Alternator help

It will help if you show a picture of the regulator, it should be an electronic version of the early regulator.

The regulator has three connections, ground, IGN, and FLD. The IGN connects to a blue wire that hot when ignition is on. It is on the supply side of ballast resistor, not coil side. The FLD is connected to the green wire that goes to field connection on alternator.

Since you have a voltmeter, check the voltage at the IGN terminal, it should be close to battery voltage when running, typically 13.8 to 14.4V. If that connection is poor, the regulator will be unable to supply enough field current for regulation.