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Faced with the challenge of using both a conventional TNC and the need to interface the 756PRO to the computer soundcard, I designed a switchable interface. It connects to the ACC1 8-pin accessory connector and offers either an extension of the connector, for use with the TNC, or 4 phono jacks to provide audio in, audio out, FSK, and PTT. In addition, it provides transformer isolation of the audio in from the external sources.
FOR SALE: I have one fully-builtup board available postpaid in the US for $50. In addition I have several of the boards without components available for $20 each. The board requires about $20 in parts, some from Mouser and some from Digi-Key. I do have some of the parts available so if you buy the board I can quote you a price for what I have in stock.
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The interface uses a 6-foot ACC1 cable to permit positioning it conveniently. The cable is shielded and is purchased prefabricated from Mouser. |
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On the left is a standard DIN8 connector. When the selector switch is not closed, this is an exact extension of the DIN8 ACC1 connector on the back of the Icom transceiver. The middle phono connectors are audio in and out, independent of the microphone input and the speaker output. The right phono connectors are for PTT and FSK. |
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The two black boxes are sensitive relays. I looked at using a rotary switch for this function but it was very expensive and posed wiring and mounting problems. By use of the relays, a single SPST switch makes the selection and can be controlled remotely from the board itself. The relays are powered from the rigs 13.8v line and have diode protection to allow solid-state switching. |
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Close examination will reveal that a 130 ohm resistor is used in series with each relay coil. Since the relays are nominally 12v and the rig supplies at least 13.8v, it was decided to drop the voltage down to 12v. This limits current consumption of the relays. The diodes shown are weird smt 1N4148 equivalents. They, including their solder pads, are completely round. As such, they are really a nuisance to solder. Note the 1:1 audio isolation transformer. Since the rig input is high-impedance, it is terminated in a 1k resistor to minimize noise inherent in high-impedance circuits. Note also that the switch connection at the bottom right is bypassed by a .01 µf ceramic chip capacitor. While the smt resistors are all 1206 size (1.2 x .6 mm), this is 805 size (.8 x .5 mm). It is certainly tiny! |
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