x4 bad ground, depending on the year the bike might only have 3 wires from the regulator, (2 for the crankcase plug and 1 to the circuit breaker) later ones have 4 wires, same as before but with a ground wire.
If you have the earlier 3 wire type, when you replace the regulator look on the bottom and you will see a hole built into the casting, connect a 4th wire to this hole with a self tapping screw and run it directly back to the ground connection on the battery. this does away with the unit grounding out through the bolts holding the regulator to the frame, these bolts corrode, insulating the unit and stopping it from dumping excess to ground and then it burns out. I had the same problem about 9 hyears ago and went through a batch of regulators, to the point of keeping a spare in the tourpak, since making this mod I have never replaced another unit. As an extra precaution use dielectric grease on the self tapping screw.
Hope this helps your buddy
Alien