Your rotor gets power on the same brown wire that feeds the reg/rec. If the rotor has a short to ground that will blow the fuse. Did you check the ohms of the rotor from slip ring to slip ring as well as from slip rings to ground? Should be about 5 ohms ring to ring, infinity to ground.
On the 81 the charging system was ungrounded at the brushes. The earlier bikes 70-79had the grounded brush.
Leo