The tension spring was what was wrong with my Spartan when I bought it rather cheap because it didn't work. Does your machine HAVE a tension spring at all? Mine did and it was not working. I found info to fix it here:
http://www.tfsr.org/pub/technical_in...echanism_2.pdf This was the site I found
http://www.tfsr.org/publications/tec...achine_manual/ and then tension mechanism was the topic. Use the info for the Singer 99 or 66. I had to follow the directions exactly to get it to work right. It was a little screw on the side of the machine next to the tension needed to be done right. Sew classic might have a spring if yours is blown out - you still have to install it like the instructions.