Nescaf mod

Description: The NESCAF audio filter is a compact and cost-effective circuit that enhances amateur radio setups. It is particularly beneficial for radio receivers or transceivers that do not include an optional CW filter. However, one limitation of this filter is the absence of a bypass path for audio during CW transmission, which would allow users to hear the CW sidetone generated by the radio through its speaker. A straightforward solution to this issue is presented through an altered schematic of the NESCAF audio filter. The modification includes a bypass path implemented via a reed relay that connects the audio input signal directly to the output audio amplifier when activated. The relay coil is energized through the ground return path provided by the Morse code key. To minimize interaction with the radio, a silicon diode and a Zener diode are included in the circuit. A 10K resistor is added to reduce gain and match the audio level when the filter is in use, while a coupling capacitor blocks any DC current. Grounding the "Morse key" terminal operates the bypass relay. The implementation is not critical, allowing for experimentation with component values based on user needs. When adding this feature, it is recommended to use a reed relay with a 5V coil, selecting one that draws minimal current. The 9V regulator (U2) has a 100mA current limit, and a DIP package reed relay such as the Clare PRMA 1A05 is suggested. The 5.1V Zener diode is crucial as it prevents excessive voltage on the relay coil, keeping current low and limiting the floating voltage at the Morse key to below 5V, which is standard for modern radios. Verification of the floating voltage on the specific radio model is advised to choose an appropriate Zener voltage diode. The added circuitry is depicted with the reed relay highlighted in blue, positioned above the filter chip, and all connections for the additional circuit are shown with thin red wires. To operate the bypass circuit, the "Morse key" terminal should be connected in parallel with the radio's key input (hot wire and ground) and the Morse code key, with attention to polarity.

The NESCAF audio filter circuit is designed to be an effective enhancement for amateur radio operations, particularly for users engaged in CW (Continuous Wave) communications. The primary function of the filter is to improve audio quality by filtering out unwanted frequencies, thereby enhancing the clarity of received signals. The addition of the bypass circuit addresses a common limitation faced by operators who wish to monitor their CW transmissions.

The modified schematic incorporates a reed relay, which serves as the switching mechanism for the audio path. When the Morse code key is pressed, it completes the circuit, energizing the relay coil and allowing the audio signal to bypass the filter directly to the amplifier. This design ensures that the operator can hear the sidetone directly from the speaker, providing immediate feedback during transmission.

The inclusion of protective components, such as the silicon diode and Zener diode, is critical for safeguarding both the relay and the radio equipment from potential damage due to voltage spikes or excessive current. The 10K resistor serves a dual purpose; it not only helps to match the audio levels but also ensures that the signal remains within acceptable limits for the audio amplifier, preventing distortion.

When selecting a reed relay, it is essential to choose one with a low current draw to avoid exceeding the current capacity of the 9V regulator. The recommendation for a 5.1V Zener diode is based on its ability to clamp the voltage at the relay coil, thus ensuring reliable operation without risking overheating or damage.

Overall, this modification enhances the usability of the NESCAF audio filter, making it a valuable addition for amateur radio enthusiasts who require improved functionality in their CW operations. The straightforward nature of the circuit modification allows users to tailor the design to their specific needs, making it an adaptable solution for various radio setups.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 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 Zener diode is important, as it insures that the relay does not get much more than 5V on its coil, keeping the coil currrent low.

Additionally, it limits the floating voltage at the morse key to less than 5V, which is normally the floating voltage on modern rigs. You may verify this on your radio and select a different Zener voltage diode to suit your radio. The following picture shows the added circuitry, with the reed relay (in blue) sitting on top of the filter chip.

All connections that are part of the add-on circuit are made with the thin red wires. Operating the add-on bypass circuit is a matter of connecting the "Morse key" terminal of this circuit in parallel with the radio`s key input (hot wire and ground) and on the the morse code key. Please observe polarity.