Want look what is inside a foot controler of sewing machine? This video helps you do it and to understand how to repair a foot control pedal. But please be aware that disassembling of any electrical appliances should specialists only.
If the sewing machine has become slower to work, or works with jerks. One of the reasons for this is that the speed controller is faulty.
This type of rheostat consists of many graphite round plates. They are very fragile, be careful.
The principle of operation of such a rheostat is that the denser the plates are pressed against each other, the more electricity passes through them. Soot on the surface of these plates prevents the normal passage of electricity. The soot needs to be removed. This is easy to do if you rub each plate on both sides on paper. But do it carefully, the plates are very fragile and break easily.
The position of the ceramic rheostat also affects the operation of the electric motor. Adjust it so that the sewing machine does not start at the slightest touch of the foot, but only after a slight pressure on the pedal.
There should always be a gap between the platinum and the graphite pusher. Otherwise, the electric circuit will be permanently closed and this may result in a fire of the electric motor. Therefore, after operation, always unplug the sewing machine.
And the last piece of advice is not to do this work yourself; contact a specialist.