Joe,
I believe the 'root of the problem' is that your cable ties the shield
to pin 8 rather than the mic plug shell. That is poor engineering practice.
Yes, the K3 makes the situation worse by tying pins 7 and 8 together and
running them to ground through an inductor, but I believe the real
source of the problem is the connection of the cable shield to a line
that is also used as a signal return. Normal practice would connect the
cable shield to the connector shell.
The K2 does not have the same problem because there is no inductor in
the path for either pin 7 or pin 8.
This is a classic example of 'the pin 1 problem' that Jim Brown
continually refers to.
73,
Don W3FPR
Joe Subich, W4TV wrote:
There have been persistent reports of "RF feedback" in the
microHAM microKEYER II and MK2R/MK2R+ when used with the K3.
The problem seems to be related to the design of the K3 front
panel mic jack and a work around has been found for microHAM
interfaces.
The easiest solution for the user is to modify the microHAM
DB37-EL-K3 cable by cutting the wire currently connected to
pin 8 of the Foster (microphone) plug. There is no problem
when the DB37-EL-K3 cable is used with the Elecraft K2 or
when the DB37-El-K3 is used with a "breakout" cable (In-line
Foster jack, RCA and 3.2mm plugs) for the rear panel mic and
PTT inputs of the K3.
This same issue can appear with any external interface that
connects via the microphone jack. For those interfaces, the
microphone return (pin 7) must be separate from any shield on
the microphone cable. The cable shield and "PTT ground" must
be connected to the shell of the Foster mic jack (chassis);
they can not be connected to the "mic ground" (pin 7).
The root of the problem appears to be in the wiring of the
K3 mic jack. Pin 7 (mic ground) and pin 8 (PTT ground) are
connected in parallel, through a common RF choke (L4 on
the front panel board), then to the circuit common (ground).
Note in the K2, both PTT (pin 8) and mic (pin 7) returns
are connected directly to the circuit ground through P1
and the K2 does not have the "RFI" problem.
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