My Amana Radarange over the stove microwave started to omit a "burnt rubber" smell and stopped defrosting or heating anything all of a sudden. Model #MVH15W. Still works otherwise, lights on and turntable turns etc. What could it be?

You're describing a common smell that goes along with a magnetron tube that has failed. The magnetron tube makes the microwaves, and will eventually fail if you keep a microwave long enough. They can fail in a couple of different ways, and one is to short out accompanied by that burning smell. It's probably not worth trying to repair your oven because of its age. I don't believe the magnetron is available any longer. If it is, it will be costly, and replacing the tube is best left to a professional because of safety concerns. Your most economical choice would be to replace the oven.