Hello Erik, Just had the same problem and fixed it.
1. Turn mixer upside down and release the 2 screws holding the curved elevating hinge. This will release the black motor cover, pull down cover to access the rear face of the knob.
2. The knob carries a "Kenwood" label. The label has to be removed by releasing the two plastic latches you can see underneath the label ends on the rear face of the knob. Push each gently towards the spindle and towards the top of the knob, and the label will come out stuck to the plastic part. Be carefull - it is not a strong object.
3. Now you can see the knob spindle, a circlip and two slots, one bigger than the other, which when engaged into the two white "teeth" on the plastic cam will operate the ON/OFF switch and control the motor speed. What seems to have happened is that there must originally have been some sort of spacer or washer under the circlip, which has broken/disintegrated and fallen away - I could find no sign of one in my mixer. But that missing spacer/washer allows the knob to come out along the spindle so that the driving teeth do not engage in the two slots to operate the controls anymore.
4. Remove the circlip (don't lose it!) and pull off the knob. You will find a spring underneath - leave it in place.
5. Obtain a washer (I made mine out of nylon on my lathe) of outside diameter 9.5mm, central hole 6.5mm and thickness 2mm.
6. To reassemble, push knob onto spindle and engage the two teeth against the spring force. Test rotation of switch to ensure the ON/OFF position is correct.
7. Have someone help to hold the knob down in this position or use sticky tape to temporarily hold the switch down. Slip on the new washer and refit the circlip on top of it - a bit tricky, and needs fine pointed pliers. The knob should be operating correctly again now.
8. Re-insert the label carrier into the knob and screw the curved elevating hinge back in place.
All should now be well. Hope this gets your mixer to go again Erik. Best Wishes. Neville Beale, New Zealand
Neville Beale, New Zealand, January 2006