2 things that will blow the fuse
1 most common is the safety interlock circuit.
One thing to remember is to always shut off the unit before you open the door.
2 the magnetron or power supply is shorted, this is high voltage (CAUTION )
there are 3 parts to this, transformer, capacitor, magnetron some parts can and will hold a charge and I repeat CAUTION
normally if it has a burnt smell when you open it to replace the fuse it is time to go shopping or seek out an appliance repair shop.
If not, its probably the door switch replace them in a pair. The use of a multi meter is used to test and set the switches.
Good luck
rtwelec
December 2008