[Nuvistor]

This is a photo of a slider pot, there are different designs but look similar. The slot in the pot is where the control knob slides along.
To clean you only need to spray a little of the Electrolube or whatever you get into that slot then work the control back and forwards. You can do this without dismantling from the top of the control panel. The spray should have a long spout to get into that slot.
Have the machine unplugged from the power and allow a little time for any of the solvents to evaporate before switching on.