The OP should do a load/voltage wave form test on the alternator. The basic output is three-phase AC, which is converted to DC via the diode trio and rectifier bridge. If one diode opens up the alternator will still function, but with one of the three phases lost, alternator output is reduced by about a third at all rotor speeds.

An open diode can be seen on the voltage wave form output with a high resolution scope, and any decent electrical shop should be able to quicky diagnose.the problem,

If it has a open diode the least expensive solution would probably be to buy a rebuilt OE spec alternator.

Correctly diagnosing the problem and replacing the failed part or assembly with new or rebuilt OE equivalent is an easier and cheaper way to repair than a back yard re-engineering job, but it happens all the time, usually with less than satisfactory results.

Duke