[CQ-Contest] Contest Station Switchbox

[Pete Smith]

I'm in the process of building up a combination switch and junction box for
my CONTEST station, which has as its objectives to simplify and improve
wiring reliability and do away with a lot of discrete boxes on the desktop,
behind the radio, etc.

Tentatively, my plan is only to do DC and audio switching inside the box,
which will be aluminum, to serve as shielding against any stray RF.  I plan
to include in it switching to select either my MFJ-1278 of my microphone to
feed the audio input of the transceiver, opto-isolated keying and PTT
circuits, and audio shaping circuits to roll off high-frequency noise in
the headphones.  I *may* also try to include isolated audio and PTT
connections to and from my DVP, which is the classic K1EA board in the
shack computer. 

In thinking about this, I'm concerned about creating ground loops and
inducing hum into the audio lines, in particular.  My DVP has had a
virtually disabling problem with hum that has resisted 2 years'
intermittent efforts to fix it, and I'm not interested in propagating the
same problems (or other similar ones).  If I run audio circuits with one
side grounded to the box, for example, it seems it would be easy to develop
a few millivolts of AC differential that would readily get into sensitive
places like the microphone input on the transceiver.

Would I be better off building the box in plastic, and using two conductors
rather than one for the audio lines, with the objective of isolating each
circuit from the others?  Or are there construction practices that,
properly followed, could let me have both RFI protection and ground-loop
avoidance.  Is there a published source for the appropriate ground rules to
be followed?

Thanks in advance!




73,  Pete N4ZR
Loud is good


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