Well, I hope this helps...
I did a bit of troubleshooting tonight and determined that the problem must be worn brushes (probably).
A check of the book then told me that the brushes have about a 60,000 mile (100,000 km) lifespan. I wish I'd read that first!
In any case, I'll be removing the alternator even though my Midget has only 48,000 miles, at least I'll be eliminating that as a possibility. ...or perhaps I'll put an oscilloscope on it first, just to confirm...I'll let you know...
On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"
'68 Sprite 1275
'76 Midget 1500