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On my '78 R100/7, the gen light comes on as usual at idle and it used to go off within a few hundred rpms or rev. But the past couple days now the idiot light stays on for a couple three k of rpm. It's a bit sporadic too. At night now, it was easy to see. When pulling out at a green light, the gen light was on until maybe 3k rpm, then all was good. When stopping, sometimes it was ok, but then the light would kick on and stay on almost like no amount of revving would get it go off.
What do you think, voltage regulator time, or maybe something else?

Check your battery post connections, then ground at the tranny.
Intermittent failing sounds like a connection issue.

Check that the brushes are not getting hung up in their holders. Do the springs have enough pressure to hold them from bouncing against the slip rings?

Kurt in S.A.

SteveD wrote:...follow Jeffs suggestions.

Absolutely.
On a case by case er, case.
After three days of close scrutiny by wary members.

ME 109 wrote:Check your battery post connections, then ground at the tranny.
Intermittent failing sounds like a connection issue.

Thanks. Nothing looked out of the ordinary. Cleaned and tightened battery and tran. ground connections to no avail.

Kurt in S.A. wrote:Check that the brushes are not getting hung up in their holders. Do the springs have enough pressure to hold them from bouncing against the slip rings?

Kurt in S.A.

Both the brush and spring are appear to be in good working order. I pulled each alternator connector and cleaned same to no effect. Gen light stays on until 2k + revs.

SteveD wrote:The GEN light is still working so simple things first...follow Jeffs suggestions.
When mine developed similar symptoms, the rotor eventually died and the GEN light stayed off. That happened within 2-3 weeks of regular riding, with the GEN light going out regularly over 1800rpm, usually higher.

Yep, those symptoms seem to be going on here. Any deals on rotors anywhere?

Everyone should keep a spare rotor handy, just for when you need to do some elimination tests.

Is it worse when hot, or cold?

I’ve had a couple of rounds with my alternator. I’ve found that there are several things that will make the light stay on too long. Even too much carbon dust from brush wear will ground out the brush holders.
What I’ve learned from my battles is before you start throwing money at a alternator....First, remove the stator assembly, (it’s the only way you can really inspect the inside brush). Thoroughly clean the dang thing, and shine up the slip rings if they need it. Next, make sure the bushes and springs are working perfectly and then carefully put it all back together. Once you have no doubt that the brushes could not be causing your dilemma then you can start looking at other parts.