I'd like to add one other possibility to Pete's excellent summary. On my 73, I have frequently seen alternator inop lights come on intermittantly, or even solid, due to unbalanced voltage regulators and other wiring faults (for example, lose diodes in assembly at top center of forward firewall - alternator warning circuit board). Here is an excellent thread on the subject of alternator problems. Bob Cook provides some good troubleshooting steps.
http://www.337skymaster.com/messages...=&threadid=604
Kevin