On my 1976 DT175 the voltage regulator pictured in the bulletin is under the seat, near the battery. The wiring loom has provision for it to be there. Probably worth comparing pre- and post-75/76 wiring diagrams and see if you can do a better job of it than the way Yamaha suggest

Purely for argument's sake: if one were to try to replace the rectifier I would imagine any suitable generic rectifier diode of the correct voltage and current rating? How about the regulator? 6 volt zener diode + resistor?

Going a step further is there anything to be gained building a decent combined regulator/rectifier system common on "modern" bikes?

Correct. Previously posted wiring diagram is for a 1976 DT175. 1974 and 1975 diagram below. (1975 could be slightly different... not sure... all my manuals that have diagrams say the A and B models are "similar" with the diagrams marked "A".)



The Service manual has a "component list" for the 1976 DT100 and DT175C. Only the 175 has the regulator.



Finally, manual has component list for 1974 and 1975... no regulator listed.



The "combined ABC" service manual is a bit confusing... It's basically the 1976 manual that Yamaha edited to include the earlier A and B models. So some of the things they show are the "one year only" 1976 DT175 modifications... engine has a few different internal parts, fuel tank is another... and the more complicated handlebar light switch that connects to a voltage regulator is as far as I know a "one year only" (1976) on the DT175.

74 and 75 also had the lights on-off switch on the handlebar... With switch off, AC headlight power from stator doesn't connect to anything. Switch on uses the AC wire from the stator to power the blue wire (instrument lights) and also the "high-low" part of the switch that sends power to the headlight high and low beam respectively. That blue wire is where you'd want to connect in a voltage regulator per the Yamaha Bulletin.

Probably no use to you but I know for a fact the the 1975 DT250B & DT400B DO NOT have voltage regulators. The 1976 DT250C & DT400C DO have voltage regulators and are mounted on the air filter housing.

No, that is quite useful!

Pre 1976 is a good starting point for assuming the 6v bikes don't have regulators. However, as in some markets 1975 bikes did get them, always worth checking first. Also always worth checking that a previous owner hasn't already fitted one!

Good point! I am sometimes closed minded and only think of the U.S.A. market and forget these bikes were distributed to various markets around the world, with each market dictating different environmental and safety regulations.

That's odd... I can't find a rectifier anywhere under the gas tank...

Do you have the mounting bracket and/or a red wire and a white wire in the vicinity, as per JimA's photo in post #9 on this thread?

John Paul, my apologies, I should not have posted off of the top of my head because I was wrong. To be clear my 1975 DT 175 does not have a voltage regulator. And the rectifier is not a part of the headlight wiring. When I went back to edit my post I should of been more clear. Sorry for the confusion.
Jim

Yes, just to confuse things more the '76 European DT175 has different lighting wiring than the US model. My ignition has off-on-lights-parking and on top of that I have a switch to turn on parking (as running light) while the ignition is on and the main lights are off...

Fun times looking at wiring diagrams until I got a Haynes manual!

Some MOT testers in the UK have been known to prepare to fail bikes for non working lights, until they realised that the ignition switch had to go round another turn (or more likely, a rather disappointed owner pointed this out to them)! I suspect that this applied/applies to various makes and models.