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Interfacing a Icom IC-28, 38 or 48 mobile for remote base service
By Lee N3APP
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From an email from Lee, N3APP to Repeater-Builder:

These mods provide connections from an Icom mobile to a repeater controller.   From
the controller to the radio you have PTT and TX audio, and in the other fdirection you 
have COS and RX audio.   I had no need for PL decode as my radios did not have that 
option.    Note that these radios are mobiles, complete with the duty cycle limitations 
inherent in a mobile design, and are only useful as a REMOTE BASE, not as a repeater, 
they will NOT do continuous duty!

1. Remove the cover where the speaker is located.

2. With the radio facing you, find the memory battery.

To the right of the battery are two inline plugs, one is a 4 pin and the other a 3 pin.

Note: +12volts is available on the red wire of the 3 pin plug.

3. The COS signal is on the third pin back of the 4 pin plug on a red wire. This pin is 
active high at about +4.5vDC. Use a common NPN transistor with a 4.7k base resistor as an 
isolation stage. This will drive a relay or connect to your controller's COS input but is 
now active low. The simplest thing is to use this signal and program your controller for 
active low. If you need active high,use another transistor as an inverter.

4. Transmit audio and PTT are taken off the rear of the mic jack, use a 10k pot on the 
mic audio.

5. Receiver audio is taken off the speaker with a 10uf capacitor and the volume control 
will affect the level. I have not found an alternative audio tap point that is not 
controlled by the volume control with enough output to drive a controller (see note 

6. Now add a 4" fan to the heatsink as the transmitter RF power amplifier will burn itself 
up if you talk too much. Remember - it's a MOBILE.

Enjoy and happy repeating / remoting...
73 from Lee, N3APP

Note from Mike WA6ILQ
If you don't want to use speaker audio, a simple opamp circuit can buffer the audio and set it to a volt or two p-p of AC to feed to a controller.   There is probably a +8 or +9v audio supply voltage available in the radio (I do not have an Icom service manual available to me, or I'd scan a schematic for this page).

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This page last updated 12-Feb-2005

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