[Elecraft] Wiring the Shure 450 mic for K3

[Don Wilhelm]

Iain,

While that is true in many respects, the following facts are also true:

1) The K3 mic connector shell is grounded - so purists should connect 
the shield wire from th emic element to the shell of the Foster plug.
2) The K3 original design used a 100 uHy inductor in the ground path 
(pins 7 and 8) of the mic jack input.  That has been corrected in the 
current build of K3.  Those who have earlier K3s should short out Front 
Panel L4 as described in the Elecraft Application Note "Front Panel 
Microphone Circuit Modification".
3) The above will be a benefit, but it still does not completely solve 
the 'pin 1 problem' (although it comes as close as practical (see below).
4) To truly solve the 'pin 1 problem', the microphone jack shell should 
be connected directly to the *enclosure* at the jack location (rather 
than to the board ground) - that is difficult to achieve with the 
enclosure mounted over a PC board.  It was not a problem in older 
equipment where the jacks were fastened to the enclosure itself rather 
than fastening to the PC board.

If some users have a need to do so, it would be possible to add a solder 
lug on the inside of the Front Panel and connect it by a short path to 
the microphone jack shell - that would be the proper "pin 1" solution.  
But since that would require a mounting hole (for the solder lug) in the 
K3 Front Panel metalwork, I doubt that many owners would be willing to 
do that.  There is space to mount a solder lug on the bottom flange of 
the front panel metalwork, so some owners may be willing to add one 
there (just below the mic jack) - the solder lug can be positioned in 
such a way that it is possible reach into the left end and solder (and 
desolder) it to the ground wire around the mic jack when the front panel 
metalwork is installed (or removed).

After the change to bypass L4 is in place, mic jack pins 7 and 8 both 
have a relatively short path to the board ground plane, and in all but 
the most extreme situations, it will be sufficient.

73,
Don W3FPR

Iain MacDonnell - N6ML wrote:
>
> Hmm, but there has been a recommendation to connect the mic return
> and shield to the connector chassis, instead of using pin 7, to avoid
> the "pin 1 problem".
>
> http://n2.nabble.com/Microphone-ground-td1329465.html
>
>     ~Iain
>