[Elecraft] Icom SM-8 Mic to K3 Need wiring advice

[Jim Brown]

  On 10/10/2010 9:56 AM, G. Beat wrote:
> It is not unusual for newer microphones to separate the microphone  shield/ground for Audio from the DC ground used for Push to Talk (PTT) switching.

Yes, mics and mic cables are wired in a variety of ways. Some have four 
conductors plus a shield.  Some have two tiny coaxial cables plus an 
overall shield. Some have only two conductors plus a shield. Some have 
three conductors plus a shield.

When wiring a mic to a ham rig, we must figure out which of these we 
have. To do that, we must get our words straight.  The return for a mic 
capsule is not MIC GROUND, it is mic RETURN or COMMON. The return for 
PTT is not PTT GROUND, it is PTT RETURN or PTT COMMON.  These points 
will nearly always look the same with a DC Ohmeter, but at RF they may 
have significant inductance between them, and that inductance creates a 
Pin 1 Problem.

Cable shields can act as receive antennas, so they can carry RF current 
(like from our transmitter). The ONLY proper connection for an overall 
cable shield is the CHASSIS.  If you connect it anywhere else (for 
example, to a circuit board, where it wanders around and eventually 
finds the chassis), you have a PIN 1 PROBLEM, which is an open door to 
RFI (RF in the shack).

If you have only one cable shield, it MUST GO TO THE CHASSIS to prevent 
RFI.  Even though that shield may be used for mic RETURN and PTT return, 
it still must go to the CHASSIS.  Inside the radio, MIC return and PTT 
return are connected to the chassis via circuit traces. Those circuit 
traces are plenty good enough to carry mic audio and PTT return (DC), 
but they MUST NOT CARRY THE CABLE SHIELD.  That's a Pin 1 problem.

For more on The Pin 1 Problem, see http://audiosystemsgroup.com/RFI-Ham.pdf

73, Jim Brown K9YC