Swap the drain magnets.

With the RHS Drain magnet disconnected and a new 3A SB fuse installed, you should be able to play a game. Also, solenoid 14 (Trapdoor hold) is part of the same F104 circuit. Check that the trapdoor mechs are working 100% and there's no friction of the moving parts.

You could also check the voltages at the lugs of any solenoids. With the playfield raised and the front door closed, you should see the same DC voltage on both lugs on just about every coil.

From memory, the magnets should read 7.5 Ohm (game off and measure with the magnets disconnected).