[Elecraft] K3 -- Unwanted tone in transmit audio, all modes

[ken brookner]

I have 2 K3s:  one I built a year or more ago and one I just purchased 
with a sub receiver built at the factory.  On the new unit with the sub, 
when I transmit there is a tone in the audio.  It's pretty constant at 
about 2.8 kHz, will vary with the mic gain, is audible in the MON output 
as well as on a separate receiver.  It is evident in USB/LSB, Data, AM 
and FM in the monitor and is present with no mic plugged in.  My usual 
power supply is a 25 amp switching supply, but the issue remains when i 
switch to a 25 amp linear supply.  I can unhook everything from the K3 
except for power and antenna with no change.

The K3 that I built does not exhibit this problem hooked in the station 
the same way--e.g. unplug one from antenna, power, rs232, etc. and drop 
the other one in with the same connections.

Elecraft's thinking was that it's an interaction between the main and 
sub receiver dsp boards and they sent me a replacement sub receiver dsp 
board which I have just installed.  Problem remains and was not 
changed--still the same tone.

The new unit contains the ATU, sub receiver, digital audio board, and 
the 100 W option.

Anyone come across this problem before?

Thanks & 73,
Ken, KY5G