Topband: Signal pickup mystery

[Lee K7TJR]

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.
   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? 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?
  Maybe these tests will provide more clues.
Lee   K7TJR