1967 Fastback with original 1600 engine, dual carbs, 30amp 12v generator, original Bosch regulator. Our red warning dashlight came on and does not go out until we shut off engine. Took a voltmeter first to battery to see how things look. Engine off the battery is over 12.5 volts. Start engine, idling and drops to around 12.2 volts (hmmmm) and rev engine to ensure generator is generating and voltage at battery drops to almost exactly 12 volts. So it's not charging at all and the battery is basically supplying all the voltage for running the car, spark plugs, etc. Check the input to the regulator from the generator and at fairly high revs it's producing no more than 6 - 8 volts on my digital voltmeter and the output of the regulator (B+) is basically less than a volt or two. There is a procedure in the Bentley on how to disconnect the generator from the regulator (only for a few short test seconds cuz apparently this test can burn out windings if you keep it going) to test the output from the generator and it showed 6 - 8 volts at around 1000 - 1300 rpm and if we really revved the engine (3000rpm) it got around 10 volts on our digital voltmeter but the manual said you should get more than 30 volts with no regulator at high rpms. I'm wildly assuming that my digital voltmeter is reading the output of this kind of generator accurately and I don't need to find an old-style voltmeter with a needle that sweeps across the scale to read these voltages.
Time to find a new generator?