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.

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:

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.

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.