This is a particular problem with transceivers that have a
single V- lead and only one "pin" for the V- connection
(like
the K3). If the negative lead connection is not
absolutely
perfect other "ground" paths (e.g. the shield on a
microphone
cable) through accessory equipment will carry a portion of
the
return current.
It is nearly impossible to cure this with any rig with an
external 12V supply unless you isolate the radio power
supply negative terminal from ground, or float all the audio
line grounds except at the radio. I first discovered this
problem with an IC-751A ICOM, which was my first 12 V
operated radio. My FT1000MP was no better, so this is
definitely not a "K3-only" issue. There isn't a 12V radio
out there that will not have this problem aside from some
isolated ground Motorola two-way radios and things that will
run off 12V positive ground systems.
I use a separate 12V supply for any 12V radio, and a smaller
supply for accessories, for two reasons:
1.) If you use a common supply and the supply positive lead
ever gets shorted to ground or the negative ever opens it
can blow up foil traces in small accessories like keyers.
2.) If an accessory device gets a short it can melt wires or
open foil in other devices.
I use a low current thermally resetting fuses for
accessories and a different heavy supply with an isolated
ground lead for any 12V radio, and I use isolation
transformers when necessary on audio lines.
It's just silly to design an audio interface device that
commons the shield of audio lines to multiple pieces of gear
or to the cabinet, but that is how Ham accessories are
usually made.
73 Tom
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