several years ago when my one sister was still living, her microwave died, so i got her a new one and installed it. [one of those under cabinet units, but i forget the brand]
anyway, while the old one was in my pile of "stuff" waiting to be repurposed into whatever i could figure out what parts could be used for whatever, my one buddy stopped over.
seeing the microwave, he asked if i checked the internal fuse. huh ???????
we took the cover off [tamper proof screws], and looked around inside a bit, and discovered a 20amp [car style] glass fuse that was blown.
i replaced that fuse, and it came back to life ! my buddy is still using that thing today at his camp !
i don't know if all brands have those "secret fuses" inside, but i'll bet there have been plenty of microwaves junked because those fuses have blown.
15 minutes or so, and what are those fuses now, $1.00 maybe ? and some dollars are saved..........
of course today, it's just toss it and buy a new one.
growing up around depression era folks turned me into a weird kinda doofus i guess.