Voltage regulator, alternator or other?

Hey all

My 1981 911 SC was having erratic voltages ranging from 12.8-15.4. it had one of the original style external voltage regulators which I guessed was the problem. I bought a new solid state one from design 911 and now the voltages stay steady but don't go above 13 and gets lower the longer I drive so is draining the battery.

Could I have worked it up wrong or could it just be the alternator that's the problem and the different style regulators are just dealing with it differently? Any help or advice is very much appreciated.

Thanks

Alternator could be faulty along with the gearbox earth strap , always worth haveing a good look at this earth strap ..

Some good info here though ..

https://www.pelicanparts.com/techar...tor_Replacement/20-Alternator_Replacement.htm