OK, after many travails finding the proper bits, I opened the case up.
While there was some grease/dust combo blocking the ventilation holes
there was none in the interior of the oven.
I spent about 10 minutes poring over the innards and did not find
anything obviously broken/burnt/crisp/wrong.
So... wife gets a new microwave. And I have a nifty new set of bits to
add to the collection.
Thanks all!
73 de Donald
AE6RF
dgsvetan@rockwellcollins.com wrote:
> Donald,
>
> Thanks for the sound file. The interference definitely has a 60 Hz
> component to it. (That's the raspy buzz from when the oven is on. I
> agree that you do have a lot of other noise as well.)
>
> Since the problem is originating at relatively low frequencies, my guess
> is the following:
>
> 1. The oven is old enough that I truly believe it has a conventional
> power supply using an el-cheapo HV iron and copper transformer.
>
> 2. The AC buzz on the noise leads me to believe that any one or more of
> the following could be the cause of the noise:
>
> - Arcing within said el-cheapo transformer
>
> - Arcing within the HV rectifier
>
> - Bad or non-existing nosie filter at the power control relay that is used
> to control the HV. They may have used a "real" electro-mechanical relay,
> or they may have used a solid state relay, which is actually a triac or
> back-to-back SCRs. If the solid state relay does not have a filter
> network on the line input side, it is likely that spikes from the small,
> but definable "Off" portion of each 1/2 line cycle are radiating their way
> all over the place. Maybe there was a filter there to begin with and it
> is now history. If the actual cause is due to arcing, either the
> defective part needs to be replaced or, in the case of solid state relays,
> a filter added (or replaced) right at the input terminals of the device.
>
> Note that I did not include the magnetron itself in the above list.
> Presumably, the maggie is fed with HV DC, so it should not have an AC line
> frequency component. On the other hand, if the HV rectifier is bad, it is
> possible that there could be raw AC on the maggie, and that's not good.
> (As I understand it, early microwave ovens did not have HV rectifiers.
> Since magnetrons ARE a diode, the thinking was that they could operate as
> self-rectifiers. That thinking changed when it was realized that the
> stresses of PRV from direct line operation were causing a lot of tube
> failures. Hence, enter the HV rectifier block to save the maggies.)
>
> In any event, since the unit is many moons old, I'd open it up and take a
> look (with AC power cord disconnected, of course). Look for any signs of
> something burnt or charred.
>
> Good luck.
>
> 73, Dale
> WA9ENA
>
>
>
>
>
> Donald Kerns <dkerns@cruzio.com>
> Sent by: rfi-bounces@contesting.com
> 05/09/2008 12:55 AM
>
> To
> rfi@contesting.com
> cc
>
> Subject
> Re: [RFI] Microwave RFI - radiated (?)
>
>
>
>
>
>
> rfi-request@contesting.com wrote:
>
>> Date: Wed, 07 May 2008 16:31:30 -0700
>> From: "Jim Brown" <jim@audiosystemsgroup.com>
>> Subject: Re: [RFI] Microwave RFI - radiated (?)
>>
>>
>> Try more toroids in the stack and use close-wound turns to lower
>> the resonant frequency, using the data for coax of similar size as
>> a guide. If that helps, it's being radiated by the power cord, If
>> it doesn't, it's probably being radiated by internal wiring.
>>
>>
> OK, four (4) turns of the power cord around four (4) of the toroids
> doesn't touch the noise.
>
> That sort of says radiated rather than conducted to me.
>
> The microwave was a wedding present, so it is at least (counts on
> fingers and toes) 13 years old.
>
> I've put a recording up at http://www.kernsanalysis.com/ham/microwave.wav
>
> 7.131 MHz, AM mode 9.0 KHz filter (note how noisy my location is even
> without the microwave...)
>
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