With the game on, every board's +5V test point read zero and the MPU LED never lit, although all the rectifier board test points and fuses were good.

No +5V on any board and no MPU LED: bad return pin in the solenoid driver board J3 plug

Cause: One pin in the Molex plug that carries the unregulated 12V feed (and its return) from the rectifier board to the solenoid driver board's J3 had failed. The driver board's 5V regulator gets its input that way, so no 5V reached any board.

Checks

• Measure TP5 on the solenoid driver board; it should be about 12V DC (the regulator input). Zero there points to the feed from the rectifier board.
• Measure at the connector pins, not just the test points, following the schematic from the rectifier board to J3 on the driver board, including the return pin.
• Check each pin in the J3 plug for continuity and a tight fit.

Fix

• Replace the failed pin (or the whole set of pins) in the J3 plug and reseat it.

The rectifier board and power wiring carry transformer AC and the high-voltage display supply; take care when measuring on a powered game.

Systems

• Bally MPU AS-2518-35

Games

• Paragon

Related tips

• Fuses blow with a brand new rectifier board: cold joint on BR1
• Rectifier test point ranges with fuse and rectifier
• Rectifier test point reads below its range
• After a while all solenoids except flippers drop out until power cycle: rectifier board
• Rectifier board test point voltages
• Backbox GI out and fuse F6 blowing: burnt rectifier J3 plug and a shorted bridge
• Where the 43 V solenoid supply leaves the rectifier board
• Controlled lamps dim and TP1 reads about 2.8 V instead of 5.4 V: half the bridge open

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