This microwave has a rotary knob which is used to manually set the cooking time, with a digital display. Over time the knob has started to work very unreliably: some days one can rotate and set the cooking time as normal, other days rotation of the knob increments the time very very slowly or not at all. I have tried to access the knob and clean any mechanical contacts but it difficult to get at and I am not even sure if it is a mechanical issue. I cant correlate the erratic behaviour of the timing knob with any other setting on the microwave. Any ideas how to repair this?
I would wager even odds the problem is temperature related. I've no idea about this model, but these sort of devices no longer use mechanical contacts. The knob is typically connected to a reluctor wheel and a pair of sensors detect the notches in the wheel going past to increment or decrement the timer. There's different types of sensors, one type uses a coil of superfine wire. The wire can develop a break, but as long as the temperature is right, contact is made and it still works. A change in temperature starts to open the break and it works intermittently. More change and it stops working.
These sensors are not repairable, they could conceivably be replaced, though I'd be surprised if you are able to get spare parts for this machine. Furthermore, if you happen to reassemble the shielding incorrectly, you could create a very dangerous condition where the device emits high levels of radiation completely undetectable to you and your family.
Even though you may be quite mechanically adept, between the radiation risk and the probably unavailability of parts, I suggest you simply find a replacement unit before this thing fails completely.