Nescaf mod

Posted on Feb 6, 2014

The NESCAF audio filter is a small and low cost circuit that works great. It is a good addition to any amateur radio setup. It is especially useful with radio receivers or transceivers that lack an optional CW filter. I have made a product review of the NESCAF audio filter on my Amateur Radio Blog. One feature this filter lacks though is a bypass

Nescaf mod
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path for the audio when transmitting in CW. This would allow to hear the CW sidetone geneated by the radio through its speaker. So I have come up with a simple solution to this issue. Referring to the altered NESCAF audio filter schematic diagram below, all added connections and components are shown in wider/bolder black color. The bypass path is provided by a reed relay. The relay simply bridges the audio input signal to the output audio amplifier. The relay coil is energized via the ground return path provided by the Morse code key. The silicon diode and the zener diode insure that interaction with the radio is kept to a minimum. On the audio path, a 10K resistor is inserted to reduce the gain and match the audio level seen when the filter is inserted. A coupling capacitor is also inserted to block any DC current from flowing. Grounding the "Morse key" terminal will operate the bypass relay. Nothing is really critical in the implementation, and you may want to experiment with the component values to suit your needs. The following are things you should consider when adding this feature to the filter. Use a reed relay with a 5V coil. Try to select one with the least amount of current drawn from the coil. Remember that the U2, the 9V regulator, is limited to 100mA of current. The relay I use is a DIP package reed relay (Clare PRMA 1A05) I had on hand. You should be able to find an equivalent relay very easily. The 5. 1V...

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