I an just getting into doing portraits. I have two Britek HS-2000 strobes. When I attach my Nikon D70S with a sync cord to the Britek lighting, the strobes will only fire when my pop-up flash on my camera is up. When the pop-up flash is up, the extra light from my pop-up flash washes out the photo. How do I get the strobes to fire without my pop-up flash?