Dave Barton emailed back, pointing to the "Failure to Charge" section
of his web page:

http://www.davebarton.com/AdjustableVoltage.html

He says if the old regulator works, usually the problem is usually
poor contact between the brushes and the slip rings.  Sometimes it
starts working after some "determined revving," or one can clean the
slip rings or pre-seat the brushes.

I'll give it a try!