All contacts/grounds have been cleaned, no broken/naked wires, etc....

So, I tapped into the yellow MAIN HEADLIGHT CIRCUIT wire coming off the lighting coil. I am using an OEM 6 volt Stanley regulator from the later model 77+ bikes. Instantly blew headlight when bike cranked up (headlights are $36 a piece).

Am I tapping into the wrong wire? I have the wiring diagram and am a little confused.

I tried this on a 74 DT250, tapping into the dark blue (high beam) wire inside the headlight shell as directed by the service bulletin, but blew the low circuit bulbs in headlight and tailight, so I obviously need to tap into the main lighting wire coming off the lighting coil on that bike.

My question goes back to this bike, the DT3. Best i can tell there are YELLOW, GREEN, GREEN/RED. All these are charging circuits for lighting....but seems like the YELLOW is not the one I need to be tapping into with the voltage regulator?

Did you ground the regulator?

Yes, from one of the mounting bolts to the frame.

Service bulletin does not say to tap into the "high beam" wire... it says to tap into the AC power wire after the light "on-off" switch which is usually dark blue. The picture in the bulletin shows this is the wire powering the instrument illumination lights... the bulbs that light up the gauges at night. Those bulbs get power only with the headlight switch on and has power when the beam selector switch is on high or low beam.

The yellow off the stator is the AC power for the headlight. It changes to dark blue on the "out" side of the headlight on-off switch. The headlight on-off switch is located in the key on early models and in the handlebar switch on later models. That's why Yamaha has you connect to the instrument light bulbs because the method works equally well on either switch location.

The reason Yamaha doesn't connect right off the stator on the early bikes is because on the early bikes you can turn the headlight off. If you connect there... before the headlight on-off switch... then you're asking the voltage regulator to absorb all the AC power from the stator without a headlight to help. This could overheat the regulator. If you leave the headlight switch on all or most of the time when riding, it would be okay.

(On later bikes with the headlight always on, the regulator is connected right off the stator.)

I found and submitted another service bulletin on headlight bulb blowing troubleshooting that might help? I know Mako is busy and I haven't seen it in the Tech Library yet? I will try to attach a copy.

I see DEET had responded... took me too long to finish my reply I guess.

You've got the best helping you! I'll stay out of it to avoid confusing things.

So, I understand your explanations on everything. I did try the "after the switch" dark blue wire on my 1974 DT250....and it worked fine for the high beam bulb and stoplight portion of the brakelight....however....when I switched to low beams....it burnt out the low beam portion of both headlight and tailight. I always run headlight on....so I will stay tapped into the yellow wire right off the stator on both bikes. I will try the DT250 next. I did have good luck this 2nd time on the DT3!!!....I am using the tailight as my test. So far, running light and "stop" light both are working and not burning out!!!

I really didn't do anything different on the DT3. I can say that the headlight that burned out was an oem unit...and may have been previously compromised....but I did see it flicker on...then immediately burn out on my first attempt.

Now that the brakelight High/Low is working on the DT3...I guess I will try a new headlight and see what happens.

Thanks for the help and AWESOME bulletin attachments. I have those...but they are a lot less clear in the service manual. MUCH easier to read....plus, I think the second page of the second bulletin is not included in my downloaded service manual.

To follow up on DEET's suggestion...is the ground you made good? ie, bare metal to bare metal and not a painted or corroded or rusted ground?
Not suggesting you don't know what you are doing, but sometimes it's the easiest thing...

Yes, 100% on everything all else electrical. It's the first thing I do when I get an old bike....remove and sand all grounds, remove and sand all electrical connections. Doing this first takes alot of the guesswork out once you get into diagnosis.

As stated....I didn't really do anything different...except I did free up and clean the turn signal part of the combo switch. For some reason...everything as far as the brakelight seems to be working and not blowing. Went for a test ride to get the revs up...all seems well. I'll install new headlight next and hopefully things will be ok. FYI, turn signals are off the bike and I am running the OEM battery just for a voltage dump...just in case....it has been said that a battery absorbs some of the extra voltage on these original bikes...not sure if that's true...but thought I would mention it.

Now for the WEIRD sidenote. I have a 1972 LT2 with NO REGULATOR, NO BATTERY, all stock wiring. Headlight or tailight has never blown!!! Turn signals are off. Every single other bike from this era has blown out lights. So how in the hell did they sell so many bikes back in the day and this issue never get resolved until 1977? I figured folks would be pissed as hell and knocking down the doors over this......but I guess most folks back then took all the lighting off the second they got it home huh?

"So how in the hell did they sell so many bikes back in the day and this issue never get resolved until 1977?"

Just a guess, but when did the headlight on become mandatory in many states? Maybe people just didn't ride after dark back then?

Are you using the stock dimmer switch?
The stock switch is designed to turn on both elements before turning off the other.
That way, the voltage doesn’t spike between switching.
My other thought is that your regulator is not working .