Voltage Regulator --->>> ~15.2 VDC @ 4000 RPMs

With ignition switch ON, the brown wire will supposedly have full battery voltage at the ignition switch.

Would check for full battery voltage on the brown wire at each connector between the ignition switch and the R/R.

There may be a voltage drop due to a corroded connector.

Good Fortune!

Thanks. I was confused about checking at the main fuse: there isn't actually a brown wire there - one of the fuses has a brown/white & when I first looked at it I thought it was red/white. doh.

Sorry to have confused you Martin. Check the Voltage across the battery, and at both sides of the main fuse, Keep your Meters Black lead at the battery Negative for the tests. The Main Fuse is one of the Most common places for Voltage Drop on these old bikes, thats why I suggest to start there. It also feeds power to the whole bike, except the Starter Motor itself, which runs directly off of the battery cables, using the starter solenoid as a high Amperage Switch.

It was my mistake - a guy (me) can't expect every detail to be spelled out. :blush:
It took me a while to figure out how the brown wire on the R/R functioned - I think I know how to proceed, starting w the fuses & working from there. I'll test some of the easier points first which may speed up pinpointing where the drop is occurring. fingers crossed. Thanks.

You'll get it, as you already have a meter and know how to use it. Thats a big step in tracing these little gremlins down.