Mine did similar some time back. The screws holding the front bearing in its housing came out and damaged the field windings. Found the alt is similar to one used on a Holden Barina, local car re-badged. Fitted field coil, loctited scews and tested ok. Been good for last 4 years. If you take it to an old time auto electrician I am sure it would be repairable. So many newer repairers only want to replace parts instead of repairing. Nigel.
Found out it was NOT the alternator.
The bolt the holds the shaft that drives it had come loose from the engine.
Replaced bolt, secured it with locktite.
Done 3.000 km now without problems.