The dynamo looked suspect, so I replaced that one for a spare one, but that still didn't fix the voltage.
So next I looked at the regulator box and found that the voltage coil screw was turned almost all the way in, leaving no discernible gap. So I adjusted the gap and found the voltage almost too high afterwards. Maybe I should have looked at the regulator first, but I never had many problems with these. Until recently that is.
After some more adjustment I decided to limit voltage to approximately 13.5 volts, to prevent damage to the battery while not running the battery down when we need to drive a long distance with the lights and wipers on. Meanwhile I'll put a new regulator box on my wish list as the old one looks a bit tired. I know 13.5 volts is low, but since I have experienced an exploded battery caused by too high a voltage I like to stay on a fairly low charging voltage.