That may be because the wire insulation has failed - or it could be that the battery in the ohmmeter produces a slight magnetic charge in the coils that fools the meter.

Every time I see a new post to this thread I wonder if the troubleshooting steps given in the manuals in the Tech Library have ever been followed in order?  (I find the earlier manuals to be helpful...  Yamaha often published more theory and detail in the earlier manuals and places like the 1969 "Shop Guide"...  all available in the Tech Library.)

And if the steps have been followed, what was the failure point and the details of the failure point? 

I realize that the above may have been done...  but all I see posted here are readings from tests that I've never seen in any manual...  could also be my reading comprehension is lacking and I missed understanding which part of the dynamo, yolk, armature, or regulator troubleshooting steps have failed?

Might just be dumb luck, but following the steps given in the manual in order has always identified the problem area for me...  never had to invent my own tests...  most of the time it has been the brushes, brush contact surface, or the regulator in my experience.  Once I came across a bad armature that was shorted internally.  An electric motor shop confirmed the armature was bad. 

 

So tonight is the night, got back to it and was determined not to leave the garage untill I had either an answer or success, and it was success! After following all test procedures,again, and when that showed nothing I resorted to "my own tests" and found the ground screw to be probably .020 to long touching a coil and shorting out.
Fired up the bike and it charges,to much mind you but that is tomorrows job to adjust the regulator.
I want to really thank everyone for your help and my son says big thanks as well! 

 

Great news and I'm chuffed for you and your son that you got it sorted.

We should put your problem/solution in the collective memory bank, as one more thing to check if someone has a similar issue. I had a RD400C that wouldn't charge. I could see which check was "failing", but it was not immediately obvious why. Turned out that there was a severed wire in the rotor. This was repaired and, fingers crossed, all now seems ok.

Congratulations. 

If you could elaborate on how you found a shorted coil that was not revealed by the test procedures it may be helpful to others.. 

Sure, I thought there was a continuity issue with the info supplied in the manual and from DaveHunter so I took everything (connections,brushes, disconnected regulator) and systematically started to reassemble one thing at a time and checking continuity with every install of component. When I tightened the ground screw it went bad,loosened it then it was good. Installed shorter screw and problem fixed.
I am a tool maker by trade and specialize in automated machines so trouble shooting is common for me.

So I think I need a regulator for this bike, the cut out coil shows no action even with power and ground attached direct to reg. With all the experience on here I want to get a good replacement, any suggestions on what to order and from where?

I’m not too familiar with that model but I believe that voltage regulator fits many Yamaha models and there are used and N.O.S. On eBay from $15.00 to $220.00.
Another note is I see your thread title says 1975 DT125C, a DT125C would be a 1976 model not sure it would make a difference in part numbers for a voltage regulator, any of the major Yamaha parts vendors can give the part number to see exactly what models it will fit.

thanks Lizeec,will check it out may very well be a 76, not sure ,its my sons bike and that's what he has always referred it as a 75.

It is a 1975, I looked at the stamping on the frame, or one of the stickers. It's a DT125B. Congratulations! I wonder who supplied the screws?