R26 not charging

[BSA Bob]

My R26 suddenly stopped charging with the red light on. It had briefly stopped charging when revved hard on a ride but came good, the other morning it wouldn't charge when I started it.
I'm not confident to diagnose the problem, the manuals are vague about what the issue might be.
It has a standard generator & ignition set.
I've removed & cleaned the brushes & cleaned the commutator, they look OK & I can't find any wiring faults.
I connected a voltmeter between terminal 61 on the generator & ground. No reading. The engine stopped when the battery negative lead was disconnected.
The battery's good, measuring about 6.3 volts. No difference in reading across the terminals with the engine running.
I've cleaned the voltage regulator. I noticed that there are no gaps between the 2 contacts "b" in fig 212 of the German language repair instructions manual on this site.
And lastly, is there a diagram of the electrical connections in the generator unit itself? 

[BSA Bob]

[Heiko]

Here it is:

https://cms.bmw-einzylinder.de/index.php/literatur/anleitungen-und-abhandlungen.html?id=99

[BSA Bob]

Thanks for that, Heiko.
Do I infer from another post that you work on steam locomotives? Wonderful machines, I can talk about them all day.

Unfortunately I still can't get an idea of what the R26 fault is.
I've checked & cleaned the brushes & commutator again. Using a meter there's current flowing between the commutator & terminals- ground, 15 & 61.
I bridged the terminals 51 & 61, this blew a fuse on the + side of the battery.

Does this give any clue as to what might be wrong, alternator or regulator?

[BSA Bob]

As per the piece Heiko sent, I tried to repolarise the alternator.
I connected a wire from the + side of the battery to terminal 61 with the ignition off, & with the engine idling.
The regulator immediately started smoking at points "b" I mentioned in my first post.
The 15 amp fuse on the + side of the battery didn't blow.

[Heiko]

Thanks for that, Heiko.
Do I infer from another post that you work on steam locomotives? Wonderful machines, I can talk about them all day.

Thank god, no!  I am an authorized expert for rail vehicles and railway operations manager.

[Dieter57]

And for this to Heiko ?