How I modified the Yaesu FT-747 to incorporate an internal USB soundcard to allow it’s use on digital modes. The modification has a minor downside, this being that the microphone is open while transmitting signals via the USB soundcard. The only way around this problem without building an audio switching circuit is to remove the microphone from the radio.
I have detailed another method of allowing the FT-747 to used with digimodes using an external interface on this page.
The USB Soundcard Dongle
This little mod started off as an experiment after I found one of those cheap USB sound card dongles laying around. I originally bought this dingle many years ago when I needed a quick and cheap method of getting audio to/from my PC. Obviously, this cheap Chinese piece of hardware isn’t hi-fi quality. For it’s size and price, the quality is plenty good enough for digimodes.
What parts are needed to perform this mod?
First we need a USB soundcard dongle, these can be found on Amazon or eBay very cheaply. The only requirements are that it has audio out (speaker) and audio in (mic) sockets.
You will also require either a standard USB cable, an old USB mouse or keyboard cable or a USB extension cable. Also needed are two 3.5mm jack plugs, a short length of screened audio cable, a 50K mini preset, a 1K resistor and finally a 600 ohm 1:1 audio transformer.
A soldering iron and a few basic tools are required to carry out the work.
Modifying the dongle
If using a USB extension cable this next part can be omitted as you can simply plug the dongle into the extension cable.
The first step is to remove the case from the dongle. Normally the two halves of the casing can be prised apart with a small screwdriver. Don’t worry about breaking the casing, this will be discarded anyway.
Once removed from the case make a note of which way round the USB plug is attached to the PCB. In the dongle used here, the white plastic contact carrier is on the bottom, thus pin 1 is nearest the bottom of the picture. The standard USB can be found here. Now carefully remove the USB connector from the PCB.
Take your USB/mouse/keyboard cable and cut off the end that’s not required. Prepare the end of the cable and solder to the dongle PCB. Most USB cables use standard wiring colours which are Red (pin 1 +v), white (pin 2 data -v), green (pin 3data +v) and black (pin 4 ground). It would be wise to just do a quick check with a multimeter to ensure the format has been followed.
The remaining text applies regardless of which method of connecting the USB cable used.
Dongle to radio wiring
Solder a piece of the screened cable to each of the 3.5mm jack plugs. If using stereo plugs, connect the tip and the body to the wire, the ring can be left unconnected. Make sure you have enough cable to be able to route around the inside of the radio.
Assemble the 50K preset, 1K resistor and the isolation transformer as shown in the schematic. This assembly may be mounted on a small piece of stripboard or similar, alternatively, it may be possible to solder the preset directly on the transformer depending on the components used.
Here can be seen the tiny circuit covered in heat shrink. Connect the input of this circuit to the dongle audio out socket using one of the 3.5mm jack plugs. The output is connected to the microphone input on the rear of the microphone socket.
Before fitting heatshrink or whatever form of insulation you have decided to use, set the preset to approx. mid-range. The preset is used as a voltage divider so as not to overload the microphone input circuit. You will need to test the radio in transmit mode and make any minor adjustment to the preset if required.