Topband: Signal pickup mystery

[Pete Smith N4ZR]

Replies interspersed - thanks for all the ideas.

73, Pete N4ZR
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On 3/11/2012 4:47 PM, Lee K7TJR wrote:
> Hello Pete,
>> Now here's the mysterious part.  If I remove the DC power from the
>> preamp, the 20-meter signals drop from S9 to barely audible. This is
>> also noticeable, but just barely, on the 1550 KHz signal. Is it possible
> that the preamp, which is between the feedline and the primary of the
> binocular matching transformer, is somehow amplifying the common mode
> signals? The shield of the coax connects to the shell of the preamp, and
> from there to the secondary of the matching transformer (the 75-ohm
> side). Is it possible that common mode signals are getting back into the
>> preamp input through the matching transformer primary?  If so, any ideas
>> on how to clean it up?  Or should I just get rid of the preamp out there
>> and do my amplifying in the shack?
>     It seems to me that your preamplifier is amplifying whatever it is being
>   fed. A common mode problem would likely be after the preamp and in the
>   feed-line with no connections to the relay box. You could certainly prove
>   this by removing the input to the preamp and replacing it with a
>   termination. Power up and look at the signal levels.

Excellent idea.  Easy to do, also.
>     I am more inclined to think the signal is being introduced into the
> preamp
>   by your relay box. Does the relay box short or terminate unused inputs, or
>   are they allowed to float?

They are grounded.
> If they float the contact capacitance could
> inject
>   signals into the output. Have you terminated the active input? You could
>   try shorting all the inputs to see if the signal is being picked up by the
>   open inputs. Also the power supplied to the relay box could inject signal
>   by the coil to contact capacitance. Do the signals go away if you remove
>   the power to the relay box while leaving the preamp active?

All good thoughts!
>    Maybe these tests will provide more clues.
> Lee   K7TJR
>
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