When your fuse blows when you press the start button, it typically indicates you have a component shorted in the High Voltage system. It is possible that faulty door switches can do this as well but it is more common for the trouble to be in the HV system. This system consists of 4 main parts. The Transformer, Capacitor, Rectifier (Diode), and the Magnetron. If either the mag or the rectifier short out you will usually get a louder more pronounced hum from the unit when it runs and no heat, but it usually won't blow the fuse. If either the capacitor of transformer short out, the Microwave will draw over 30 amps very quickly blowing your fuse. The more common of the 2 parts to fail is the capacitor. Frequently when a transformer goes bad it will also smoke or burn up. Anyone attempting to repair their own Microwave must remember that they can hurt or kill you. A high Voltage system in a microwave oven can carry anywhere from 3000-5000 volts and shock the life right out of you, even with the unit unplugged. The high Voltage capacitor is like a High Voltage battery and can hold a charge for quite sometime. If you are not a qualified technician I would highly advise you to keep yourself out of these units.
James Bennett / A.C.& M. Service, May 2007
