Hi and welcome to FixYa
. I am Kelly. High voltage capacitors can and do fail when a microwave is used for long periods of operation, when the unit is operated with no internal load and sometimes for no reason at all other than the capacitor had an internal failure such as corrosion or a break down of the metal strips inside of it where the strips can make contact with the the case. I have been working on microwaves since the 70's and often there is no apparent reason for the failure of the HV cap.. They just sometimes fail.
However...... there is also a power diode in the microwave that is connected directly to one terminal of the HV Cap. If the power diode has failed it can cause subsequent failure of the HV Cap. The Sharp Power diodes can be found here: http://www.repairclinic.com/Sharp-Microwave-Parts?s=t-FH-DZA119WRKZ-%3d%3db113a6c41
You use and ohm meter on resistance to check the diode AFTER removing the terminal of the diode from the capacitor. NOTE: the capacitor must be shorted across the terminals at least 3 times with a screwdriver
while holding the handle only to prevent a horrible shock.
Once the diode terminal is disconnected from the HV Cap read resistance in BOTH directions by reversing the test leads on the terminals of the diode. One direction will read resistance and the other direction it will read open / infinity. Any other reading is a failed diode.
You check the capacitor using an analog (scaled vs digital display) ohm meter on R X 1,000 or R X 10,000 and watch for a jump on the meter just as you touch the terminals EACH time you reverse the leads. No jump or... steady resistance = bad Cap.
Sharp Capacitors can be found here:
Just note that either a CAP or Diode failure can cause the main fuse to blow at the end of the power cord in the fuse holder.
Fuses are normally either 15 or 20 A Slo
fuses and can be found here:
Thanks for choosing FixYa