Notwithstanding the foregoing, there are two bits of elastic-trickery magic inside an alternator. One is the much discussed diode or rectifier pack, which turns wiggly AC into nice smooth DC, but there is also a regulator pack, which controls the output voltage by twiddling the field winding strength. (circuit diagram of alternator for you, if it helps) If your charge light gets dimmer / brighter / stays on / won't come on and your alternator isn't chucking out 14V+ and your battery is going flat, then I think it is the regulator circuit myself.
Overall, have the bugger out and get yourself to an auto-electrical specialist place - they can test it, fix it and replace it if needs be. You can self-repair alternators, but I (speaking only fot me) wouldn't bother.