The MPU (a replacement board) never boots: its LEDs stay on and nothing else happens. The 5 V test point on the solenoid driver board reads only about 4.3 V and the 5 V adjustment pot barely changes it.

MPU won't boot and driver board test points read low: replace the solenoid driver board

Cause: First the rectifier board outputs were wrong (fixed with a new rectifier board and repinned connectors); after that, the original solenoid driver/regulator board still could not regulate 5 V properly.

Checks

• Measure each rectifier board test point against the values in the manual before anything else.
• Repin the old Molex connectors between the rectifier, driver board and MPU.
• Measure the driver board test points; 5 V must be close to 5.0 V for the MPU to start.

Fix

• Replace (or rebuild the regulator section of) the solenoid driver board; a replacement driver board fixed it here.

The driver board carries the high-voltage display supply (over 190 V); take readings carefully.

Systems

• Bally MPU AS-2518-35

Games

• Paragon

Related tips

• After a while all solenoids except flippers drop out until power cycle: rectifier board
• Controlled lamps dim and TP1 reads about 2.8 V instead of 5.4 V: half the bridge open
• No +12 V at rectifier board TP3
• Solenoid board 5 V collapses under load: open 12 V bridge on the rectifier board
• Rectifier test point reads below its range
• Fuses blow with a brand new rectifier board: cold joint on BR1
• Solenoid driver TP5 under 13 V
• Rectifier board test point voltages

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