Another wire you might want to check is the ground wire from the back of the alternator to the block. This wire is necessary because the alternator is mounted on rubber bushings. If is broken, the return path for the battery charging current will be open. Hence, charge light on and only battery voltage readings. I would think the only other problem would be a brush not making contact. Brush assembly is easy to remove. I would not expect all the diodes to fail at once. If one fails this usually results in the light glowing only dimly at idle. To test the alternator independent of the regulator look up "full field test" in a shop manual. Again, its easy with an external regulator.