1st post but long time lurker on this site.

I have a 1980 2l van and have an issue with the main beam....when I activate it through the indicator stalk and red light comes on the dash only one lamp works....ie one lamp full beam the other completely off! Dipped head lights and side lights work no problems.
My lamps are original 1980s round versions as is the wiring.
I think my van is a very early Air-cooled model as no fan blower or any other mod cons......at all!!!!!
I have checked the earth crowns and wiki so hopefully some one on here can shed light onto my ahem current issue.
Thanks in advance.
Samuel