Electrical question.1985 750.

So I'm building up this bike I just got  when I try to hook up the battery to attempt to start it only will light up the instrument panel when I push the start button.it will turn over if I jump the relay...but if I try to have this small black relay that's the size of a Lego cube..(two circle on top)... That's the best description I have cause I don't know what it's called or does. If it's plugged in then I blow the starter fuse .....any idea what's the problem? Bike has 20,000km and looks fairly unmolested...any help here would be fantastic.thanks

Sorry I see this is wrong forum