Is the spring actually broken? I've seen those just in a wrong position or get gunked up with oil - if it is there, do like Joe said, follow the directions in the TFSR book for Singer 15 tensions. I copied out the page then enlarged it to make it easier to see at a glance. If you do it according to their directions it should work if all the parts are there. You can get the spring from Sew-classic if it is missing or sprung.