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Microwave repair
If the lamp and the turntable run and a cup of water stays cold after one minute, the fault is on the high-voltage side. Unplug the unit and leave the cabinet cover in place.
No heat
The magnetron, the high-voltage diode, and the capacitor sit behind the cover. The capacitor can hold a charge after the plug is pulled. That is a technician's job, not a Saturday project. Tell the shop whether the turntable turned and whether the light stayed on during the test. Those two facts separate a door switch from a magnetron.
Turntable
Lift the glass plate and the plastic roller ring. A cracked ring or a hub that is off center stops the plate while the oven still heats. If the ring is intact and the coupler on the motor shaft is stripped, the motor underneath the floor is the part.
Keypad and door
Dead spots on the pad are the membrane keypad. A door that latches but never starts the countdown has a failed door switch. A microwave that runs with the door open is unsafe. Unplug it and do not use it again.
Sparking inside
Metal, foil, and a fork will arc. So will a burned waveguide cover, the small card on the side wall of the cavity. If the cavity arcs with only a cup of water inside, unplug the microwave. A cracked cover lets food splatter into the waveguide and the arcing gets worse each cycle.