Any of the above plus defective battery, internal failure of the alternator beyond the brush/regulator assembly (or if not internally regulated perhaps the regulator itself), the ignition switch, and the ever popular disintigrating wire harness. All of them happened to me with my 1980, like a bad alignement of the planets, only for real, not mumbo-jumbo. The wiring harness can do some totally illogical things that only make sense if you know about the posibility of it failing. I had to fix the alternator 3 times, each a different failure before figuing out the wiring harness issue. After the wiring harness repair, I ended up replacing the alternator because the end frame was worn out.