Hi, I tried to pick up part 2 of your chat to save you some time but another expert got it. Anyway here goes:

You said it sounds like a spark coming from the microwave. This symptom is usually caused by either a bad capacitor or a wiring problem... unless the power diode cooked and is allowing a short to the chassis. (
I really need a model number to pass you exact part information but for reference purposes I am going to use this one: JVM3670SK (counter top model)

The parts to check with power off and high voltage capacitor grounded / shorted across the terminals at least 3 times.

1. The capacitor (Item 75 of this link:)
http://www.appliancepartspros.com/partsearch/model.aspx?model_id=5218871

a. With the power diode disconnected read resistance across the terminals. You should see a momentary jump in resistance followed by the reading moving to infinity. You should see this EACH TIME you reverse the meter leads on the contacts. No jump = bad cap, always reads resistance with one side of the cap terminals disconnected = bad cap.

2. The power diode has failed (Item 77 above link)

a. With one of the 2 terminals free you check for resistance in ONE direction only. If you reverse the leads and still read resistance the diode is bad. If you do not read any resistance in either direction the diode is bad. In other words... one direction / polarity it reads like a wire... other direction the meter should read above 240,000 K ohms.

3, There is a possibility that the magnetron failed but.... with out knowing both of the above components are functioning you can not for sure determine if the magnetron has failed. (Item 61 above link)

There are a few tiny thermostats inside the microwave that are about 1/2 inch in diameter that could stop things from working but...they all will read "0" resistance at room temp across the termnials. If any of them read open it may be as simple as a failed thermnostat. Broken terminals on a thermostat may cause sparking. (Very rare!)

My concern is that you state your hearing sparking... It still leads me to either the Cap of the Diode as the source of the sparks.

Thanks for choosing FixYa,
Kelly