You will need to remove the accessory cooling fan and then the whole cover. There are two security torx screws, the ones with a pin in the centre of the head drive recess; can't remember the size. Once off you will be able to see 4 microswitches behind the control panel. One of mine was just a hair out of place although how I'm not sure because they appeared to be well set. I ohmed all the switches as I originally thought one might be faulty but they checked out ok.
So after that I checked the magnetron with these instructions:
http://www.ehow.com/how_5982436_chec...magnetron.html
http://www.gallawa.com/microtech/mag_test.html
Hopefully yours is fixable that easily. It's at least worth checking.
Good luck
Clark