Wow, the ignorance expressed here is astounding. I can see why "people" are warned away from repairing their microwave.
A magnetron is not "radioactive", it emits nothing when not powered and it emits radio waves when powered, it is basically a form of "radio tube" and most emit the same freq. as your old cordless phone and wifi router, just a lot more energy, thus why a microwave can interfere with your old cordless phone or wifi. There is no "nasty radiation".
The only danger in a microwave is the HV capacitor which can kill you so learn how to safely discharge one, it is not hard if you have half a clue. All the ones I have ever dealt with self-discharged as designed but don't bet your life on it. When replacing any component inside make sure to route all wires as you found them. When replacing the magnetron (most can be purchased new for $30-$50) be sure the gasket (usually inc. with a new mag.) is in perfect shape and in place and that the mounting tabs/area is undamaged. If the mag. mount/area and the microwave in general is not bent/damaged then it won't be leaking any "radiation". If there is heavy arcing/burn damage around the mag. mount area this may be a cause for concern re leakage, on some models it is an easy repair, on other it is not and the amateur may want to stop at that point. Do not crush or break the old magnetron, some of the older ones can produce a dust from the ceramic insulator that is very harmful to breathe in.
As to quality, I have seen magnetron failures in one year old machines and the brand doesn't matter much, recent microwaves use magnetrons manufactured in one of two factories that supply most of the world and the standard/QC is not what it was 20 years ago. There are other factors leading to some machines failing more often like waveguide design issues (GE/Kenmore over the range models were infamous re this) but nothing the consumer or salesman will be able to know.
Maybe something burst from the microwave like a light bulb or a circuit popped? If you feel like it is working fine otherwise you should concerned with local repair person except to mess with it.
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