 or the wiring between them is at fault.
• Running the solenoid board from a bench supply confirms whether the board itself is good.

Fix

• Replace the original bridge rectifier for the 12 V on the rectifier board, even if it tests good with a meter.

Transformer AC is live on the rectifier board while you measure; keep fingers clear and power off before removing the board.

Systems

• Bally MPU AS-2518-35

Related tips

• Fuses blow with a brand new rectifier board: cold joint on BR1
• Solenoid driver TP5 under 13 V
• Rectifier board test point voltages
• No +5V on any board and no MPU LED: bad return pin in the solenoid driver board J3 plug
• Controlled lamps dim and TP1 reads about 2.8 V instead of 5.4 V: half the bridge open
• Where the 43 V solenoid supply leaves the rectifier board
• MPU won't boot and driver board test points read low: replace the solenoid driver board
• After a while all solenoids except flippers drop out until power cycle: rectifier board

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