Repairing a Kenmore Sewing Machine?I recently acquired a vintage Kenmore sewing machine, model 1251. It looks like new, and hasn't been used much. I cleaned and oiled it, and it sews like a dream! However, the clutch knob is stuck, so I can't disengage the needle arm for winding bobbins. Even when loosening the screw on the knob, it won't budge. Please help! Thanks!