Thanks for the link to the Youtube Video.

That is the same symptoms as I had and I have found that the potentiometer terminal relative to earth is about 125VAC. As you can see from the Video that she received an electric shock when she touched one of the terminals of the potentiometer and this is why.

I think the mode of failures for these speed controls is one of the low voltage signal cables shorts to earth.

Unfortunately, this is a design floor and the microprocessor side of the board should be supplied through an isolation transformer and optoisolator used to switch the MOSFET transistor on. Thus this keeps both halves of the electrical circuit separate and relative voltage to the earth of the potentiometer, display and hall effect sensor should be zero.

I have managed to chip away some of the encapsulation material and found that a circuit board track has fused (open circuit), a 15v Zener diode has short-circuited and the STM8S microprocessor has been damaged.

I don't really want to replace the controller with another of the same design.