charging system blues

1971 Duster V8 - have a 3 wire alt w/a electronic volt/reg. amp meter indicates normal chg/dischg condition. at idle it shows slight discharge then increase r's it shows on the chg side - but across batt only shows 12v on a voltmeter. amp meter indicates chg voltmeter across batt indicates none. going to look a little closer today. suggestions-direction appreciated. I've read FABO for 2 days getting ideas. thanks, mike.