The microwave starts, then stops after a few seconds, and smells

Remove the cover after pulling the power plug. Check the transformer and large capacitor for signs of overheating. If they look ok, then remove the sheet metal wave-guide from the area where the energy enters the cooking cavity. Under the wave guide is a fan-like device called a stirrer that has a plastic bearing. This stirrer bearing is commonly arced over (when the oven is operated with nothing to heat in it). Usually the bearing looks good, but has arced internally. Replace the stirrer as a unit. The machine can be operated with the case off and a cup of water for a load, but keep you hands clear. Look for smoke.