[Roadsters] battery not charging

[Tim]

First off: I am an electrickery novice. Big time.

This is a new condition this spring: 70 1600 w/ gm alternator upgrade 
done three or four years ago. Alt is putting out over 12 volts at 600 
rpm idle; more at higher rpm. Battery is over 12 volts once charged with 
battery charger. But car won't start if battery does not have most of a 
fresh charge from the charger. Amp meter will go into the negative if I 
turn on the head lights but never goes into the positive any more.

Is this my regulator going (gone) bad? How do I test it?

Thanks

Tim