Fuse blows on microwave switch on

one of three things shorted could cause this: H.V. diode,capacitor,or the magnetron tube, take screw driver and short terminals on capacitor before touching anything inside the case, then chk resistance on capacitor should show infinite after needle settles, diode should show hi resistance one way and a lower resistance in the other direction(use the RX100 scale not the RX1) on both parts diode should not show very low resistance or it could be leaking,mag is a different story,it can be chk'd but in a different way