Steve Kissell wrote:
Initially, I thought I detected a black powder residue near the
microwave door

First of all it's really dangerous to cook black powder in your
microwave, save that for your flinklock! :-)

An appliance parts store nearby suggested that I have a technician (they
recommended) look at the magnetron.

Question:
Would that be the circular part about the size of a silver dollar and
3/8 " thick that is located in the base of the oven at its center?

No it's that's not the magnetron. If you can't recognize a magnetron,
you should not be in there. If you put it together incorrectly you can
expose yourself to both high voltage and high power RF radiation.

However, you can "fix" it yourself. I had a magnetron burn out and I
looked into replacing it. I found three sources on line for the
particular magnetron that I required, cheapest one was $100. I bought a
new microwave for $70.

I know magnetrons are cheap to the manufacturer (like $10, perhaps
less), but I could not find a onesies-twosies distributor that was
cheap. Considering that you *will* have to have someone qualified
install it, *will* be cheaper to buy a new oven. Seven years is a good
run in this modern not-built-to-last world.

-Greg