Icom IC-F6011 audio input

On a recent gig I was introduced to the Riedel Riface. It’s a custom rack-mount chassis for two Motorola GM360 radios, so they can interface with an audio console, intercom system, or even themselves to use as a repeater.

One of the other guys in the company figured he could build one for cheaper than the commercial version. It uses two Icom IC-F6011 base units. Getting audio out of the Icoms is easy, there’s a 3.5mm stereo jack on the back.

Getting audio into the unit is more of challenge.

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The only audio input is via the RJ45 jack on the front. The pinout is as follows:

  • Audio +: Pin 6
  • Audio -: Pin 5
  • Tx contact +: Pin 4
  • Tx contact -: Pin 7

The input is unbalanced. If you are running a balanced signal into it, leave ground floating. I had some bad hum with ground tied to negative. It went away when I cut the ground wire at the radio end.

If you send too much audio into the input, it overloads and cuts out. I found a -40 dB pad is enough to send a normal line-level signal into the radio. The circuit I used follows:

Icom-XLR-to-RJ45-pad

The final problem is that if you keep the Tx switch closed on power-on, the radio won’t transmit. To get around this, you can unplug the RJ45 connector when you turn it on. For unattended operation, this isn’t ideal. The alternative is to use a 555 timer circuit to delay the connection of the transmit contact.

555 timer icom

I ended up using a 1 µf electrolytic capacitor and a 4.7M ohm resistor at 12 volts for a delay time of about 6.5 seconds.

Interestingly, the transmit contact isn’t a dry contact. There’s about 5 volts across it. This means it’s easy to use an NPN transistor (I used a 2N4400, not a 2N4416,) turned on by the 555 timer, to connect Tx + to ground. The 555 and the Tx – shares a ground.

I was able to fit all of this into a very small box from AllElectronics.

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About Edward Carlson

Film and television Director of Photography working in Los Angeles, CA.
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