I’d start with the line fuse:
Cause 1
Line Fuse
If too much current passes through the microwave circuitry, the line fuse will blow. If the line fuse blows, the microwave won’t start. To determine if the line fuse is at fault, use a multimeter to test it for continuity. If the fuse does not have continuity, replace it. In addition, you should investigate and resolve the problem that caused the fuse to blow. If you don’t resolve the underlying issue, the new fuse will blow too. (Caution: The microwave oven can store thousands of volts of electricity in its high voltage capacitor, even after the microwave oven has been unplugged. Due to the potential for electric shock, it is extremely dangerous to replace the electronic components in a microwave. Only a licensed technician should replace the line fuse.)
Parts
Line Fuse for your Quasar Microwave
RepairClinic Item #757856
Ceramic fast blow fuse, 15 amps, 250 volts
https://www.repairclinic.com/RepairHelp/...