Because of operational safety we recommend that an additional 20 Ohm resistor is installed between the line out and potentiometer. This way you will avoid a short circuit of the sound card output at the final position of the potentiometer. Thanks for that information, Wolfram, DF3UT.

G-38 Sound card interface, instruction manual

Acknowledgements

We would like to thank Bert, DG5KAJ who provided the casing and several other parts for our first project and Wolfgang, DJ7HE for contributing the circuit board materials. We would also like to thank all members who were involved for their input who gave up quite a few weekends and made this all possible.

Disclaimer of liability

This interface was tested by Gerd (DO1KGS) on a FT-817 and by me (DL2KEB) on a FT-990 and TS-790e and it worked perfectly. We will assume no liability for any damages that are caused through the operation of this interface, be it through errors in the set up or through an incorrect operation. All interfaces underwent a functionality test and were delivered at self cost price. We will gladly help you with any problems you may have but we cannot provide any warranty.

Description

The interface is to be installed between the computer and transceiver and thus enables the operation of new digital operational modes on short wave and UHF/VHF. A lot of free programmes can be operated with this interface, e.g. Digipan, WinPSK, MSSTV, MTTY, Throb, IZ8BLY Hellschreiber, IZ8BLY MT 63 Terminal and lots more.

The interface mainly consists of two 1:1 NF (tone) transformers and an opto-coupler. The NF transformer ensure for a galvanic separation of the signals that are transmitted between the computer and transceiver. There is one transformer for the transmission branch and one for the reception branch. Because we are dealing with passive components there is no need for operating voltage for the interface. There is a trimmer located in front of the NF transformer within the transmission branch. This can be used to weaken and adjust the input signal so that the microphone input is not overdriven. This is necessary because the microphone input can only cope with a level of 20mV whereas a lot of sound cards can deliver a level of 1-2V.

The PTT circuit consists of an opto-coupler of the type CNY-17-IV or its equivalent and a few other additional components. The operating voltage for the opto-coupler is taken directly from the computer through the serial port. Herewith the programmes can automatically switch the transceiver on air, this does not have to be done manually anymore.

A circuit diagram and description of the components can be found further below.

Connection

First of all you have to unscrew the top of the casing. You do this by removing the four screws on the top. Please take care when putting the casing back together again: the threads are cut out of soft aluminium and the thread could be destroyed if too much force is used. Place the interface in front of you in such a fashion that the connector sockets are on the left side. The set up is the same as the set up on the circuit diagram, i.e. the upper NF transformer (small black block) is for the signals that come from the transceiver to the computer; the bottom transformer is for the signals that go from the sound card to the microphone input. Below that you will see the opto-coupler (small black beetle) for the PTT.

Two jacks have to be soldered on to the delivered cable: one microphone jack and one loudspeaker jack. The jacks and configuration differ with each transceiver, so it is advisable to check the description of the equipment and obtain the correct jacks. Then the following signals have to be connected:

- Loudspeaker output
- Loudspeaker earth
- Microphone input
- Microphone earth
- PTT
- PTT earth

Some transceivers have so called AFSK or package jacks. If you decide to use this type of connection you will probably have to replace the 10 Kilo-ohm resistor which is located in front of the lower transmitter with a link. Otherwise this input could be understeered which means that you would have no output signal. Please check in your equipment manual which input level this input requires.

It would be best if you note down which colour belongs to which signal, the cable has eight wires. Always use one wiring couple of the same colour, e.g. green and green/white, for a specific signal, this makes it easier for finding any faults. Solder the compatible signals and drill a hole in the casing, either in front or on the back – whichever you prefer. Do not connect the earth wires of the cable with the casing as this causes a ground loop.

Please find below the wiring diagram:

Sound Card Line In   Sound Card Line Out Serial Port Com1 oder Com2

Speaker or Mic Speaker Ground Microfone Microfone Ground PTT PTT Ground