ALLOWS OPERATION OF PSK31, RTTY, SSTV & MANY MORE DIGITAL MODES
 
Since I am required to go on job training trips a couple of times a year, I wanted a portable souncard interface to use with my Icom 703. I'm not real big into the digital modes, but operating from a motel room leaves a lot to be desired using minimal antennas, so I figured that since PSK31 does so well with low power it would be nice to be able to operate the digi modes while on the road. I wanted an interface that was small enough to pack into the same bag with the 703, so I built this interface into an Altoids tin for compactness. It has a dedicated cable that allows it to be plugged direct into the accessory jack on the back of the 703, and will also do the same on my Icom 706MkII. I have used it to operate PSK31, and some RTTY from a motel room with fair success running 5 watts into a mag mounted MP-1 antenna on my car parked outside my room door. The interface will work with any of the soundcard programs I have been able to find so far, and these include PSK31, RTTY, SSTV, Hellschreiber, CW (RX only), MFSK16, and many more.
 


Below is the schematic of the interface, and as you can see it is a fairly simple circuit. I've listed the part numbers for the audio transformers from Radio Shack, but I used a couple of 1:1 transformers salvaged from old external modems that I had on hand and they work just fine. I just like to use parts I have if they will work ! The circuit board I used is just a scrap piece of copper clad board I had on hand and cut to fit inside the Altoids tin. I used a Dremel tool to cut the pads to solder the various parts to and then mounted the board in the tin with double sided mounting tape after the interface circuit was complete. I used regular RCA jacks and shielded cables from the interface to my Dell Latitude laptop computer, and a shielded multiconductor cable going to the 703's accessory jack. The interface includes 2 small gain adjustment pots on the board that allows you to set the transmit output power, and the receive audio gain. The PTT circuit uses a simple transistor switch using a standard 2N2222 npn transistor. Some folks will say that using this circuit eliminates the ground loop isolation, but I have used this circuit with my Icom 706MkII at 100 watts on RTTY with no problems, and it is a similar PTT circuit used on some of the old serial interfaces used a few years back such as the old Hamcom interface. I have never experienced any problems with this circuit with up to 100 watts, your mileage may vary ! All in all, it's a very effective little interface that allows operation on the soundcard digital modes without spending a bunch of money (I've probably got $2 invested in mine) on and interface. And, you can tell the other guy, "interface here is a homebrew" !
 

 

73 !