I have an interesting intermittent charging problem on my 1980 Spada.
The charge drops to 0 (zero) and the battery light comes on if any of the following electric components are used: Headlight (but ok if only sidelight), Brake light when using front brake lever (but ok if using foot operated brake) and finally the neutral indicator light! The problem started about 150 miles into a 1,250 mile journey round England and Wales a few weeks ago; I completed the trip ok - basically by not using the headlight 
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Anyone got any ideas where the problem is / where I should start tracing the fault?
Check your battery is holding 12.5 v when standing and 14.5 v when revving above 4000 rpm roughly At the same time check your battery connections are good and tight and the earth lead is clean going to earth. The earth leas under the battery tray never hurts from removing the bolt and cleaning up the hole and everything.,
If this doesn’t solve it then-
If the first you have a battery problem, if the second a charging problem still.
If second take off alternator cover and inspect the brushes, they have a wear line on them.
The copper slip rings will be a dull brown colour, this is correct.
If ok check all of the wire connections are tight and bright, clean.
Then check the 3 phase going into the rectifier for the same.
Check especially that the earth wire is fully grounded, I added a separate earth wire.
Always check earths first.
Thanks Ian - given the intermittent nature of the problem I think you may be on to something regarding dodgy earths so I’ll start there …
With electrics take it slowly and try one thing at a time.
If that doesn’t work put it back to standard.
This only falls apart if you have 2 faults on the same system, ask Eddie Cox, that is when someone like me earns his corn or gets lucky.
Both are the same in the end.
as above and also check the fuse box and spin the fuses around, that is if you are still using the old 16amp ceramic continental ones as they can give trouble
, in fact just a hour ago whilst riding my V65, (same fuses) the charge light came on when I turned the headlight on, back home I removed tank and pulled the 12 and 15 pins connector blocks apart, pushed them back tighty together. then span the fuses around in their holders, jumped back the bike and everything was working fine.
Now this where northwest and myself differ.
I still have the 31 year old ceramic fuses and holder on my Mk111.
I think they are better than the later Ford or blade type fuses.
But remove your negative battery lead and clean the terminals and fuse ends can solve problems though I doubt this one.
I had exactly this happen going home from Inn in the Country camp.
Alternator rotor gone high resistance. Measured as 40-something Megohms, (should be 3.5 Ohms); but I suspect that when running, more current than that suggests was actually getting through, because it just managed to power the ignition system (and keep the Voltmeter around the ‘12’ mark) but naff all else.
I measured at the slip rings, so suspected broken wire.
This was not the first occurrence, previously, again coming home from Wiltshire branch camp, what I now know as the same thing happened for about 20 minutes while negotiating Marlborough high street. I assumed it was simply because of having the headlights on (110 Watts plus 20 Watts sidelights, plus ignition etc.). But no indeed it was a harbinger of impending demise…
Well if you need to replace a rotor or stator then West Country Winding do exchange items which are wound tighter to give better output than originals for less than new price.
^ …and wot 'e said. 
You have my sympathies where intermittent faults are concerned 
Several possible causes as noted above.
This makes an interesting read
http://www.thisoldtractor.com/guzzitech.dk/pdf/charging-around-the-circuit-john-noble.pdf
Good luck
Steve