As per the other answer, you need to lock the gearbox. The easiest way to do this is (if you have one) to use the mincer attachment. Assemble the mincer tightly with the cutting blade in place, and insert into the front of the gearbox. This should provide enough resistance to allow the large pulley to be unscrewed (anti-clockwise) by hand. You can also jam the gearbox by using a _very_ large flat bladed screwdriver in the slow speed outlet, but put some kitchen paper (or similar) over the end of the screwdriver so as not to damage the output drive shaft.

The tension on the belt is adjusted using the three screws/nylon motor mounting bushes which secure the motor to the casing. The belt should be slightly loose - i.e. you should be able to squeeze it slightly with your fingers where it comes away from the small pulley on top of the motor. Be careful not to over-tighten the motor screws as it is possible to strip the threads in the top of the casing. Most importantly, there should be a white plastic washer which fits tightly on the top of the small motor pulley once the belt is on. This stops the belt riding off and damaging the top cover.

Take the central nut off. Try and remove the planet hub by wiggling it with a downwards motion. Check that the gear (cog) on top of the beater socket is not damaged (sometimes they’re made of plastic and have broken or split). If necessary fit a new gear (available online) and secure with a 5 mm starlock washer/push-on fastener. Whilst its off you could clean off any old dirt/grease using a cloth with some white spirit, and re-lubricate around the teeth on the underside of the gearbox with a smear of grease. Also check that the hole in the middle of the hub is still “D” shaped - these can wear down with age. Replace the planet hub and refit the nut using a new washer (plastic or fibre) between the planet hub and the nut. An M6 nylon washer should be the right size. Do the nut up with a spanner, but do not overtighten as you don’t want to strip the threads. Note that there may be a small gap between the hub and the nut if the new washer is thicker than the original. This will not affect operation.

The Kenwood Chef is primarily a domestic food mixer, not a kneading machine. When kneading dough you should keep the speed as low as possible and not exceed quantities in the user manual. Finish kneading by hand if the machine cannot cope. Check that the locating pin between the top and bottom castings hasn’t broken off. It’s also possible the gearbox could be damaged if the machine has been overworked on dough.

The burning smell is most likely coming from the main power resistor on the speed control circuit. It is a 450 ohm, 15 Watt, wirewound resistor which becomes quite hot in use - more so at low speed. A slight odour is not a fault. Sparking from the motor brushes and/or speed control contacts can also result in a slight electrical smell. If the machine has been heavily used it could be worthwhile getting the motor cleaned and serviced to prolong life.

To service the gearbox you should take it fully apart and clean out all of the old grease. This usually requires soaking the parts in white spirit, especially if some of the old grease has become toffee-like. Most of the gears have shim washers, take extra care not to lose them when cleaning out/disposing of the old grease. The gearbox should be re-packed with 133 grammes EP2 lithium grease. Clean the sealing faces and reseal with silicone sealant (the small tubes available from some pound stores are ideal for this). Nowadays, food safe grease is usually used around the teeth of the large ring gear for the planet hub, although the original install was the same as for inside the gearbox. Do not put food safe grease inside the gearbox; it doesn’t provide the same degree of lubrication to the gears at high speed and will result in damage.