The fuse for the regulator also powers the turn signal, reverse lights, and meters (not the tach). Make sure these curcuits are working properly.
The field winding current is supplied through this fuse. Perhpas the field current is too high. Measure the resistance of the field coil. It should be about 2 ohms.
15 volts is actually ok. The spec in the manual allows from 14.3 to 15.3 volts at 68 degrees F.