Electronics > Repair

Microwave oven with no rectifier

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jmelson:
My wife's beloved 1984 microwave oven died a couple weeks ago.  Fuse was blown, after some digging the HV capacitor had a shorted section.  It was a 0.41/0.44uF dual-section cap rated at 2600 V.  There was a cook/defrost switch that connected both sections in parallel for cook mode.  The thing that has me stumped is I can't find a HV rectifier anywhere in the circuit.  Yes, I can see that the magnetron could act as a rectifier, so you could build an oven that runs without a rectifier.  But, one issue with doing that is that the transformer secondary would have DC in it that could saturate the transformer.  The CLASSIC microwave oven circuit has the transformer feeding a capacitor that then has a rectifier with the cathode grounded and the cap/anode junction connected to the magnetron cathode.  This circuit approximately balances the current in the transformer.  The circuit in the existing oven seems to apply the raw secondary to the capacitor AND the magnetron cathode in parallel, causing the capacitor to have a lot of 60 Hz AC current flowing in it.
Has anyone seen a microwave oven with this circuit?
Thanks for any hints,
Jon

CaptDon:
Having only a half wave rectifier does not cause a D.C. imbalance in the transformer. In a typical microwave oven the rectifier and the diode action of the magnetron form a voltage doubler circuit. The capacitor charges on one half of the cycle and the the capacitor voltage is in series with the transformer secondary voltage of the other half of the cycle. I have never personally seen a microwave without the diode. Some ovens use an inverter and the diode is a bit hidden within the core of the inverter.

vk6zgo:

--- Quote from: jmelson on November 18, 2024, 01:05:11 am ---My wife's beloved 1984 microwave oven died a couple weeks ago.  Fuse was blown, after some digging the HV capacitor had a shorted section.  It was a 0.41/0.44uF dual-section cap rated at 2600 V.  There was a cook/defrost switch that connected both sections in parallel for cook mode.  The thing that has me stumped is I can't find a HV rectifier anywhere in the circuit.  Yes, I can see that the magnetron could act as a rectifier, so you could build an oven that runs without a rectifier.  But, one issue with doing that is that the transformer secondary would have DC in it that could saturate the transformer.  The CLASSIC microwave oven circuit has the transformer feeding a capacitor that then has a rectifier with the cathode grounded and the cap/anode junction connected to the magnetron cathode.  This circuit approximately balances the current in the transformer.  The circuit in the existing oven seems to apply the raw secondary to the capacitor AND the magnetron cathode in parallel, causing the capacitor to have a lot of 60 Hz AC current flowing in it.
Has anyone seen a microwave oven with this circuit?

Thanks for any hints,
Jon

--- End quote ---

Such designs were common in the 1970s/early '80s.

wilfred:
Which fuse? Mine which broke some months ago wouldn't heat food but everything else worked normally. Mine had a mains input fuse and a high voltage fuse for the magnetron which had blown. The cause was one of the door switches had failed and was visibly charred. There were two other safety switches but this one did the heavy lifting. The HV fuse was hidden inside a 10cm long enclosure that completely covered the fuse holder. I can't remember if the switch failed open or closed.

But mine was only twelve years old and is probably a generic design like every other off brand oven from China. Thankfully spare parts are in plentiful supply every other week on the roadside.

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