Motorola Mitrek repeater conversion by KF8KK

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Mitrek Conversion
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Converting the Motorola Mitrek FM transceiver for use as a ham radio repeater isn't brain surgery nor rocket science.

It shouldn't come as a surprise that there are many different ways that people go about making the conversion.   Most conversions have certain merits, and some are better than others. 

These pages detailing my conversions are placed here more to jog my memory of what I've done when it comes to preparing another Mitrek for repeater use in the future as opposed to any attempt to advance the state of the art.   Surely there are flaws in the methods used in the conversion, as I am human just like the rest of us.   This conversion works for me and it is an evolving process.

If you attempt to follow these 'pointers' please beware that you are on your own and I assume no liability for what might happen.  The photos show actual Mitreks that are in use at the time this page was written (summer 2004).


This is a Mitrek modified for the external input for the receiver.   

The receiver connects to the BNC connector coming out through a hole in the side.  The transmit signal still comes out the UHF connector on the front of the unit.   

This is a 30-50w Mitrek which can be easily seen by the shorter length of the unit.  The 75-100 watt units are a bit longer and the heat sink fins are longer.

The inside of the Mitrek shown above has the PL decoder board on the left (mounted from the top).

Between the shiny chrome PA cover and the PL board is the helical resonator section.  The six resonators on the left are for the receiver, while the four on the right are a filter for the transmitter.  

You can tell which band Mitrek you have by taking a quick look at the helical resonator section.  This Mitrek is a UHF model (good for 440mHz ham use).


There are two 'band splits' of VHF-Low Mitrek.   

One covers the 42-50mhz segment and can be easily converted to the six meter ham band (it'll tune there without any changes, but there's a few capacitors to change for optimum receive).

The 30-40mhz split Mitrek can be converted for use on the 10 meter ham band.   Be careful to get the band-split that you desire.  

This Photo shows a VHF-Low Mitrek.

In the space where the helical resonators are on a UHF model, the Low-VHF units have typical coils inside shields.   The top seven coils in this unit are part of the receiver 'front end'.   The lower four (with the bright blue and red tops) are part of the 'extender' option. 

On VHF-Low you will find that man made noise is a big problem.  The 'extender' is Motorola's name for a noise blanker.   The Mitrek noise blanker ('extender') is excellent and renowned.  If you're running a rig like an FT100D or IC706 you will notice quite an improvement on switching to a Mitrek with the extender.   The extender is an option-- and on many (mostly the lower powered versions) VHF-Low Mitreks you will find that this section of the transceiver has a circuit board without the parts mounted on it.

This particular unit has had the power amplifier removed and a PL encoder put in the space.  I have modified this to feed the 1w output of the 'exciter' to a Micor external power amplifier.



Here is a VHF-Hi Mitrek.  

The helical resonator section is quite different and has a rectangular hole in the middle where an optional preamp can be inserted.

Also in this view is an old Heathkit RF probe.  You can see the DVM the probe is plugged into on the left.  This is an inexpensive and invaluable tool when it comes to bringing the transmitters of some of these down to the ham bands from where they were in the commercial service.   Of course, a spectrum analyzer is preferred, but there's a big difference in the price-- and this is quite adequate for the hobbyist.

This photo shows the Mitrek on the bench with a special cable connected which I have made that allows me to easily get to the connector pins to verify that my connections internally to the Mitrek are working.     If you plan to convert a few of these it's worth the time to get a test cable made up.

You can also see the Mitrek control head on the left under the bench.  I've also got a full Mitrek cable set which I use to verify that the units function BEFORE I start the modifications.

It's incredibly important to check to see if the radio worked before tearing into it.   Failing to test the radio first can cause you hours of anguish later on if it's not functioning.

Thankfully, the Mitrek control cable connects via a connector that's the same used with old Motrac and Mocom70 transceivers and is relatively common and cheap (used). 

The first modification I perform is to take the receiver RF line from the T/R relay and cut it at the relay.   Make sure you cut the cable to the receiver.

I then drill a hole on the left side of the Mitrek and pass the receiver RF input through the hole and put a BNC connector on it.

Photos of this will be forthcoming.


The second modification I perform is to jumper pin-1 and pin-25 on the connector that connects the 'interface board' at the bottom front of the unit.

This jumper provides for the 'normal' operation of the PTT line.   

Normal meaning that the PTT is brought to ground when you want the transmitter to transmit.

Ground pin-13 on the connector and it transmits.

In order to operate full duplex (receiving while transmitting) you need to remove diode CR1 on the transmitter and CR403 on the receiver.

Photos of these will be forthcoming.


The photo above shows the jumpers which allow you to isolate the pins on the main connector from the internal circuits of the Mitrek.  These are handy for changing the intended use of the connectors pins.

This view shows the main connector on the upper left (the black piece).

On the 'interface board' (the vertical board that the main connector is soldered to) there are jumpers for each of the pins.  Jumper 'L-9' connects in series with pin 9 on the connector.  Removing these jumpers allows you to reassign the connector pins without worrying about what was connected to the connector previously.  

Jumper J-11 ('A') is where the discriminator output is found.  This is where I usually take my received audio from (pin 11).   If I need to have the received audio muted (when no signal is present) before it leaves the Mitrek I wire in a small relay across where J-11 used to be.  'D' in this photo shows such a relay.

'C' is a squelch pot wired in on this Mitrek.  One side of the squelch pot gets connected to J-11 ('A') and the other side to ground.   The center pin of the squelch pot goes to pin14 (or J-14).  If you were to get your received audio from the pin-11 discriminator output there is no need to wire in a volume pot.

FYI--  while the Mitrek schematic calls for 25k pots, I've used everything from 10k to 100k pots with fine results.  I think Motorola chose 25k because they got a good price on a huge quantity of them.  Unless you are a purist (stop reading and go away if you are) there's no reason to sweat a 25k pot.

In this photo you see J-11 next to 'A' with a red wire that goes to the squelch pot.   At 'B' is a 0.001mfd bypass capacitor to shunt any RF on the audio lead to ground.

The white wire just to the right of 'C' is the squelch connection from the center of the squelch pot.

FYI--- the diode shown at 'C' and the '5.7k pull up resistor' at 'D' (between the diode and the +12v terminal is part of the COS output.  The grey wire is the COS line from the switching transistor described later on this page.

This image shows a nice 10turn 10k pot which is used here for the squelch. 

I simply found a convenient ground spot and tacked one side to ground.   The red wire goes to the discriminator output on pin-11 and the white wire goes to the 'squelch input' on pin 14.


On the receiver side of the Mitrek you can see the spot that I use to get the 'carrier present' signal from the squelch circuit in the Mitrek.  

The green 'A' points to what is known as 'point E' on the schematic.  In this photo there is a pin at the spot for attachment of a slide on connector.  Not very many Mitreks have the pin, most just have a larger soldered hole in the board (as seen in the next shot).

At 'B' you can see a common NPN transistor (I use 2n2222 because I have plenty of them-- no other reason).

Connect the emitter to ground, a resistor between the base and that 'point-E', and then take the collector to your output pin (I use pin-10) and your controller.   If your controller is like mine, you might want to use a pull-up resistor (5.7k works for me) to the +12v line.   I add a diode for safety and to allow me the option of piggybacking other receivers on my COS line to the controller.

With this setup the COS line goes HI (+12v) when there is a signal present on the received frequency.

Here is another shot of the 2n2222 COS transistor.  You can see the 5.7k resistor connecting the transistor base to 'point-E' (denoted by the green 'A').

The green 'C' shows where I picked off the ground for the emitter.   The green 'B' shows the body of the generic NPN transistor.   You can see the grey wire from the collector going to the output connector area (shown as 'D' in the photo just below).

When you wire in this transistor, try to keep it fairly low to the board so that installing a PL unit does not cause problems.

I choose 5.7k because I have plenty of that value.  In practice anything from 2.2k up to 10k is likely to work.

In this photo you can see the protection diode (optional) at 'C' and the 5.7k pull-up resistor at 'D'.

With the squelch pot installed, and the npn switching transistor wired in, the COS for the receiver is finished.

I have wired the COS this way to feed NHRC, CAT, MCC RC1000/RC100, Arcom, and various other homebrew controllers and never had a problem.

For those wishing to use PL receive, please see the next page.


Mitrek Conversion page 2