I repaired a non functional on/off switch on mine by leaving the pots/switch soldered to the board, I undid the screws holding the lower chassis to the front of the unit, then you can pull out the board with the controls on.
I then opened the back of the switch by bending up the three tabs. Watch out as there is a spring and 4 parts inside! Open carefully and take careful notes/pics of the arrangement of parts before letting it all come apart. Theres an aluminum casting that engages with the knob, which sits on a pivot between the switch case and a brown plastic rotor with a torsion spring attached. On the rotor there is a white plastic 'dog bone' that actuates the switch contacts.
I cleaned the switch contacts with a bit of very fine oxide paper and then simply re-assemble...
Actually re-assembly is a total PITA due to having to get the 4 bits into place, while spring loaded and then getting the cover back on... I used some paper strips to hold the bits together and then pulled the strips out, once in place. Also a pin to position the switch rotor as you lower the back cover on.