Solving problems with a Singer 4423 Heavy Duty Sewing Machine which runs on by itself

In our workshop recently we had a Singer 4423 heavy duty sewing machine that just refused to stop running, even when the customer did not have their foot on the pedal. Normally, in a standard domestic machine, the culprit is the foot control where a small part has burnt out and needs replacing. This was not the case with the Singer 4423 and at first, we thought it was the motor but on closer inspection, it turned out to be a burnt-out motor block. Quick replacement and the machine had stopped running on by itself.