There is a lot of great engineering in the stock system... Yamaha (and others) "balanced" the fixed output(s) of the fixed magnet alternator against the load to keep the voltages in range.

The system generally works very well until the load changes... like a bulb burning out or even a bad connection... allows the voltage to spike.

There's a lot of analogies that come close to the principle.... one is to think of the voltage as speed and the bulb, battery, etc. load as a hill. Yamaha designed the lighting system circuits to have the exact power needed to prevent being able to exceed a specified voltage (speed). Remove the headlight load while the engine is running and the switch is selecting the "nighttime" circuit and suddenly you have less load so voltage goes way over 6v... it's like you made the hill less steep so now you can go much faster up the hill.

Problem is remaining bulbs can only take a certain amount of "speed" (voltage) before they blow...

The regulator is AC. You mentioned accidentally connecting it to the blue taillight wire which is DC. Hooking up a Yamaha regulator to DC is one of the few things that will damage it.

The only reason there is AC and DC circuits is needing a battery to power lights with engine off... no such thing as an AC battery that I know of so they had to rectify the AC output from the mag to DC to provide battery charging for stuff that needs a battery.

And as far as why it took so long? Electronic semiconductors were NOT readily available and cheap when these bike were new. Until the early 1970's, many electronics like televisions still had vacuum tubes instead of transistors! (I still remember having to wait for the TV to "warm up" before you could watch it... and the hum and glow of the tubes inside... and I remember my Dad buying our first color TV in about 1977 that didn't have vacuum tubes) Advances in electronic tech finally allowed a reasonably priced AC voltage regulator to be produced that could handle the power switching needed.

MarkT excellent way to explain how the electronics work on these old enduros           

I'm starting a new thread as the bike I am currently working on is a JAPAN SPEC DT250. The wiring could possibly be totally different than US spec?? Only three wires (Green,yellow, black) coming out of stator. I have a Japanese wiring diagram that someone here graciously provided.....but I have not been able to translate it from Japanese. The key switch only has two positions, so I wonder if that is the same as US spec??

'72 US Model DT2

The yellow goes directly to the voltage regulator, I made the short blue jumper and the other end of the blue is to one of the bulbs for the tach.

The black is attached inside the headlight bucket to one of the screws holding the voltage regulator in place and the other end tapped into the black for the high beam indicator light.

OK, awesome. I have found a "similar" wire on my JAPAN SPEC DT250a. I will give this a try if direct off yellow stator wire does not work.

So, I tested ohms on both old and new regulator. With meter set at 20M the Stanley read 6.7, the new finned regulator read 15.8. Can I assume Stanley is bad?

GOOD NEWS: The newer finned is hooked up at yellow wire right off stator and SO FAR, everything is holding at tail lamp bulb. No spikes, etc. I do have a 12v bulb in, but the Stanley blew that one too. 6V bulb is on the way. I won't harm anything by running a 12v tail lamp bulb in the meantime will I?

I will test a new headlight bulb once I have put some street miles on this and see if the new regulator holds up.

If it doesn't, I will go into headlamp shell and try the blue wire solution....with hesitancy :(.

Thanks for all the help on this folks. This thread is old because I moved and things go pushed to the back burner. Just so glad to get my JAPAN SPEC 74 DT250a back on the road!!! Leaking petcock now of course...grrrr. On that note, is the extra "gasket" o-ring type thing that goes on outer side of valve gasket very specific, or can I just use an apporpriately sized generic o-ring, or square o-ring? I ordered a few new valve gaskets but couldn't order the outer gasket separately.

You can go to any auto parts store and get a 1154 bulb for your taillight it’s 6volt that’s what we use