Frigidaire Microwave stopped microwaving, but convection works

My 4 year old over-the-oven microwave stopped microwaving today, but the convection part works. Everything else seems to work fine. i noticed some faint ‘smoke’ inside the oven at about the time it may have died.

What could be the problem? diode?

Please help.

BTW, the plate DOES spin too.

>>>UPDATE>>>

Door switches are good.
I’m actually getting 120VAC at the inlet side of the high voltage transformer when the oven is (trying to) heat–but it doesn’t heat. I assume this means the board is good.

I have checked the capacitor and it looks good as does the resistor within the cap. I checked out the magnitron which looks like it is fine when I do a continuity check, but looks a little worn and has some signs of rust. Is this normal?
I get ~0.6ohm on the inlet side of the high voltage transformer when everything is disconnected.
I think I checked the diode properly.

Anyone have ideas as to what to do next?
Replace the diode?