Since my last message, I have been improving the XR-2206 setup and it worked nicely for some iCW QSOs.
Please find the present schematic at:
1.) The supply voltage V_CC has been reduced to 9 V in preparing the addition of a standard voltage regulator, which may be driven by 12 to 15 V.
2.) The circuit for keying the Amplitude Modulation Input Signal (PIN 1) has been changed.
3.) The rise/fall times of the circuit shown are at about 8 ms each.
4.) Trimmer resistors R_S (PIN 16) and R_D (PIN 14) have been added and are adjusted for minimum THD.
5.) The output to Sound Card 2 (in fact the input to Mumble) now is by an audio transformer
6.) The harmonics of the output signal are down by about 55 db. That's not much but may be improved
by a low pass filter in future. The capacitor C_R parallel to the secondary winding (inductivity about 290 mH) of the audio transformer already achieves a slight improvement in getting harmonics attenuated.
7.) The output "To Audio AMP (M.T.) -- for the monitor tone -- is fed to a TEA2025B audio amplifier not shown in the present schematic.
8.) The simple circuit for keeping the carrier down (down to about - 55 dB) at "KEY OFF" around R_CS works fine.