1976 DT400 charging?
RT325 wrote: Not sure myself unless its got a single wire AC regulator & its holding it back due to being faulty, Thing 'is' with your bulbs, if someone's snuck a 12 volt in somewhere it'll hold it back, or if [like us] you have a replaceable headlight bulb & its got a higher wattage fitted, Lower wattage bulbs means brighter lights [weird i know] & consequently more left over for charging the battery. I now see you say minus battery. How does it compare with lights turned off.
Headlight on or off it's the same. Does the bike have 2 separate charging circuits? I'm thinking yellow wire for headlight and green/ red for battery.
Daytime charging (DC to battery), nighttime charging (DC to battery), and Headlight (AC).
That's if it's stock... if someone swapped out the ignition for a later model with "always on" headlight you'd be missing the daytime charging.
Are you checking with engine revved up?
Any difference between lights on and off?
Your problem could be the handlebar switch. The "on-off" for the headlight also switches between the day and night charging. Could have a bad connection in the switch.
I think you said you tested the rectifier... which is just a diode... I just had a bad one that seemed to test fine but wasn't working right. No voltage at all came through on mine. So if you've checked all the connections and everything is good you might try swapping it.