Microwave oven

It sounds like the magnetron is shorting or pulling too much current when it is powered up.

Is for myself personaly, when my personal microwave oven fails, if it is not a small easy fault to service, I replace the complete oven. Many of the models now carry a 3 year warranty. If you amortize the cost over the 3 year period, the cost is very cheap.

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What do you mean by "going bad"?

Was it heating during that 1 minute?

There's only 4 parts there - HV transformer, HV capacitor, HV diode, magnetron. It's unlikely that the HV transformer could go bad in a way that would not result in a blown fuse or lots of smoke.

The magnetron could be defective and show up only with HV applied.

What abouut the condition of the waveguide and the rest of the oven?

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The capacitor showed a short to case after running for about 2 minutes, but would again test OK after being off for a while. The wave guide and the rest of the oven look A1. The magentron look good also. No ;I got no heat in that 1 minute that I ran it. I had a cup of water in it, and it stayed cold. I am beganning to think it is the magnetron even though I can not prove it.

The capacitor showed a short to case after running for about 2 minutes, but would again test OK after being off for a while. The wave guide and the rest of the oven look A1. The magentron look good also. No ;I got no heat in that 1 minute that I ran it. I had a cup of water in it, and it stayed cold. I am beganning to think it is the magnetron even though I can not prove it.

The capacitor showed a short to case after running for about 2 minutes, but would again test OK after being off for a while. The wave guide and the rest of the oven look A1. The magentron look good also. No ;I got no heat in that 1 minute that I ran it. I had a cup of water in it, and it stayed cold. I am beganning to think it is the magnetron even though I can not prove it.

I used to fix them years ago. I made a resistive divider which I could use to measure the high voltage at the cathode using a conventional voltmeter. If the heater windings are OK and the voltage is about 2500 (IIRC) then replace the magnetron.

Cheers.

Bill.

It appears that our microwave oven has lost cooking/heating power over the years. Is that possible: Can the magnetron put out less power as it ages? How can one test the heating power of a microwave oven: Perhaps by heating a known quantity of water to a desired rise in temperature over a prescribed time period? Are there any such measures for a microwave oven of a known original/rated power?

Rick

Magnetrons do lose their output over time.

On Sun, 19 Oct 2008 02:22:41 GMT, "Richard G. Gould" put finger to keyboard and composed:

Notes on the Troubleshooting and Repair of Microwave Ovens

7.1) Testing the oven - the water heating test:

- Franc Zabkar

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It's not a magnetron, it's a klystron. It's a tube and tubes get weak. But more likely, one of the power sources has become weak.

Three power sources are required for reflex klystron operation: (1) filament power, (2) positive resonator voltage (often referred to as beam voltage) used to accelerate the electrons through the grid gap of the resonant cavity, and (3) negative repeller voltage used to turn the electron beam around. The electrons are focused into a beam by the electrostatic fields set up by the resonator potential (B+) in the body of the tube.

the

heating

prescribed

I don't know where you get your information, but microwave ovens use magentrons. A klystron is a rather different sort of microwave device. For one thing, it's an amplifier, not an oscillator (unless you misalign it, especially the B cavity).

I know about these things, because I used to install and align klystrons. I could tell you a few "Jedi tricks".

....Just remembering a few years of tweaking 1kw klys'es at 0-dark-30 in the military and watching the beam current like a hawk.....

ISTR some commercial process heating chambers that used multiple klystrons for outputs.

I suppose they still use Klystrons as deep space power amplifiers such as in Apollo missions. I think the standard power was 20 KW and made by Collins. A Hydrogen Maser was also used as a low noise amplifier and was used as the primary frequency standard, Cesium second. I just updated my learning from the web. A Cesium standard is basically a Xtal oscillator buffered with a atomic feedback loop and has poor short term stability. The Maser has the best short term stability and porr long term.

greg

Apollo missions.

and

porr long term.

Klystrons & Klystrodes were common in UHF aTV transmitters. The last I used were a set of 65 kW, made by EEV on us Ch. 55. They are being replaced by solids state transmitters.

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The Apollo mission was not "deep space". Nevertheless, I installed several Varian klystrons in NASA's STDN.

Well the moon was a bit farther away. I don't remember what things were tracked by the maser. Maybe it was just a backup, but easily overloaded on strong signals. All the 3 main 85 foot MSFN/STDN stations had masers. After Apollo, ERTS or Landsat was the main preoccupation. Tracking the Alsep packages on the moon was also done for a VLBI experiment. After that the stations were also recording Helios orbiting the Sun, a very week signal. You really had to play with it to get phase lock.

Here is a link to the NASA bible for the stations. Its a big file....

greg

The waveguide on the output of the mag (and it is a mag - ignore what clodhopper says below about it being a klystron) can become coated with resolidified vapourised grease and clag from the food that's been cooked in it - especially if it is a combination type that has a conventional oven in it as well. This can result in substantially reduced microwave cooking efficiency.

Arfa

Absobloominglutely

The first thing to do with an oven which appears to have lost perfomance, is to give the cavity a good clean, including the inside glass of the door and the waveguide cover and to check that there`s no nasty carbonised gunk up the waveguide. Simply replacing the waveguide cover perks them up a bit.

It`s also important to check the spade connectors on the magnetron, these sometimes become loose and erode away making intermittent contact for the heater current. My own Panasonic was suffering from this - about one in four cookings would fail, replacing the spade connectors cured the problem.

It is possible (IME) for magnetrons to lose power over time, sometimes the antenna cap gets eroded away and sometimes you find cracks in one of the magnets.

Ron

Yes they do loose power over time ! It depends upon how fast the filament/cathode looses the ability to produce electrons. Often Thoriated Tungsten is used in the heater ! If it gets hammered it rapidly looses emission.

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