The interface has a built-in digital VOX circuit, activated by the digital audio transmit tones, and therefore does not need a USB port or a serial port for operation. One stereo audio cable from the computer audio output (speaker or earphone) to the interface audio input, and a multi-wire cable from the interface to the transceiver microphone or accessory jack are all that is needed for transmitting. For receiving, one additional audio cable, from the transceiver audio output (speaker or earphone), to the computer microphone or line input is all that is needed. A transformer isolates the transceiver from the computer which will prevent most ground loop problems. In rare cases, it may also be necessary to insert a transformer in the receive audio line, which is separate from this interface.

MATERIAL LIST

(1) Circuit Board (available from KH6TY, 335 Plantation View Lane, Mount Pleasant, SC, 29464, for $5.00 and SASE)

(1) 100K, 1/4 watt resistor (Mouser 291-100K-RC)

(1) 47K, 1/4 watt resistor (Mouser 291-47K-RC)

(1) 330K, 1/4 watt resistor (Mouser 291-330K-RC)

(2) 4.7K, 1/4 watt resistors (Mouser 291-4.7K-RC)

(3) 1.0 uF, 50V, electrolytic capacitors (Mouser 140-XRL50V1.0-RC)

(1) 10 uF, 50v, electrolytic capacitor (Mouser 140-XRL50V10-RC)

(2) 1N4148 diodes (Mouser 512-1N4148)

(2) Audio or switching transistors, 2N2222A, or equivalent (Mouser 863-PN2222AG)

(1) 600:600 ohm isolation transformer (Mouser 42XL016-RC)

(1) Panel mount 1/8" stereo audio jack (Radio Shack 274-249 or Mouser 161-3402-E)

(4) 1/4" #4 sheet metal screws for PCB mounting

(1) Altoids tin or Radio Shack project enclosure (270-1801)

Solder all parts to the circuit board as shown on the circuit board legend, schematic, and pictures, saving all lead ends cut off to use for tie points, P1-P6. To make these tie points, squeeze a cut off lead between the fingers, forming a long-legged "U", insert in the holes, and solder. If you use a metal-cased transistor for Q1 and Q2, note that legend is reversed, indicating the emitter lead at the top, but should be indicating the emitter lead at the bottom instead. Just insert the transistors so the emitter is nearest to the bottom of the circuit board and the center lead (the base) still goes to the center hole of the three holes. For plastic, inline transistors, just be sure the emitter is in the hole nearest the bottom of the circuit board, the base is in the center hole of the three holes, and the collector is in the top hole.

Drill a 1/4" diameter hole in the center of each end of the enclosure. Mount a stereo jack in one end, and mount the circuit board with #4 screws, with the end with P1 and P2 facing the stereo jack. Attach P2 to the jack ground pin and P1 to either remaining pin.

Make up a 4-wire cable with a microphone plug to match the transceiver, and note the color of the wires for 8 VDC, PTT, Ground, and Microphone or audio input for the transceiver. Insert the other end of the cable into the remaining hole in the interface enclosure. Strip all four wires 1/4" and tin.

Turn the transceiver power off, plug the 4-wire cable into the transceiver and confirm which wire is ground. Solder the ground wire to P6.

Turn the transceiver power on and measure the voltage on the remaining three leads. Solder the lead with a steady 8-14 VDC to P4.

Now short each of the remaining two leads to P6. Solder the one that switches the transceiver into transmit to P5.

Solder the remaining wire to P3.

Use a cable tie where it exits and where it enters the enclosure to strain-relief the 4-wire cable if desired. If mounting in an Altoids tin, use a rubber grommet to prevent the tin from cutting the insulation on the cable.

ADJUSTMENT: Plug a stereo audio cable from the computer earphone or speaker jack into the stereo jack on the interface and place the digital software into transmit. Adjust the Windows speaker volume slider until the transceiver goes into transmit and then move it about 20 percnt higher. This should usually result in the right audio drive level for the transceiver without overdriving the microphone input. If the audio drive level is too low, decrease the value of R1, or if it is too high (overdriving the microphone input), increase the value of R1.

RECOMMENDED SOFTWARE: Fldigi (http://www.w1hkj.com/NBEMS) or Multipsk (http://f6cte.free.fr/MULTIPSK.ZIP)