I know that when a fuse blows in a microwave, it means that something is wrong and needs to be fixed.
When it first happened I checked the continuity on all the door switches and the thermal fuse. I replaced the thermal fuse basically because I did not understand the spec which said that the continuity test should be positive only when the thermal fuse is bad. I figured this out when I got the new one and tested it. In any case I decided to reassemble the thing and put a new (regular electrical) fuse in. Worked like a charm. For about a month (maybe two). This time I brought it down from over the stove to disassemble it and check the capacitor and some other thermal fuse. Huge pita. Won’t do that again. Everything checked out, at least to my Youtube educated self. Put it back together and bingo, good for another two months. Now I have a supply of fuses and simply replace them when they blow.
The microwave is on a dedicated circuit with a 20 amp breaker. That breaker has never tripped. The fuse in the microwave is also 20 amp.
Maybe make sure nothing is shorted. Check for any scorch marks on the inside of the case and chassis, wires that are pulled right around any sharp corners where the insulation may have rubbed off. Check near things that may vibrate when the motor(s) are running or the door opens or closes etc.
It’s possible that when you opened the case, did whatever you did and then closed it back up, you moved a wire so that it’s no longer shorted, but eventually worked it’s way back to the same spot.
I have no idea if you can run a microwave with the cover off, but if you can, you might try that and watch to see if you can spot a wire heating up (turn off the lights, see if anything is glowing) or point in various spots with an IR thermometer and see if any spots are considerably warmer than you’d expect them to be.
There’s only four main parts in a Microwave:
Transformer
Rectifier
Capacitor
Magnetron
One of them is failing. It’s not dead yet, but it’s causing the unit to draw more current than it should. Possibly a shorted winding in the transformer.
Check the outlet also. I recently had a microwave go bad when the outlet developed an open neutral (though the actual fault was at the panel). A $7 outlet tester will work for this.
And I would additionally check the voltage to make sure you’re getting full 110V. I don’t know about microwaves but other appliances can be damaged with low voltage.