Do you know how to use a multimeter?
If not, first choise is swap the fuse junction box.
If yes, there is a yellow wire in pin 9 bottom left of small fuse box plug which comes from one of the yellow stator wires that trips the headlight relay, which once tripped holds itself in the on position.
Disconnect the regulator plug with the yellow wires in it, also disconnect the black plug under the carbs holding 5 wires 3 of which come from the stator and do resistance checks from the fuse box yellow wire to each of the 3 yellow wires at the regulator plug and to the 3 yellow wire at the plug under the carbs. One from each plug should go to the yellow wire at the fuse box.
I have heard one person did there's check and found they were ok but on closer inspection of the wire there was only one strand of wire connected, so resistance checks seemed OK , but with only one strand of wire connected it was not allowing enough power through to trip the headlight relay.
For Tech Questions Bike Model letters are more useful than saying the year, for uk owners anyway, as registration year is not always year of manufacturer model.