Microwave wasn't working at all, I replaced the fuse and everything seemed okay but it stopped working again almost immediately, its a MagicChef, model MCO165UW made in August 2010

SOURCE: Microwave oven blowing 20 amp ceramic fuse

Chances are if the varistor fails, it fails drastically causing a short circuit across the supply. It will sit across the supply (it will look like a thick ceramic capacitor) and you can check for continuity after isolating that at one end. If it shows low resistance (it should show almost open circuit with a Digital meter), replace it.

It need not be the culprit. If you are familiar working with High voltage circuits ( they are lethal inside a microwave ovens as the current capability is high here), you can discharge the doubler capacitor and isolate the high voltage diode to check for shorts in the HT area.

SOURCE: Goldstar MIcrowave stopped working when light bulb blew out.

when that happen to me i change the bulb and the fuse in of the microwave f the don't work send it to the company that made it and they should tell what wrong.

SOURCE: keeps blowing fuses

This problem is just like my own microwave, replace the shorted HV capacitor

SOURCE: No power. I changed fuse but microwave blows it immediately.

Be very careful! Standard microwaves have 2100volt capacitors and commercial ones 4000 volt capacitors.
*Decharge your capacitor(s) before doing diagnostics*.
You are now testing the high voltage side (as it works on convection). The main components are a transformer, a capacitor, a diode and a magnetron (havent seen a triac in a panasonic residential). If you have a proper meter that can test uf, decharge the capacitor, then test the capacitor, if it reads 0, you need a new capacitor (10$ used typically, 14$ new from ebay). To test the magnetron, decharge the capacitor first, remove the spade connections, test each leg to the frame of the microwave... if it shows measurable resistance, the magnetron is bad. The transformer is not something you test as I personally dont have a 4000 volt meter. If the diode, the magnetron and capacitor are okay, then the transformer is bad. To eliminate any other function, disconnect the low voltage to the transformer run a micropower cycle and check the fuse.
Hope that helps. My first instinct is the capacitor.

SOURCE: GE Dual Wave II microwave oven blowing fuse

I have the same issue and the same unit. I wonder if there's a recall...