68 Barracuda alternator question

seems to have quit charging - after a weak cranking start-up and a short drive, battery is drained below 12V and with car running my volt meter is only showing the remaining low charge left in battery where previously it would show pretty solid 14V or so. SO.. when I put a voltmeter on the output stud of alternator with engine running in buries the needle off the scale.. that was strange..but maybe not if it's dying..Anyway am I on right track thinking I need to replace this old alternator? It's likely not original to car but it is obviously an oldie..any comments appreciated.
thnx