Hi @noltian ,
Was there anything metallic inside the oven when it was on and it sparked?
A humming noise would indicate that the interlock switches are OK as they’re there to prevent the microwave from operating if the door was open but I suppose one could be faulty.
It could also be the HV diode or worst case the magnetron.
Be safety aware when working inside a microwave oven as the HV capacitor can store potentially lethal voltage (>6000V DC) for months even when it has been disconnected from the power for that long. It needs to be discharged correctly before commencing any work in the oven.
Was there any metal accidentally placed in the oven when it sparked, you didn’t say?
Something has failed so you would need to use a DMM (digital multimeter) to try and find out what has happened.
Microwave ovens are extremely dangerous to work on if you don’t know what you’re doing. If you don’t have a DMM and don’t know how to use it then don’t try. It’s not worth it. Contact a reputable, professional appliance repair service and ask for a quote.
If you know how to use a DMM then disconnect the power from the oven, remove the cover and then discharge the HV capacitor before doing anything else!
Videos on YouTube show how to do this. Personally I think that it is not the best way. Placing a screwdriver or an insulated pair of pliers across the HV capacitor’s terminals to discharge it can damage the capacitor. Also I have seen the results when someone used too small a screwdriver and it literally welded across the terminals with a big flash. Use a 10M Ohm 10W resistor test lead to safely discharge the capacitor. It only takes about 5 minutes and after that use a screwdriver to short out the terminals to make doubly sure that it has been fully discharged.
Once that is done, inspect the components for any obvious damage, burnt etc and then test them using the Ohmmeter function of the DMM.
Test the HV diode, HV transformer, magnetron, interlock switches and the fuse - part #4010 Interior parts which is located next to the noise filter. Although the fuse failing was not the problem. It was only doing what it is supposed to do i.e. protect the device from further damage and also the user from possible harm e.g. fire or electrocution