Sharp R-980A Convection oven - arcing without any metal (except turntable) and smoking inside.

There is a shorted happening when the magnetron release energy. The problem of that is the waveguide already not in focus (already have a big hole on it) hence it scattered the microwave wave elsewhere inside the unit and also to its own metal casing. That cause's to arcing when operating.
Waveguide hole needed to be replace and it need some cleaning & painting of the affected metal (usually near around the magnetron waveguide). Once it's done the unit will not arcing again.
I do hope I answer your problem. Thanks for asking.