I don't really feel up to "walking you through a testing regimen", but maybe in the Zen sense I can show you the path, and you can walk the path yourself. In addition to the fuse, there are other switch-like elements, all in series, which may fail. Sometimes there is a thermostat, or thermal fuse, to make sure the oven cavity doesn't get too hot. Always there are interlock switches, to make sure the door is closed before energizing the microwave generating circuits. Often times there will be printed circuit diagram, printed on paper, pasted to one of the inside surfaces of the metal cover. This can be helpful. There are also resources like this one:http://www.repairfaq.org/sam/micfaq.htm Keep in mind while working that you do not want to touching the insides of your microwave while there is the possibility of these circuits becoming energized. This suggests that you should do your poking around, disconnecting, reconnecting, etc, while the oven is UNPLUGGED. The repair FAQ linked above has some other safety tips, e.g. for discharging the HV capacitor, etc. In fact it might be prudent to read that guide first.
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Step 1 is to discharge the large capacitor you see. If you don't understand how to do that don't even take the cover off. The energy stored in that thing after being unplugged can kill you faster than you can say ooooops! The failed component is often obvious. Look for signs of short like arc burns where wires connect and in particular on the magnitron.
http://www.doityourself.com/stry/repairmicrowave One word of caution, don't even think of messing with the internal parts if you don't know what a high voltage capacitor looks like and how to discharge it. The capacitor in a microwave oven stores several thousand volts and holds that charge even when unplugged.