76 kz900 charging problem

hey fellas i got a 76 kz 900 and im having charging issues. the bike does fine all day but wehn i turn the headlgiht on and ride the next time i go to start the bike it wont start the battery is dead. so i know the stator is some what working atleast any ideas?

IS it only with the headlight or does it drain the battery in a 30 minute run as well?

Regulator properly grounded?

Run through the electrical checks first...