Hello Peter, I operate digital modes (mostly RTTY) using a K3/0-Mini and a
remote K3. I operate direct to my own K3, and on RemoteHamRadio.com
A description of how I do it is available on my QRZ.com page for
K6UFO. Under "Articles" near the bottom of the page, look for:
K6UFO's How to Operate Digital Modes on RemoteHamRadio.com
To respond to some of your particular questions:
1) The received audio level is dependent on the AF knob, because only the
"Speaker" audio is being returned from the remote K3, and it varies with
the AF knob.
2) There is no audio signal on the K3 TX site using the USB audio
codec because it does not carry audio, only rig Control, PTT, CW
keying. Not audio.
Your transmit audio needs to go (attenuated) into the K3/0-Mini Mic
input, or into the AUX/Mic input on the Control RRC-1258. Otherwise there
will be no audio to transmit at the radio. I do not know of any way to
use the USB connection to do this with the default settings for a K3/0-Mini and
RRC-1258 remote system from Elecraft.
You cannot monitor the actual transmitted audio with the K3/0-Mini
"MONitor" function unless you have specifically enabled that in your
configuration by settiing up Full Duplex and Continuous RTP Tx.
(RRC User Manual page 59.)
Otherwise, you are hearing a locally generated signal from your HRD/
laptop. You would know if you were really monitoring the transmitted signal
by the time delay.
I think you have assumptions about how it should work that are not the
same as the actual functioning described in the K3 Remote Owner's Manual
and the RemoteRig_RRC1258-MkII_Users_manual
Good luck,
Mark K6UFO