The kind of failure described in this section could be due to failure of the magnetron tube, the power relay that supplies the magnetron tube, or the high voltage supply for the magnetron, especially if the turntable, fan, light and controls all still appear to be working.
I recently resurrected a dead G.E. Profile built-in microwave oven whose power transformer had failed in a most smoky and smelly fashion. The display and touch pad control still appeared to work, but it didn't heat anymore. The fuse didn't blow. After the oven was taken down and the skins were removed, the failed part was visually obvious. Even though it was 5 years old and well out of warranty, we were able to get a replacement transformer from G.E., and after about a half hour of reassembly, I was able to plug the oven into a heavy duty extension cord and start it up on the bench. I wiped the the oven cavity with household ammonia to remove the smoke residue as much as possible. A bit more work and it was mounted on the wall again and back in service, although it took a couple of weeks for the remaining acrid smell to fully dissipate.
Magnetron tubes are very reliable and not very likely to fail in less than 10-15 years. You'll probably get tired of the oven before the tube burns out. The high voltage transformer is the most likely thing to fail. Since the transformer has both high voltage and low voltage secondary windings, it is possible for the high voltage winding to fail, yet the low voltage section to still work and supply power to the electronic controls. The next item after the transformer is the silicon rectifier used in the high voltage DC supply for the magnetron.
The filter capacitor in the high voltage section is usually bridged by a high-value "bleeder" resistor to discharge it when power is turned off. It should be safe to work on within a few minutes after power is removed from the oven, although to be sure, one should short the capacitor terminals together with a screwdriver or other insulated tool before doing any work. ''Unless you are equipped with proper high voltage test gear and are trained in working with high voltages, you shouldn't work on a live magnetron circuit, as it can kill you.'' The reason I felt safe in tackling this job is that I'm an electronics engineer with 40 years of experience, including training in vacuum tube circuits.