It is probably the foot controller like xxxxxxxxx said. He called it a "wiper", because a brass arm moves across contacts fastened to Nichrome wires. The longer the circuit, the faster it runs. There may be broken wires. I have one just like that. It can't be repaired by most. Too expensive for what you're left with. The new solid state controllers are so much better and under $20 at Sew-Classic.com. I am replacing all of mine. They won't get hot or potentially cause a fire, shock or other. There is a do-it-your-self tutorial on their website. They are also 1/3 the weight of the old, clunky, ones.