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A short pictorial demonstrating modifications to the Sanwu SW-HF07 USB sound adapter for use with VKLink nodes.
A search on eBay finds what appears to be a suitable CM108 based USB sound adapter. The eBay pictures suggest the chip is genuine, but theres only one way to find out. I ordered 3 and they arrived fairly quickly from Hong Kong, and all for $17AU. Cheap.
Visual inspection checks out OK. Chip is marked CM108 - bit hard to see in the photo. Good start.
lsusb identifies it as 0d8c:000c
Which looks up as
At this point its safe to say the adaptor runs a genuine chip.
Plugging the adaptor into working VKLink Pi produces the expected results.
Let the mods begin. Firstly to provide strain relief to the soldered joint to the IC pins, take some wrap wire and feed under then pins of the chip to solder to pins 13 and 48, thusly. Use caution to not bridge the adjacent pins on the IC with solder. If in doubt, check with multimeter.
That done, lift the capacitor nearest the edge of the board, nearest the green socket. This breaks the path between the ring contact and the rest of the circuit. We use the ring contact to provide PTT signalling. Install a 1N4148 or similar diode to the ring contact of the pink socket.
Next, break the track between tip and ring contacts on the pink socket. This allows the use of the ring contact for COS input from the receiver. RX audio is brought in through the tip contact.
Use a BC549 or similar and a base resistor and attach suchlike. I used 4k7ohm. This is the PTT output. Collector to ring contact and emitter to ground contact. In this photo the COS input wire has been attached to the diode. This can wait until later if it makes things easier.
Make the connection of the wire to pin 13 and the base resistor.
Thats it, the modification is complete.
Its the readers responsibility to provide correct interfacing to the nodes transceiver. The COS input logic direction can be defined in software, but the input is expecting a pull to ground. In my particular case I needed to install a pull up to 5V resistor to the ring contact to prevent the input floating. The software would not detect change of state on the input with received signal. Level adjustment can also be done in software, but I prefer to run RX level at 0 and trim the audio level externally. I run the TX audio level at 500 in the tune menu, and trim the level into the transmitter. YMMV.
It would seem there are some variations between boards of the same manufacture and model. Where the track gets cut between tip and ring contacts on the pink socket on the outboard side as described above, some boards have a track between contacts on the inboard side of the socket also. Two of the three boards that I ordered had this feature, but the original board I modified did not. Simply take a sharpened small flat blade screw driver or similar and break the track so that tip and ring are disconnected on both sides.