I have a Natioanl NN-6207 Microwave,I have replaced the magnetron and the fuse, every time I switch the microwave on it makes a loud humming noise and blows the fuse. The magnetron has two terminals F and FA which one goes where and could this be the problem. Please help

F and Fa could be the problem but.... You must check the power diode and the high voltage capacitor. One of these is most likely the reason the fuse failed in the first place. Ground the HV capacitor by holding an insulated screwdriver well back on the handle. The locate the Power Diode that will have at least ONE terminal that connects to the capacitor. Power doides are either 3'8 inch wide and 2 inches long or round an liik like a bit resistor with no color markings. ' Check the HV capacitor by rading each terminal to the case of the capacitor (no shorts allowed)
Then read across the capacitor terminals with the meter set on R x 10K. Watch for an initital jump in resistance and then it drifts up to infinity. Reverse test lead positions and you must get the same result. Resistance and then drift to infinity. Any other reading.... infinity in both directions, short to the case or short across the 2 terminals = bad Cap.
If after you perform the above tests and you still have the problem ... knowing you might pop another fuse there are 3 door switches. 2 of them will read resistance / continuity across the terminals when the door is close and one of them will read continuity across the terminals when the door is open. One of the 3 switches has 3 wires on it... the 3rd wire shorts the magnetron power when the door is open and the power relay / HV circuit is powered. This is a built in blow the fuse to prevent magnetron operation with the door open.

I think your are going to find that either the power diode or the HV Capacitor has failed.

Thanks for choosing FixYa,
Kelly