Which regulator are you using?  Is the case grounded?  Where did you connect to yellow? 

Can't help much unless we know what you know. 

Many regulators are crap...  service bulletin shows best way to connect which is basically blue wire to gauge illumination lights...  If you connected at yellow right at stator it should have worked...  yellow in headlight bucket probably not. 

Just googling away to find a regulator in the system as didn't come out until after "74 & this "77 DT400E shows it & sure enough is into the mag yellow but starts off as yellow & white from the Reg. Must get a hard time when lights are off unless its from a market where lights are on all the time. Might need to save & expand it. Man i still hate trying to follow wiring & worse without color. I see yellow becomes yellow white again heading to power the dip switch. "Headlight high beam" is always "yellow" on yamaha but "not related to the mag lighting yellow". Confused--i've not even started yet lol. Think was Mark said if its into the yellow it should save the lights regardless [or similar wording]. So was it lacking earth where you mounted it. If not frame mounted on steel then its good to run an earth from Reg body to the frame or better still the loom earth at the coil mount normally.     
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