Just tried my '66 GTO's mechanical voltage regulator on my friend's
'69 Firebird and it started charging his battery again. Even though
he had bought a new voltage regulator a month or so ago. (And then an
alternator.) His does not have the condenser installed on the voltage
regulator like mine does, but there's a terminal for it.

I'm wondering if the condenser is necessary, maybe to keep the
contacts from burning? I'd heard it was to prevent radio
intereference, which we don't care about. I can't find anything in
the service manual about it. If it's necessary to prevent contacts
burning, maybe his new voltage regulator fried its contacts.