What voltage with the headlight fuse in? We are starting to run into electronic voltage regulator failures, just on age I guess. Do the regulator diode check. Here's some more ideas, Check voltage drops, running at idle, across each fuse, voltage drop across the ignition switch. Check the back of the headlight bucket at the 3 grommets for cut or scraped wires. Check connections and battery ground strap. How good is the battery? Ohm the rotor. ohm the stator yellow wires. Guess what I am saying is check everything looking for a smoking gun.
I was able to do some testing tonight but didn't test anything with the bike running because it was getting late. Here's what I got with the headlight fuse in:
After sitting for 24 hrs battery voltage is 12.2v with the key off, 11.39v with key on. Btw...my headlight comes on as soon as the key is turned on...always has.
Key on - voltage at the brushes is 10.3v
Key on - brown wire at ignition switch is 10.5v
Rotor slip rings test at 4.6 ohms
I did the diode test. When the reg/rec was replaced eons ago, the factory connector was replaced with bullet connectors and each white wire was labeled 1, 2, 3.
Diodes 1, 2, 3:
Positive (red) probe on the red wire, Negative (black) probe on the three white wires = no continuity
Negative (black) on the red wire, Positive (red) on the three white wires = W1 8.84 ohms, W2 10.28 ohms, W3 8.76 ohms
Diodes 4, 5, 6: (for this I tested both the brown and black wires and got identical results)
Positive (red) on the black wire, Negative (black) on the three white wires = W1 10.19 ohms, W2 9.57 ohms, W3 10.23 ohms
Negative (black) on the black wire, Positive (red) on the three white wires = no continuity
I checked the wiring running in/out of the head light bucket and didn't see problems with cut wiring.
I'll do some more tests tomorrow with the bike running and post those results.