[ICOM] Running an IC-725 and R75 together

[Gary Fiber]

Chris Mangus wrote:
> Has anyone ever run an IC-725 and R75 together, using CI-V and a T/R 
> switch?
>
> I'd like to take advantage of the better receiver...
>
> Thanks,
> Chris / N7WBZ
>
Should not be too hard.
If you want the R-75 to follow the tuning of the IC-725 or vice 
versa...operate like a transceiver then:

Connect the CI-V ports together, a mono jumper will do for this; no 
CT-17 required.
Make sure the Transceive function is enabled in both radios...This is 
generally the Icom default anyway.
As you turn the main dial and change mode on the IC-725, The R-75 should 
set itself to the exact same frequency and mode.
As you turn the main dial and change mode on the R-75, the IC-725  
should set itself to the exact same frequency and mode.
It will make no difference which dial or mode setting buttons you use as 
the other radio will follow. You can try this out without doing any 
fancy antenna stuff to make sure the basic tuning and mode change works. 
Just as you might do already turn down the IC-725 volume control.

If you want them to operate as separates, then don't connect the CI-V 
but do the other below.

The R-75 has a MUTE input. You need to ground this input when 
transmitting with the IC-725.
If you are not running an amp you can try connecting the amplifier 
keying output on the IC-725 to the mute on the R-75. They both happen to 
be RCA connectors.
Or you could use an external relay set up to short the mute input of the 
R-75 if you are already utilizing the amp keying out on the IC-725.

Even though the mute will desense the R-75 I would still open the 
receiver antenna connection to the R-75 to protect the front end.
In so doing that you may want to construct an external relay to open the 
antenna to the R-75 when transmitting.

A 12 volt DC, DPDT relay will do. If also switch an amp you can use 
something like a 4PDT relay and use the extra contacts for switching the 
amp etc.
Try to limit the current a bit, I would say use one that has a coil 
resistance of around 140 ohms or higher. That will limit the current to 
around 100 milliamps or less.
Place the actual antenna center across one center contact, the receiver 
antenna to the normally closed contact so it is NOT connected when you 
are not transmitting. This contact will open when you transmit. Connect 
the IC-725 antenna to the normally open contact on that same side of the 
relay so when you transmit the antenna is connected. I think the relay 
will switch fast enough so it will not hurt anything. Putting a snubber 
diode across the coil will make the relay release faster not switch 
closed any faster so far as I know..BUT will reduce and transient 
voltages generated by the relay coil so its not a bad idea to utilize one.

It will be ok to use a metal box and coaxial chassis connectors using 
the box for the ground side of the coax.

Connect the receiver mute across the normally open contacts on the other 
side of the relay  so they will close and mute the receiver when 
transmitting.

The coil of the relay can be connected from Pin 7 to 3 on either ACC 1 
or 2.. What will happen is every time you close the CW key, or the PTT, 
Pin 3 goes low, thus current will flow from Pin 7 through the relay coil 
closing the relay. Now the receiver antenna should disconnect and the 
receiver will be muted until you release the CW key  or PTT .

Hope this helps and Have fun with the " Twins "

Gary K8IZ