Good morning, Well what's probably happened was the magnetron shorted. Once it shorts it will continue to blown the internal in line fuse like you said, the problem is this is so old the part is no longer available in most cases, As it is your model isn't coming up so I cant be sure though ,A magnetron can list from $60.00 to $160.00 plus you add labor to that and your looking at more money to repair than replace. (not to mention the wait for the part) In my opinion after 30 years of appliance repairs its best to replace the microwave , Chances are that there could be other damage to the appliance that will cause other issues down the road like the high voltage transformer. Its up to you however, I hope this info helps though, thanks and best wishes, Bryan
this is what the magnetron looks like .