Your words “went dead completely”, “fiddling” “buzzing in the headlight” - brought back for me an incident with my /7 on the road many years back.  After hours of searching and analyzing, turned out to be nothing but a bad battery. Something inside my 13 month old WestCo battery had failed and was causing VERY strange electrical issues, including those you describe. 

I used to own a /7, and mine did that a few times. I was mystified for a few day until I took the fuel tank off. Under the tank on the left side is the starter relay. It is fitted in a kind of socket and held to the frame with 2 bolts. I removed the bolts and pulled the relay and holder away from the frame a tiny bit and turned it over to look at the underside. At this moment, a red wire jumped out of its segment in the multi connector. That red wire provides 12 volt potential to the relay and thru a parallel connection in the relay, to the rest of the system. My bike was inter mitten for 3 or 4 episodes and then completely flat lined. What had happened was that the brass terminal in the multi-plug had fractured but could still make contact to the corresponding spade on the relay. There was actually nothing wrong with the relay , just the wire terminal in the multi plug.

1. What James has written you about is doubly true if you have a /7 with the brake master cylinder under the fuel tank. Not only was the stock wiring of the /7 starter relay questionable from the get-go, but if 40 years of caustic brake fluid has been dripping on the starter relay connections, then you have double trouble. 

Here's a schematic of the /7 starter relay wiring and you can see that ALL the power for the entire bike passes through that relay, and (to make matters worse) the chance of mishap is doubled because the relay MUST make... not 1 but... two (2) excellent connections or nothing works.

2. The original owners of these bikes rode them everywhere, all the time. That meant in thunderstorms and all sorts of bad weather. Now 40-50 years later we are finding oxidation on the electrical connectors ALL OVER THESE BIKES. Your issues will probably never fully go away until you go through the entire bike, contact terminal by contact terminal and treat each one with an anti-oxidation compound. There are several of these compounds that chemically treat each connection, but the one I use and recommend is No-Ox ID by Sanchem. You can find this on Amazon. It only takes a tiny amount on each electrical contact, relay terminal, connector and grounding point. It chemically cuts through the micro-corrosion, and also blocks additional water for making matters worse.

You can read this older article: LINK

Previous post updated May 6.