[Racal] RA6793A Microprocessor Noise

[Dallas Lankford]

My RA6793A has suffered from uP crud, mainly above 20 MHz, from the day =
that I received it.  Another 6793A that I examined had the same problem, =
which indicates it is a common problem.  At some point I noticed that my =
phased antenna array could null the crud, indicating that the noise was =
being radiated by the receiver and picked up by the antenna.  I tried =
shielding the speaker wire (though it was already shielded) by wrapping =
aluminum foil around it.  No luck.  But then the aluminum foil slipped =
and accidentally grounded itself to the 6793 chassis at about its mid =
point without being grounded at any other point.  The noise was reduced. =
 So I used some shielded twinax, longer than I would normally use for a =
speaker connection, with it grounded only at its mid point to the 6793 =
on its rear panel.  This reduced the uP noise considerably.  My phased =
antenna array again determined that the receiver itself was the source =
of the remaining noise.  I attempted to determine if the front end was =
involved, but got negative results.  To take a break from my efforts, I =
decided to do some weak signal listening with headphones.  To my =
surprise the remaining microprocessor noise went away.  I didn't like =
the idea of leaving my headphones plugged into the 6793 as part of my uP =
noise elimination solution, so I wired a 10 ohm resistor to a headphone =
plug.  That worked just as well; no noise.  I also noticed that with =
just headphones connected and no speaker connected there is no uP noise. =
 But I often like to use a speaker with my receivers.

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