Raspberry Pi to iCom IC-F121S interface

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K6XTC
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Posts: 14
Joined: Sat Oct 26, 2019 12:09 am

Raspberry Pi to iCom IC-F121S interface

Post by K6XTC » Sat Nov 02, 2019 9:51 pm

I created this to interface a Raspberry Pi (or similar board) to an Icom IC-F121S 2 meter commercial transciever for APRS.

Audio out is coming from Pin 3 on the RJ-45 MIC jack. Normally it is not enabled and requires you
short a solder pad on the main board.

Pin 3 connects between the Audio pre-amp and the main amp. Documentation online says that it bypasses the volume control but I have found that not to be the case in reality. I've had to crank the volume all the way up to get decent volume to the sound card. This will make the main speaker blast so you'll need to insert a unwired plug into the external speaker jack, or disconnect the speaker internally, to disable it.

I supposed you could also leave the jumper open and run a wire from the speaker to Pin 3 directly as the schematics show no other components between the pin and the jumper pad. Or run the wire to the external speaker jack and just not use Pin3 (I wanted to keep it to a single plug).

AFO Jumper Location.jpg
iCom F121S MIC pin3 Schematic.jpg
iCom APRS interface prototype.jpg

icom aprs-control_schem.pdf
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K6XTC
Site Admin
Posts: 14
Joined: Sat Oct 26, 2019 12:09 am

Re: Raspberry Pi to iCom IC-F121S interface

Post by K6XTC » Sun Feb 02, 2020 6:53 pm

I just discovered that my APRS receiver had gone deaf. Possibly due to having the volume cranked to the max all the time, possibly from something else.

I have a couple spare radios so on this one I soldered a jumper between the AFO pad and the speaker out (See attached image).

It doesn't take much volume before Direwolf starts complaining the audio level is too high so I'll have to reduce the mic volume in software to compensate. Maybe, once I get caught up with a few more projects, I'll look at adding a volume limiting circuit.

You will need to disconnect the internal speaker or stick an unwired plug in the speaker out to disable it.


iCom 121S audio jumper.jpg
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