Look out for red speed warning lights on Japanese Domestic Market (JDM) bikes - now those have really caused some folks confusion!

Lol... I could well imagine! Thankfully I don't need to get an MoT any more

Up under the front part of the fuel tank, near the triple tree, left side(as I recall), small unit mounted to the frame.

Pretty certain that's the rectifier (not combined units on these old bikes).

I wrote one of my classic "too much information" answers back in post #12.

Simple answer is NO, the 1974 DT175 did not have a voltage regulator (unless a previous owner added one. )

I submitted a Yamaha Service Bulletin several months ago that had several troubleshooting steps of why bulbs might be blowing (different than the "how to add a regulator" bulletin). I don't think it ever made it to the Tech Library. I'll try to dig it up and resubmit.

several years ago i solved my headlamp issue by installing a Hella h4 12v headlamp. still working great!

Yes, others have suggested this as well!

Does it work as a half decent headlight, or is it just there as a substitute for a regulator?

maybe as much as twice as bright and i am running a standard 55w bulb

Thanks.

On the face of it that sounds like a "killing two birds with one stone" fix!

pntrbl and Hammer did the 12v headlight many years ago on their RT's... should be detailed threads on what they did that can be found with a forum search.