I had an old built-in microwave that I tried to keep working for 10 years. It kept blowing fuses. It is usually blown by the door switch not working correctly. The door usually has 3 switches to make sure the door is not open when the magnetron is running. It blows very fast. It is a direct short to ground through the fuse and through the door switch. I would "bend" my switch arm until it finally did not work anymore. Mine was too old to buy a new switch. But if you can find the parts, then it should be under $10 to fix the door switch.

If you change the fuse (usually mounted inside on the bottom metal plate) and it
blows again, try to see if there is any correlation to opening/closing the door when it blows.

The only other parts I have changed is the magnetron capacitor/diode combo ( about $17 from foxatlanta appliance ). But, usually the clock display still works when these go out due to a voltage spike.