Modifying Motorola MITREK Radios for Repeater Operation
By Doug Spreng W7MCF

The Motorola Mitrek radio is very easily modified for use as a repeater in the 144 to 148 MHz and 420 to 450 MHz bands. Consult the model charts in your manual to determine which models are suitable for conversion. This particular example is a 406 to 420 radio, (HUE 1073C), converted to link operation in the 420 band. This example holds true for all examples of conversions. Be sure to use the appropriate channel element since they are they are not all the same for the different bands. I send the channel elements to International Crystal to be re-crystalled since they calibrate the temperature tracking as well as the frequency.

Antenna Relay
The antenna relay is disabled to allow the front RF connector to be the receiver port continuously. Cut the wires leading from the board to the relay. Cut the coax, coming from the TX low-pass filter, at the relay and fold it back toward the PA cavity. This coax will connect to a coax connector installed at the left edge of the PA cavity.

Transmitter Output Connector
The transmitter output connector may be a SO-239, type "N" or BNC. I mount mine on the sidewall of the PA cavity. It is centered fore and aft and mounted as close as possible to the top of the sidewall. Make sure it is not mounted so high that it interferes with the fit of the cover. A bulkhead mount BNC would be ideal here since it is smaller than the SO-239 or "N". I have used both "N" and SO-239 successfully. The coax that was cut loose from the antenna relay is looped back and soldered to the connector.

Channel Element selection
The channel elements are hard wired on. You may use any of the four channel element sockets. I usually hard wire number 1. On the receiver side jumper pin 1 to pin 4 of the socket. On the transmitter side jumper pin 1 to pin 5 of the socket. This allows the frequency select wires to be used for other purposes.


Receiver and audio PA mods
The receiver and audio PA must run all the time in order for the radio to operate in full duplex. Cut diode CR1 to keep RX switched 9 volts on and disable speaker audio mute during transmit. Cut diode CR403 to disable speaker mute by secondary PTT.


"Carrier Operated Switch", (COS), output connection
Connect a jumper betwen test point "E" and test point "F2" to bring COS out the white wire. The F1, F2, F3 and F4 test points are located at the front of the main board between, and slightly in front of, the receiver and transmitter metering plugs. F1 through F4 are normally used for frequency selection. Since we have hard-wired the channel element on, these wires are available for other control functions. Test point "E" is located in front of U403, between CR404 and L401.


If the radio is to be used with a Link-Comm Controller a buffer will be required between the COS output and the controller input due to the heavy loading imposed by the controller input circuitry.

I use the simple circuit shown above. Power for this amplifier is supplied through the controller input which is internally pulled up to B+ with a resistor. You will also need to connect pots to control squelch and volume if the COS is used as input to the Link-Comm controller. Use circuit shown below. You may connect a speaker to the orange and green wires and monitor the activity. The squelch control will set the level at which the repeater will key up. If you are using a Palomar Controller or any other controller with built-in squelch controls you do not need this circuit.

Internally Mounted PL Detectors
A PL detector module such as the Comspec TS-64DS may be mounted inside the Mitrek radio. The input would go to the detected audio output, pin 1 of C403A. The detected PL output logic level would go to an unused frequency select lead test point (i.e. F1, which is the violet wire in the cable).

Transmit Audio injection
To connect transmit audio from controller to transmitter, jumper "JU15", (labelled "L18" on some radios), to pin 15 of the main board strip connector. This will connect TX audio to the PL/DPL input of the TX modulator via the black/grey, (special use), wire. This bypasses the speech amplifier and the equalization network which results in flat audio characteristics.

RLC-2 Cable wiring
Mitrek Cable wire color Function RLC2 Pin #
Black/Brown Receive Audio Pin 5
Black/Grey Transmit Audio Pin 4
Black/Orange PTT Pin 3
White COS Pin 7
Violet Internal PL Pin 2
Large Black, Blue Ground Pin 8,9

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Text copyright © Doug Spreng W7MCF 2000, 2004