Quick (maybe dumb) question:  Just finishing up a restoration on this '71 Yammy DT1-E 250 and all electrical is 'working' except the headlight, meter light circuits.  Is this one in which it is only with the engine running - source via stator, etc.?

I think so, and probably with the ignition switch in the second "live" position.

Thanks - at this point I'm going to hope you're right. I had a 1968 Hodaka Ace 100 that was that way - 6 volt system but headlight only came on when running and with the ignition switch in the 2nd/lighting position.

Magneto driven lights on those but be very sure the battery & rectifier are in the circuit or your lights will burn out at possibly any revs above idle.
Or add in a single wire bulb saving AC 6 volt regulator to save any possibility of 'money down the drain; buying light units if yours is a sealed unit.
Luckily our DT1's in Oceania in NZ' have replaceable bulbs so mistakes aren't too expensive.

Thanks for the confirmation & advice.
My OE sealed headlight works but the reflective surface internally is peeling away.
I've had no success finding a replacement that has the necessary mounting tabs and adjustment bracketry.

Keep an eye out for the same headlight lens, but where the bulb can be changed. The first year or two DT1 was fitted with this and the fitments are identical to the later sealed beam headlight.

I THINK the part number is 233-84120-61.

It is a Koito item and on the back of the lens is printed 110-31795 and it is 6v 35/35W.

This new sealed beam is on fleabay for stupid money!

Yamaha DT1 DT2 250 RT1 360 DT3 NOS Headlight Assembly 233-84120-60 #3 2330 | eBay

I have substituted a12v sealed beam and functions fine

CLEGG, interesting, did it myself on my RT1 using a basic system no indicators & auxiliary stuff but is yours charging the battery with lights on or does it drag it all to the light. Cheers.

You don't need the adjustment brackets...  simply use "w" clips from a later model or someplace like MikesXS to secure a 5 3/4" lens/reflector into the original chrome rim.  If you choose an H4 reflector, you can find replaceable H4 6v bulbs of the correct wattage.  Dozens of threads on this conversion.

If you are building an all original bike, occasionally an original new sealed beam with brackets shows up for sale...  expect to spend big $.

Thanks all for the headlight advice. Yes, 'stupid money' is right.
The non-sealed with a replacement bulb sounds like the way to go.