All the always-on backbox lamps are out while the controlled lamps work; later fuse F6 blows before the game boots.

Backbox GI out and fuse F6 blowing: burnt rectifier J3 plug and a shorted bridge

Cause: The rectifier board's J3 connector had burnt pins on the GI and other supply lines, and one bridge rectifier on that board had shorted.

Checks

• Inspect the rectifier board connectors, especially J3, for darkened or burnt pins and housings.
• Test the bridge rectifiers on the rectifier board.

Fix

• Replace the shorted bridge (or the whole rectifier board if it is badly worn) and rebuild J3 with a new housing and new pins.
• Do the usual solenoid driver board ground mods (capacitor negative legs to ground and the 5 V test point jumper) so the 12 V return stops burning J3.

Work with the game unplugged; the rectifier board carries high current and the transformer secondaries.

Systems

• Bally MPU AS-2518-35

Games

• Skateball

Related tips

• Controlled lamps dim and TP1 reads about 2.8 V instead of 5.4 V: half the bridge open
• Rectifier test point ranges with fuse and rectifier
• No +5V on any board and no MPU LED: bad return pin in the solenoid driver board J3 plug
• Solenoid fuse blows even with the outputs unplugged: fault on the rectifier board
• Fuses blow with a brand new rectifier board: cold joint on BR1
• What each rectifier on the board feeds
• Getting at the bridges on -18/-49 boards
• Rectifier board test point voltages

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