Well that was very instructive, on a couple of points.

I hooked up my voltmeter to the cigar lighter, because it's easier to monitor that while blipping the throttle. With the key off, the voltage was about 11V ... even though voltage at the battery terminals is 12.5V ! That's quite a drop. I'm sure the contacts in the cigar lighter are none too clean. Anyway...

Switch on, start. Charging light stays on, voltage stays at 11. Bring revs to 2000, charge light goes out, voltage rises to 12.3 or so, stabilizes. Drop revs below 2000, voltage goes back down, charge light stays off.

So, even though the charge light is off, the battery may not be being charged. Very good to know... guess I'll try adding a resistor to my charge lamp sometime.

For what it's worth, I finally got a round tuit (had a surplus of square ones to use up), and installed the 5 ohm resistor as listed in the Factory Service Bulletin. The charge/alternator light goes out immediately on firing, rather than having to rev the engine to 2000-2500. Quite a difference.

Only time will tell if it keeps my battery and turn signals happier. It was an easy change, once I got the instrument out. I followed the Bulletin's directions, except that I added a bit of liquid electrical tape and heat shrink to keep all that +12V more or less insulated.

Have been told for years that alternators have a series of diodes in them. One or more of them can be burned out and the alternator may still charge but until the one that controls the "charging light" burns out will it illuminate.

Wonder if the problem is with the diode pack and a defective diode here? The guys who got their light to come back on and function properly all seem to have replaced the diodes during a rebuild of the alternator.

Worth checking out...

What happens if you install one of the newer alternators in your older car but you do not add that resistor?

Does it blow up the diodes? or what?