I have been having this issue since i bought my 74 120y.

1.Problem is my dash alternator light stays on and brightness increases with revs
2. When measure with multimeter at alternator and battery i get 14.5volts when reving about 2000rpm and up so seems normal
3. When i keep reving it the voltage stays at 14.5v but at arpund 5000rpm my alternator shoots sparks every where
4. I have tried 3 alternators and 4 new and used voltage regulators nondifference
5.have changed the whole dash with wiring look as i thought its a short somewhere still same problem
6. When driving and rpm get high enough alt light get realy bright and blows fush
7. When i connect the external voltage regulator my rpms drop so inassume thats normal and charging

Know im out of ideas and i disconnect my regulator to drive and have to charge my battery everynight


Please shoot some ideas on what it could be as im out of ideas

Just one thing it was converted by previous owner to manual from auto but i have checked the wiring and cant find any issues

Thanks