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
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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