[Elecraft] K3 in NAQP SSB

[Tom W8JI]

> 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