If you have a bit of electrical know how, my dads microwave did the same thing and the fuses kept blowing. There is a power unit inside that had a 4 bridge rectifier. The rectifier is heavy duty and capable of carrying about 20 amps at 120 volts. Beware: there are also very large capacitors that can kill which remain charged even after the power has been disconnected.
To make a long story short, I bought a new (but different shape) rectifier and made a new mounting hole in the heat sink and ran some wires from it to the circuit board. To test the power supply check for continuity at the ac input terminals of the power supply. If shorted check the rectifier on the terminals marked ~. if shorted un-solder it from the board and find a replacement.