Re: RF feedback / muffled audio [ Mod posted to Elecraft web page]

Eric Swartz - WA6HHQ, Elecraft

We have just uploaded an important mod to the K3 Mod and App notes
page. This mod adds one wire to the K3 FP board to correct a problem
with VOX operation and low mic gain when Electret mics requiring mic
bias on the mic audio line were used on the front panel connector of
the K3. (Recently reported on the Elecraft reflector.)


Incorrect K3 VOX operation (VOX 'hangs') and/or low or corrupted TX
audio, when using a front panel mic which requires BIAS to be supplied
to the mic through the audio pin. (Elecraft MH2, MD2, Proset-K2 with
electret elements etc.)

Rear panel mic connections do not exhibit this problem. Dynamic-style
microphones (mics which do not require a bias voltage on the mic audio
pin) do not demonstrate this problem.

K3s with serial numbers between 660 and 782 with Rev B Front Panel PC
boards. (K3s as early as S/N 654 may demonstrate this problem, but we
think the problem starts with S/N 660.)

Other K3s DO NOT require this mod. All K3's that are currently
shipping have this corrected. All K3s from s/n 783 on have this corrected.

If you have an electret microphone which is wired for the K3 (Elecraft
MH2, MD2, Proset-K2 etc) you can test for presence of this problem by
attaching the mic to the front panel mic connector, setting "MENU |
MIC fpL bias" or "MENU | MIC fpH bias" and seeing if the symptom above
is present.


--- In Elecraft_K3@..., "edctexas" <edcintexas@...> wrote:

My serial number is 682. So this maybe the lowest S/N . This would
explain what I am hearing (and others). I need to check the front
panel PCB rev and look where the caps C97 and C98 are hidden (probably
near the FP mike connector.

Thanks to all who responded. I may wait a few days and see what
Elecraft announces as the "FIX", before fixing it myself. Of course
if it looks obvious... I have done alot of work w/ SMT PCBs adn parts
but my eyes are getting better w/ age!!

Regards and Thanks


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