Yes, the external (not in the alternator) Voltage Regulator (VR) is adjustable.
I suppose you wouldn't normally find this necessary, but my having a dual battery set-up
and losing a bit of juice through the isolator, I did. I wrote it up here because manuals and
the online wizards will usually tell you you can't. Therefore, it may be difficult to find
information on how to do it.
As always, if you screw this up rendering yourself balding, sterile, or both, I am not
at fault. Only kidding other than to say I have issued my disclaimer.
Actually, this is quite an easy procedure and can mean the difference between a
dead battery or slow start and a well functioning charging system.