You will enjoy your KX1, as I have mine. I made thousands of QSO’s. If you can “zero-beat” within approximately 100 Hz this should be plenty close most of the time. I am a musician so I just adjusted the tone a bit above high C which is an octave above middle C. Actually a D is 587 Hz, which is very close. In your case, I would see if the N0SS website is still active, which it was long after he became a SK. If you can find the schematic, it should be an easy build. I built a tone detector many years ago to drive my telegraph sounders many years ago. I used a $2 eight pin DIP tone decoder chip still available from Digikey and others. It was a LM567 and I set it up for a 80 Hz BW and 600 Hz detect frequency. It is also available for around $1 in the SOIC pkg. I put in a LED which flashes right along with the CW when my RX is tuned close to 600 Hz. The data sheets give info on designing your circuit. Adjust the level into the chip so it reads the CW, but not the noise, not at all a critical adjustment. Another idea is to build a 600 Hz oscillator, or get a tuning fork, and just match the frequency, however not all folks can quickly match frequencies. Possibly the best idea is to build a sharp 600 Hz audio filter, and switch it into the circuit when zero-beating and also it will be useful for operating in tough conditions. A deluxe version on the tone detector could even have something like a 560 Hz, a 600 Hz, and a 640 Hz Hz decoder and use a different color LED for each frequency….really an overkill, but we all like flashing lights ! Good Luck Rick KL7CW
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