Hello folks!  While I'm considered a "newbie" here, I've been riding since 1974 and have a fairly decent understanding of electrical/electronic basics.  I just picked up a sweet 1975 DT250-B today and none of the lighting is working.  I just have a few questions and am hoping some of the members here can point me in the right direction from which to start.  As of now, here are some facts:
1.  I have a brand new fully charged Yuasa battery installed.
2.  BOTH filaments of the headlight are burned out.
3.  I have a voltage regulator ordered and arriving next week.
And here are a few questions:
1.  Does a functioning (not burned out) headlight need to be connected for any of the other lights (turn signal, dash indicators) to work?
2.  Will ANY of the lighting work without the engine running?  In other words, can I test the connection/circuit to any of the other lighting (turn signals, dash lights) simply by turning the main switch on?
3.  Is the wiring for a '75 DT250 the same as the wiring for a '75 DT400?  The reason I ask is that I have found a nice COLOR wiring schematic for the DT400, but only a black & white version for the DT250.  Just seems a bit easier to use.
4.  Finally, is there a good post and or thread here that any of you may know of that would give me a basic "outline" of sorts to follow in trying to diagnose what the cause or causes of my "no lights whatsoever" problem might be?
Thanks in advance for any and all help, folks.  I'm excited to get this all sorted out so I can get this baby out on the road as soon as I can.  I look forward to learning all that I can here, as I know I've sure learned a lot over the past few years from the Hodaka forum in terms of keeping my '75 Road Toad in good running order!   Yeah...I guess I'm reliving my 1975 life a bit more each day...LOL
 

Headlight, I think, only works with engine running. Tail light, indicators and horn should work off the battery.

Have you checked the fuse (is there one fitted?! ) and that the other bulbs aren't blown?

The DT400 wiring schematic can be used for your troubleshooting, although bear in mind that the 400 had CDI, whereas the 250 had points, so the wiring for these areas will be different. Also, the wiring will differ slightly from region to region (eg between US and Europe).

The wiring colour coding for all Yamahas of this era was the same.

Thanks Gr8uncleal for the tips!  There is in fact a fuse in the main positive wire off the battery and it is good.  I'm going to check all other bulbs on the bike this morning to see if the headlight is the only one blown.  Good to know that the colors on the 400 wiring diagram will match those on my 250 as well.  I'll soldier on and see what I'm able to diagnose today.  I'm hoping someone here might be able to point me toward some type of known "tutorial" that tells me the best starting point to begin eliminating one thing at a time.  I know the battery is the source of power and I'm assuming I should just work outward from there...but I'm just a little unclear on the next component to look toward.  The main switch (with the key) on the dash perhaps?  I'm wondering if there are certain "common failures" that I might look toward first?

Red from battery goes to key switch.  Brown out of key switch runs horn, turn signals and brake light.  Brown also runs to light switch and powers the blue wire that goes to taillight. 

Yellow from engine goes to headlight switch and powers the other blue that goes to instrument illumination and headlight hi/lo.  Yellow is AC and only hot with engine running.

So yes, with a good battery you should have power to the red at the key.  Key on should power the brown and everything but the headlight and instrument illumination lights should work.  So that would be the place to start.  Make sure you have power to the key switch (red) and out of the key switch (brown).  Sometimes the soldered wires break off the back of the switch.

P.S.  Actually the first thing to check is all the bulbs.  It's a balanced system that matches the electrical load against the stator output and uses the battery as a crude sort of "regulator".  If someone ran the bike with a bad battery, it's not uncommon for ALL the bulbs to blow. 

Horn won't blow but they do get finicky and sometimes need tweaking or replacement.  Brown to horn should be hot with key on.  Pink to horn grounds on handlebar when button is pressed. 

With info like this... I don't need any manual to sort out lighting stuff! ^^^^^^
Sneezles61

Also, bad earths can give all sorts of weird results, although not usually total failure (well, unless the negative from the battery isn't connected properly!).

Thank you MarkT!  This was exactly what I was looking for!  I worked on the bike all day today.  All the bulbs were in fact good.  I started the bike up and saw no headlight or tail light.  All directionals were dim, so they were obviously running off the battery only and not receiving any output from the stator/lighting coil.  So that's where I'm at for now.  I guess I'll be running the vintage ride tomorrow without lighting.  But with your info, I now know where to check and what to check for.  Thanks again...I'll keep you all posted on my progress!  Oh...just one question...where exactly does the negative lead from the battery get grounded?  In other words, is there a "main" grounding point to the frame somewhere that I can check?

Thanks Gr8uncleal.  Would you know where I would look to see if that negative from the battery does lead to an actual "ground" on the frame?  Was there one particular point that these bikes had that was considered a "main" grounding point? 

On my 1974 DT360A, which is similar to your bike, there is a ring connector with two black wires that sits on one of the coil mounting bolts, underneath the petrol tank, so have a look there and give the contact areas a good clean. The negative terminal of my battery plugs straight into the main harness.