[Old Tele man]

...what's the voltage right at the battery terminals at idle? Should be about 12.6Vdc or higher? If not, it STILL could be a bad regulator (internal) or even bad diodes (also internal).

...if you 'slightly' rev the engine, does the voltage go "UP" to something above 12.6Vdc? If not check & clean all grounds (again). The alternator "output" voltage should go UP with engine speed somewhat, but not exceed about 14-14.5Vdc depending upon temperature.