Remove the two screws to the plastic vent strip at the top of the unit. That exposes a metal cover (upper right) held in place by two more screws. (Found that removing the screw holding the key panel in place and moving that out of the way made for more work room) Remove the metal cover. The HV diode (visible) on one end is secured to the chassis by a screw. The other end is fitted to the HV capacitor. So unscrew one end and pull the HV diode off the capacitor. The replacement fits on like the old one came off. Very simple repair. But it would be safe to short the capacitor terminals to the chassis first. Noticed that it appeared the sparking was caused by the HV diode's outer casing being worn by the asbestos covered wire next to it and laying against it. I made sure the two were not touching, before putting the covers back on. Also, the problem started with an occasional spark when the MW started. It quickly deteriorated to a full, non-stop electrical sparking. You can hear it, smell it, and see it through the top vent. You don't need to remove unit to work on it.
We needed a phillips head screw driver to remove the single screw holding the light bulb cover in place. Removing the old bulbs was a little work, because they had come loose from the sockets. We unplugged the microwave, so we could grip the sockets with a tool. I think my husband used a needle nose plier for this. The new bulbs screwed in easily, and then we refastened the light bulb cover with the phillips head screw driver.
Microwave worked perfectly but would no longer heat.
Looked up symptoms on parts select website. Ordered mag and diode. Very easy to take micro off the wall. Easily removed screws holding the skin on. Found diode. Really didn't need to replace it but since I had a new one (fairly cheap), just did so. Diode only installs one way. Can't botch it HOWEVER, MAKE SURE THE ONLY PART OF THE DIODE TO TOUCH THE GROUNDING POLE IS THE SCREW DOWN LEAD. IF THE OPPOSITE END LEAD, THAT WHICH IS THE PUSH ON TERMINAL, TOUCHES THE GROUNDING POLE, YOU'LL GET ARCHY SPARKY. The magnetron goes in very easy. Simply pay attention to how you removed it and reverse that process installing the new one. There are easy to use YouTube videos that show magnetron replacement. You can do this easily and save your microwave and $$ while doing so.
PartSelect website indicated replacing the high voltage power supply diode is the solution for "no heat" 99% of the time, but the diode was OK. There is more info at the website concerning the case of "loud buzzing and no heat" indicating the magnetron needs to be replaced. As an electrical engineer with knowledge of microwave components, this was also my best guess. However, getting to the magnetron is not the easiest job, but it's not impossible. You also have to be careful to bleed off any charge stored on the high voltage power supply capacitor. Luckily, I found my KitchenAid service manual tucked away inside the unit since I couldn't find it online.