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