Flip the machine over and pull the bottom off. Look for capacitors. If you find any, remove them. There could be two in the machine and could be one in the foot controller too. They were used to reduce interference on televisions back in the old black and white days.
http://thefeatherweight221factory.com/page10.php
Follow the big red arrow to the lower left and click on "the machine runs on it's own".
Joe