Wednesday, July 4, 2012

Soundcard Interface for APRS/Packet

Happy 4th of July to all of my American visitors (and also to everyone else, although I don't imagine that it is quite the same).  Not that I have that many visitors from anywhere, but it's the thought that counts.  It's been a few days since I posted anything, so I wanted to get this up on the site.  I've been enjoying my SignaLink USB soundcard interface for digital modes on HF, but I would like to run APRS on the same PC, and running two SignaLinks with APRS can be problematic.  Besides that, I didn't want to spend another $70 to $100.  So I decided to build my own interface.  There is a great resource for this at:  They walk you through the build options and troubleshooting if needed.  That being said, I've borrowed their schematics to post here (full credit for the schematics goes to them).

For this interface, you will need three connections, one for receive, one for transmit and one to activate the PTT switch on your radio to allow the transmission.  I am 2/3 of the way there.  I have the send/receive portions complete, and am waiting on the parts for the PTT circuit (it requires an opto-isolator, which I couldn't get from Radio Shack).

I tested the receive and transmit using DM780, which is part of the Ham Radio Deluxe suite, and was able have a QSO via PSK31 on 14.070MHz (manually keying the mic, which is annoying).  That being said, it seems to work well, and hopefully the parts will be in soon for the PTT portion.

Here is the schematic for the receive cable.  The radio that I am using (old Kenwood TR-7730) does not support audio out via the MIC at line-level, so I opted for the 1000:8 transformer to allow me to use the speaker out jack on the back of the radio:

And here is the schematic for the transmit cable:

These are fairly basic circuits and do not require a lot of time or effort to build out.

My Parts List:

  • 1 x 1:1 audio transformer (transmit cable - RS Part #274-1374 - these are getting scarce)
  • 1 x 1000:8 audio transformer (receive cable - RS Part #274-1380)
  • 1 x 100K Resistor
  • 1 x 1K Resistor
  • 2 x mono audio jack
  • 2 x stereo audio jack
Time to Completion:
  • This one took a few hours to put together, but quite a bit of time finding the right transformer.
Here are some pics:
(I nabbed the TR-7730 pic off of the net, but it looks identical to mine)

I decided to build this out as a stand-alone box with audio jacks instead of as a set of cables for neatness.  I'm an engineer by trade, and am rather stuffy about how wiring looks.  Stay tuned for the next installment in this post when I get the parts in the complete the PTT circuitry. 


Richard, KK4JDO


  1. I would like to know if I can use this article you wrote in my local club newsletter (Shortskip Please email me back to confirm (My callsign below at
    Thanks, Benjamin Straw - KC9UNS

  2. Hi Benjamin,

    I replied via email, but I also wanted to respond publicly as well. You are more than welcome to reprint this in your newsletter, all that I ask is that you credit also, since they were the basis for this project.