Monday, 4 August 2014

FT-857D Digital Interface Mk2 (Rebuild)

Yesterdays successful test of the digital interface for my FT-857D did unfortunately leave me with a rather convoluted collection of bits of circuits all cobbled together. Todays task then was to build the Mk2 interface - in other words, get it all on the one, neat and tidy board, ready to be boxed.

Last night I drew out the schematic, planned a Veroboard layout, and selected a suitable hunk of board. The only minor change was to include a level control potentiometer on the receive path. Not really needed for WSPR, but perhaps useful for other modes.

The picture above is how it all looked at close of play yesterday, not including the opto-isolator. The board to the lower right is the single transistor audio amplifier, that drives the audio detector and PTT switch (right hand side of main board). The modified el-cheapo Chinese USB soundcard hanging off the whole lot to the left.

This is the new board. As an odd off-cut the sizes actually turned out to be pretty good for the job, with the longer, lower section fitting the PTT system nicely. Here the audio isolation transformers, DC switch and opto-isolator are installed, along with the radio side ground connections. 

Further on, the Tx 10:1 audio divider, computer side grounds, and the audio rectifier diode doubler are installed. What can't be seen, is the continuous run of track breaks under the transformers and opto, forming a complete separation of the two signal sides.

And, finally, the completed board, in use. The USB sound card is affixed onto the board with hot melt glue. This is during final testing. Im pleased to say all the testing went well, with good receive spots from around Europe, and some decent DX 'heard by' spots into Isreal and the USA, all at about mid-day here in the UK on 20m.

The schematic is shown below. There is nothing new, or original in this design, and it borrows much from KK7UG's interface. The audio activated PTT circuit is a mish-mash of several designs found around and about the internet.

With a couple of simple mods for serial port controlled PTT etc, this could easily be made into a 'universal' digital modes interface.

The final two tasks are to replace the sound-cards USB plug with a wired USB connector, and to put the whole thing into a case.

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