Perhaps you have heard "It only takes a spark to get a fire going...". It turns out that it only takes a spark to kill a bike too.
In this case, the spark was the positive post of the battery hitting the frame while the bike was running. Because there is not a fuse between the voltage regulator and the battery, that meant that the regulator shorted to ground with all the current the alternator could produce at that moment (with all the available battery current thrown in just for good measure). The result? The regulator blew...but it blew shorted, not open. Therefore, unregulated alternator voltage was now going straight to the battery and every other electrical component on the bike.
You might wonder how this problem looks to the rider. Well, on a simple bike without a voltage or current meter and no electronics to speak of, there are not very many clues anything is wrong...for a while. But it looks pretty interesting in the rear view mirrors of a riding companion a half mile ahead. It looks like the headlight has become an arc welder; really bright...until it goes dark. Then the marker lights can be seen glowing like demon eyes...until they go dark too, burned out from too much voltage, just like the headlight. Fortunately, the bike died before the battery exploded.
Naturally, we didn't know at first why the bike died...but the bulbs were all burned out, and a little troubleshooting soon showed the regulator was toast. Still, the question was "Why?". When we noticed a big burned spot on the battery post and one that matched on the frame. Oops. Someone left the battery too loose in its mount the last time it was out. It shifted around enough to make contact.
Did I mention the bike died? It didn't restart either. That challenged troubleshooting.
To make a long story short, we got very lucky. A local bike shop, normally closed on a Saturday, answered our call and DELIVERED a working (used) regulator...for a reasonable price. (Did I mention we were 3 hours from home, in a little town that could not have been expected to have any bike parts in stock?)
With the new regulator installed, the bike started and system voltage came back to normal. Once the headlight, directional and and marker bulbs were replaced, the bike was roadworthy.
So that's it. Super bright bulbs...and then they burned out. The alternator produces charging voltage...but it keeps climbing as the engine revs climb...beyond the 13 to 15 volts that is normal for battery charging. Clearly a new regulator was needed...and then we got one and everything was groovy. WrenchMonster diagnostics...that turned the trick.
Isn't that cool?