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Re: [RFI] Microwave RFI - radiated (?)

To: Donald Kerns <dkerns@cruzio.com>
Subject: Re: [RFI] Microwave RFI - radiated (?)
From: dgsvetan@rockwellcollins.com
Date: Mon, 12 May 2008 16:24:38 -0500
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>

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


Donald Kerns <dkerns@cruzio.com> 
Sent by: rfi-bounces@contesting.com
05/09/2008 12:55 AM


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...)

73 de Donald

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