sparking from wave guide cover

Disconnected the power cord, removed the outer cover, shorted the two terminals of the high voltage capacitor with the blade of a screw driver with well insulated handle (very important to prevent a bad shock!!),;removed the plastic fan housing for access to the grounding screw and capacitor housing, installed the new capacitor with the reverse of the above steps (but don't reconnect the power cord yet!!}
Now, the most important step of all, reach inside the oven and remove the mica wave guide cover, clean the oven walls thoroughly and clean the inside of the wave guide with clean cloth and water, sand to bare metal any carburized spots on the exit of the wave guide, cut out any carburized spots on the mica cover, replace the clean cover , or order a new one if necessary. It is OK to use the microwave without the cover while waiting for shipment of the replacement mica cover, but be absolutely sure there will be no food spatters on or near the wave guide opening.