White countertop microwave on a kitchen counter
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Microwave hums loudly (with weak or no heat)

A loud hum is usually the HV transformer under heavy load from a shorted HV diode, a shorted magnetron or a failing transformer. All checks are in the high-voltage section.

Cross-checked ~60 min advancedBrands: All

Before you start

  • DANGER: the HV capacitor can hold a lethal charge after the microwave is unplugged
  • Homeowners must not open the cabinet; internal steps are for trained techs only
  • Trained techs: unplug or breaker off, discharge the HV capacitor with an insulated tool, verify 0 V before touching internal parts
  • Never run the oven with the cabinet off or with interlocks bypassed

Diagnostic steps

Unlock all 5 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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Microwave hums loudly (with weak or no heat) · OnCall Tech