The next step was to modify my front panel board by adding an audio output (mic plug) via a 25 x audio buffer and a zero-beat indicator which uses the same signal. The added circuit is built Manhattan-style on the upper right on the back of the front panel board, see image.
In this way three things are achieved:
- The amplified audio output is independent of the AF pot setting.
- The zero-beat indicator is also independent of the audio pot setting.
- The ‘no-wire’ philosophy of K2 is maintained, as there are no wires that need to be run to any of the other boards.
The schematic of the audio buffer and the updated zero-beat indicator is here. Note that the zero-beat indicator’s component values have some minor changes from the original design, that gave a nicer behavior of the display.
I also have added an optional signal from the sidetone circuit on the K2's control board (requires a plugable strap between the boards). This gives a visual check of the zero-beat indicator when you transmit, and it is also nice for demo purposes as both the received and the transmitted CW are output and can be decoded using e.g. a PC.