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Re: Conductive paint

 > Date:          Mon, 14 Feb 2000 05:57:56 +0000
 > From:          Roy Welch <rdwelch@swbell.net>
 > Subject:       [amsat-bb] Conductive paint
 > To:            amsat-bb@AMSAT.Org

 > Hi Fellows,
 > Does anyone know of a source for electrically conductive paint?  For
 > sometime I have had the annoying problem of RF getting into my PC
 > speakers.  Every time the radios key up there is a healthy thump from the
 > speakers depending on the antenna orientation.  I have completely filtered
 > all inputs to the small speaker amplifier in one of the speaker cabinets
 > with no appearent improvement.  Since then I temporarily used some foil
 > inside the speaker cabinet and this had considerable success in getting rid
 > of the RF.  So the RF is being directly radiated into the amplifier,
 > bypassing the filtering on the inputs.  I want to get some conductive paint
 > and paint the insides of the speaker cabinet to provide the shielding.  I
 > have looked into adhesive copper foil, but the inside of the cabinet is
 > difficult to get into with the foil.
 > Thanks.
 > -- 
 > 73, Roy

Dear Roy,

Back in my broadcast engineer days, I spent a lot of time getting RF out
of audio equipment.  I had both a AM and a 50Kw FM in the same building
as the studios.  Not a good situation.

In many cases the RF rectification occured in the output stages of the 
amplifier.  Sometimes bypassing the the input did no good.  If so, RF bypassing on
the output to the speaker did the trick.  What is happening is that the speaker
leads act as antennas.  RF rectification occurs in the totem pole output
stage of the amplifier.  A .001 cap installed as a RF bypass right on the
amplifier board usually cured the problem.

Feritte devices had to be avoided in audio applications.  The beads and
chokes I had available then would magnetically saturate when used on
speaker (and even some input leads) leads.  This caused the audio to
be distorted.

As I think back, I had little luck with shielding to prevent RF rectification
RFI problems.  Careful grounding and bypassing did work almost every
time.  Be sure to bypass power supply leads as well.

I did note one strange thing.  For some unknown reason some amplifiers 
could demodulate FM!  Demodulation by rectification of AM signals made
sense, but FM???  It did occur and the detected audio from some equipment
sounded pretty good.  Strange stuff this RF sometimes.

--73-- David WA0AUQ

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