Mitrek Mods for Full-Duplex Repeater or Link Service

by George Zafiropoulos KJ6VU

Rack mounted Motorola Mitrek for link service.

The Motorola Mitrek is a great mobile radio that was never designed to be a
full-duplex radio.  You can, however, make a few simple modifications to
get the radio to operate full duplex.  My club, The Sierra Radio
Association (, has several 30 watt
406/420 Mhz government Mitrek radios converted to the 420Mhz ham band used
as full-duplex link radios.  We typically run the radios at 1/2 to 2/3
rated power.  So, a 30 watt transmitter is set to put out 15 watts.  The Rx
is quite good and the radios have given us years of reliable service. After
working on many radios, we have found that at least 3 out of 4 radios will
duplex just fine.  But, in a bout 1 in 4 radios, the transmitter will
desense the receiver.  We run a very tight split of less than 5 Mhz between
the Rx and Tx frequency.

Keep in mind the following thoughts:

1) The transmitter was not designed to be a 100% duty cycle transmitter.
Make sure you keep the output power at about 1/2 rated output.

2) Keep the heatsink cool.  A fan and good air flow is a good idea.

3) If you are at a commercial repeater site, make sure you check the radio
with a spectrum analyzer so the transmitter is clean and use a good quality
commercial pass cavity like a Motorola 1500 series duplexer.


____  On the transmitter side of the board:  Remove diode CR1.

____  On the receiver side of the board:  Remove diode CR403.

You may want to snip one side of the diode and bend it up just in case you
ever want to return the radio to mobile service.

____  Flip the radio upside down.  Add a wire jumper between the solder
pads of pins 1 and 25 of the interconnect board connector P10.  This sets
the PTT to be active when grounded.  These pads are easy to find.  They are
a row of 25 evenly spaced solder pads along the edge of the circuit board.
Pins 1 and 25 are the two end pads.

____  Activate Rx channel element 1.  Jumper JU 611.

____  Activate Tx channel element 1.  Ground pin 5 of the first Tx channel


____  Add an external Tx RF output connector.

I drill a hole on the side of the radio and mount a BNC connector just
above the RX board.
Lazy man's method:
Locate the coax going from the Tx out to the RF connector / TR relay
already installed in the radio.  Snip the coax at the back of the coax
relay.  You want as long a length of Tx coax as possible from the Tx output
to your new Tx output connector on the side of the radio.

____  Add an external Rx RF input connector.

Lazy man's method:  Use the RF connector / TR relay assembly already
mounted in the radio.  Make sure you snip the black and red wires going
from the main radio PCB to the coil of the TR relay so it will not be
energized by the PTT.

Professional method:  Remove the RF connector / TR relay assembly and throw
it away.  Mount a new connector in place of the old one.  Of course, it
must be round and long enough to fit the existing hole.
Make sure you use either BNC or N connectors at UHF.  SMA connectors are
nice but fragile and UHF (SO-239) are not really good at UHF despite their

Coax from the external RF connectors to the Rx input and Tx output should
be either small diameter rigid cable or double shielded coax.  I prefer the
coax because it is easier to route some people prefer the rigid because it
has better isolation in high RF / desense prone radios.  For lower powered
radios, like the 30w model, use samll diameter cable for both the Tx and
Rx.  This makes the installation much easier.

If you install your own Rx coax...
Installing your own Rx cable to the preselector (Rx input):

____  Remove bottom cover

____  Locate where Rx coax connects to the preselector on the right side
next to the A+ connection of the PA.
____  Pry up the small metal cover.

____  Unsolder the connection on the left.

____  Remove the two philips screws holding the bracket.

____  Unsolder and remove the eyelet and save it for your new coax.

____  Install you new coax.

The control head cable connector on the front provides all necessary
signals to in and out of the radio.  I get a fresh new cable connector and
wire all signals through the new connector to my control system.  You can
order part #9-8016C01 from Motorola.

Connector pin configuration

|                                           |
|   8    7    6    5    4    3    2    1    |
|                                           |
|     19        18      (hole)      17      |
|                                           |
|  16   15   14   13   12   11   10    9    |
|                                           |

1       Tx Audio In
2       Tx Audio Gnd
3       Speaker high out
4       Jumper to pin 19
5       Speaker low out
6       Jumper to pin 17
7       No connection
8       No connection
9       No connection
10      No connection
11      Rx descriminator audio out
12      No connection
13      PTT in - Ground to Tx
14      Squelch pot wiper
15      Volume pot wiper
16      No connection
17      Power ground
18      No connection
19      +12v DC power

Local control options - if you want to have your repeater / link radio
operate like a duplex mobile...

____  Add a volume and squelch pot per the diagram.

____  Add a speaker accross pins 3 & 5.
Be very careful not to ground the speaker lead.  The speaker leads float
above ground and if you ground the lo side you will blow out the audio

____  Add a 47 Ohm 2 watt resistor accross the speaker to provide a load to
the audio output circuit.

I/O connector
Pin 11   ----------------\
                 |       |
                 |       |
                 R1      R2
                 |       |
Volume   (wiper) |       |   (wiper)    Squelch
Pin 15 -------->Pot1    Pot2<---------- Pin 14
                 |       |
R1   3.3k
R2   10k
Pot1 25k
Pot2 25k

Now you have a nice full-duplex Motorola Mitrek radio.  Good luck and enjoy
the radio.

'73 de George Zafiropoulos - KJ6VU