Photo 10: The front panel of a Mitrek mobile.   Visible is the antenna connector, the mounting tray lock, the model/serial number tag and the control connector.   Above the number tag is the spring-loaded top cover release, it looks like a nut driver would be used to turn it, but in reality it's a pushbutton.   The key is the standard Moto mobile key - a Chicago Lock and Key H2135.   For more info on two-way radio keys visit the keys page.   On the particular radio shown in the photo the fold-down handle is missing.

Photo 11: Don't be surprised if some radios have a foam gasket on the control connector.

By the way, you will want to print the jumper table below and stuff it into your Mitrek manual as I've never seen anything listing the interconnect board jumpers in ANY of the Mitrek manuals I've seen, even on Moto's schematic of the interconnect board (the ones I have show no jumpers or chokes, just direct connections).   I put this list of jumpers together as I traced out the interconnect board as I modified my link radios.

   +-----------------------------------------+
   !                                         !
   !  16   15   14   13   12   11   10    9  ! -------------
   !                                         ! The cabling (to the vehicle
   !     19        18       (T)       17     ! battery and the control cable
   !                                         ! to the head) comes out this side
   !   8    7    6    5    4    3    2    1  ! -------------
   !                                         !
   +-----------------------------------------+

If you are going to use a Mitrek that does not have the chokes on the interconnect board in a repeater system consider replacing the jumpers with chokes in the leads that you use as interface connections to the outside world.   Just take common carbon resistors and use them as coil forms, wrap them in 22,24 or 26 gauge enameled wire and solder the wire end to the resistor lead close to the resistor body.   Stretching the wire slightly before you wind it helps it lie flat, and a drop of varnish, hot-melt glue, or even clear fingernail polish on the starting end helps hold it in place while you wind the rest of the coil, then another drop on the tail end holds it in place while you solder the end.   Some clear heat-shrink tubing will finish the job.   As long as the resistor value is over 40-50 ohms the actual value doesn't matter as the choke will short it out.   A free source of enameled wire is the deflection yoke or the high voltage transformer from a old TV set.   Stretch it by clamping one end in a bench vise (or a fence post) and string off 6 to 10 feet and lean back with your whole body weight.   Then wind the choke with the stretched wire.

If you decide to improve the RF filtering of a radio and you intend to do all of the mods listed here you will need to replace the jumpers with chokes at pin 7, 8, 9, 10, and 18.   If you want to go overboard you can also add one more choke on pin 1 (which handles local microphone audio).   The simplest way to add them is to cut the jumper on the connector side of the board, melt the solder and remove the two wire ends, suck the remaining solder out, and install the choke into the same holes but on the the side of the board facing inside the radio.   Note that three of the jumpers (at JU6/L7, JU7/L8 and JU16/L19) are buried under a large coupling capacitor - you will need to lift one end of the cap to remove the jumpers.

PL Encoder control
This mod works on both the reedless and the reed board.   Lift the radio side of L6 on the interconnect board.   The radio side is grounded by JU611 so lifting it is necessary.   This disconnects the main board which disconnects the aforementioned C602 0.01uf capacitor.   Install a 12v reed relay with the contacts across JU2 of the HLN-4181 or JU3 of the HLN-4020 PL board, and the coil hooked between +12 and the now-floating JU6/L7 end.   Ground pin 7, the F1 pin, to close the relay.   Select the normally open or normally closed contacts depending on what the normal state of your encoder will be...   On both of the PL boards shorting the jumper kills the encoder tone, and in my case I used the normally open contacts since I wanted the relay's idle state to be the PL encoder on.   I did not use an open collector transistor instead of the relay contacts for this because using a reed relay was easier, faster, simpler, provided an absolute zero-ohm ground (the less resistance you have, the better the tone cutoff) and in my case the encoder off fuction is only used for a few hours once or twice a year (so I definitely wasn't going to wear out the relay).   I was also doing this conversion on a deadline.   Note that if your controller does not have a back-EMF snubbing diode on the digital output make sure you add one.
If I was going to do it all over again, I'd use a FET instead of the reed relay and that would allow me to key the PL encoder with the main channel RX COS signal.   You'd need a FET with low "on" resistance, and the source and drain leads would go in place of JU2, with the gate lead through a resistor to the F1 pin.   Or I might use an opto-isolator between the F1 pin and the FET.

PL Decode

Photos of the three types of Mitrek PL / DPL boards. All connections to the board are made through the 9-pin connector header which looks like a vertical brown line in right edge of the photos below.   The holes in the board provide access to tuning adjustments in the receiver.   The three boards are completely interchangeable except for adjusting the encoder level pot on the board.

The HLN-4020 reed-type board. In this photo the 9-pin connector is in the center of the right edge of the board.   The two reed sockets are at lower center, with the black plastic hold down arm above them.   The KLN-6210 encode reed is on the left, the KLN-6209 decode reed on the right.

The HLN-4181 reedless board. The long thin horizontal component at the bottom left is a TRN-4224 tone element that is plugged into the black socket under it.

The HLN-4011 DPL board. The long thin white horizontal component in the lower middle of the board is a TRN-6005 DPL code element that is plugged into the black socket under it.   Note that the TRN-6005 element is shorter than the TRN-4224 tone element and does not occupy all the pins of the socket.

In perusing the schematic of the HLN-4020 reed board I did notice one nice feature: it can be jumpered to use one reed for common encode/decode or two reeds for split tones.   I've not seen that feature in any other reed-based PL board.   When two reeds are used the encode reed is on the left of the hold-down bracket in the above picture with the decode reed on the right, when one reed is used it goes in the encoder socket and two jumpers are added.   The necessary jumpers are documented in the later schematics of the board (they are not included in the board revision D schematic but they are there are on the revision F schematic).   An early board can be jumpered for a single reed using the info from the later schmatic.

The rest of this section is focused on the HLN-4181 "reedless" PL board since I had six radios with five HLN-4181s, and only one of the HLN-4020 reed board.   Working for NASA for 6 years taught me that you don't put equipment into mission-critical service (i.e. on a mountaintop) that you do not have known good and ready-to-go spares for on the shelf.   In my opinion service trips should be limited to swapping entire units out, not fixing them on site (unless thatis the absolute last resort).   Especially if it takes you a couple of hours to get to the start of the dirt road, and another hour in 4-wheel-drive to get up to the radio site... and that's in good weather.

Repeat Audio In
Bringing TX audio (i.e. repeat audio) in from the outside world into the radio is simple because the Mitrek is a true FM radio and the actual modulation is done inside the channel element.   All we have to do is to tap and insert audio into the modulation line going to the element.   There is an easy way and a right way... Both take a 10uf audio coupling capacitor and connect it from the control connector's frequency 4 select pin through the RF protection choke to the modulation line of the transmitter channel elements.   The difference is where we tap into that modulation line.

The easy way is to just patch a 10uf capacitor onto the interconnect board between JU9 / L10 and this plated through hole.

>----------------------------------------O -JU3A- O--+--O -JU15- O---Pin 18 of 
9-pin PL Board                              (in)     !     L18       the control 
connector pin 1                                      !               connector
                                      +--O -Ju3B- O--+
               JU9/L10        10uf    !     (out)
TX audio in     choke        -! !+    !
>-------------()()()()--------! !-----O <-- plated-through hole 
control head               ^  ! !     !      on interconnect board
connector pin 10           !          !      right below and in between 
                           !          !      JU3B and JU3C.
                           !          !
(Frequency 4 select)       !          +--O -JU3C- O <--- this is where I grab my VOM
lift the main-board side --+          !     (out) !      or oscilloscope ground
of the choke and connect              !           !
the new capacitor to it)              !          gnd
                                      !
                   interconnect board !
                                 ==========
                           main board !
                                      !      O "Point B"
                                      !      !
                                      +------+---- to TX modulator

The better way is to run one end of that same capacitor to the main board.   Find "Point "B" on the schematic and then on the main board near the mic audio circuit, look around Q1004, R514 and R501, it can be a little hard to find...   The "Point B" is preferred as connecting the capacitor there bypasses the connection beteween the interconnect board and the main board, which according to the Cactus writeup mentioned above "has been found to be the cause of intermittent and high resistance connections".   They aren't kidding.
Note: do not use a tantalum cap for the 10uf one mentioned above, they are bad news when used as audio coupling capacitors.

The right way is to inject it into test point B, pointed to by the pen.   Don't be surprised that the hole in the PC board is big, it's sized for a push-on-pin.

This is the right way:

               JU9 / L10        10 uf
TX audio in    choke            -! !+
>-------------()()()()-----------! !-------- connect to 
control head               ^     ! !         "Point B" on
connector pin 10           !                 main board
(Frequency 4 select)       !                 (see text)
                           !
lift the main-board side --+
of the choke and connect the 
new capacitor to it

Note that the TX audio injected at the modulation line by either method shown above is NOT pre-emphasized by the TX audio circuits before modulating the radio.   If you need pre-emphasis you need to do it to the audio before it hits the radio.

In my case when the radio's dev pot is set properly and the repeat TX audio level out of the controller is adjusted properly the local mic audio is a little low - however if the local person "close talks" the microphone (with the lips just barely clearing the mic grille) and speaks at a level just a little above normal conversation level it is perfectly acceptable for test use.   As often as I go to a hilltop site and need to use the link radio's local microphone I can live with having to close-talk and "speak up" a little.

It is the authors opinion that having the repeat audio level be independent of the RX volume control, plus having a working local speaker and permanently installed local microphone jack on every radio in the system is very useful - it allows a local user to simply walk up to the rack, turn up the volume on any radio and monitor the frequency.   If needed he can pick up the appropriate microphone (or plug in a microphone) and talk just as he would expect to.

Carrier Operated Relay (COR)
Carrier Operated Switch (COS)
Carrier Detect (CD)
Channel Busy   (you choose the term you prefer)

I seriously suggest you use the Link-Comm RLC-MOT board (as described below) to generate the COR and repeat audio outputs, but if you chose not to, then generating a logic-level signal that indicates open squelch isn't hard, but it does need one NPN transistor, maybe two, depending on your preferences.

To bring the COS signal outside (from either the RLC-MOT or from the added switching transistor) I lifted the radio end of JU7/L8 on the interconnect board leaving the choke between control connector pin 8 and the COS signal source.

   Original !  Added carrier
    Mitrek  !  detect circuitry
   --------------------------------      JU7 / L8 
            !                             choke         pin 8
            !         control        +---()()()()-----------<
            !        connector       !                  F3 select
            !          pin 4         !
            !           +12v         ! <--lift this end of the
            !            !           !    choke in the radio from 
            !            !           !    the existing circuitry
            !            !           !    and hook to new     
            !            !           !    transistor collector
            !       10k  R           !
audio    !  !            !           ! <--open collector COS out...
mute     !  !            !           !    low with squelch open / signal present
switch   !  !            !           C
Q406     C  !            +-------- B
    -- B    !            !           E
         E  !            !           !
         !  !            C           !  New transistors are almost
         +-------R----- B            !  any NPN silcon - 2N4401s,
         !  !   10K      E           !  2N3904s, 2N2222s, etc.
  R414   R  !            !           !
150 ohm  !  !            +-----------+
         !  !            !
        gnd !            !       pin 6 or 17 of the control 
                        gnd  <-- connector, or another 
                                 internal ground

Yes, the second transistor inverts the first and is not really needed, but I prefer open collector outputs from radios, and I prefer active-low signals between the radios and any external circuitry, especially repeater controllers.   Why? well, I prefer open collectors since most controllers have their own internal pull-up resistors, and a open collector output is therefore universal - it will work with a +5v logic system, a +12v logic system, relays, a LED (for testing), anything, and all with no modifications.   I prefer active low signals becasue if a radio loses power or otherwise fails, the controller doesn't "pull up" the dead signal to a false positive, resulting in the system being keyed until the timeout timer disables the system TX (you DO have a separate backup timeout timer, don't you?), thereby rendering the entire system unusable.   Also when you are working on the system you can do a large amount of testing with nothing more than a clip lead shorting the open collector outputs to ground without worrying that you're going to blow something up (picture a site visit some day... PL decode not working? gound the PL decode lead and see if the PL indicator on the controller front panel lights up.   yes? the problem's gotta be inside the receiver chassis!).   Also with open collector interfacing you can shut off the system, disconnect or un-cable the bad radio/device/etc. and then turn the power back on and everything else in the system still works because the unplugged open circuit looks like the idle open collector...

If you absolutely have to use a high going signal just leave off the second transistor and use this:

   Original    !  Added
   Circuitry   !  Circuitry      JU7/L8
               !                 choke         pin 8
               !            +---()()()()-----------<
               !            !                  F3 select
               !            ! 
               !            ! <--lift this end of the choke
               !            !    in the radio from the
               !            !    existing circuitry and hook
               !            !    to the new transistor collector    
audio     !    !            !
 mute     !    !            !
switch    C    !            ! <--open collector COS output...
Q406 -- B      !            !    high when squelch open / signal present
          E    !            !
          !    !            C
          +---------R---- B
          !    !   10K      E
    R414  !    !            !
 150 ohm  R    !            !
          !    !            !
         gnd   !            !
                          gnd

Note that the above COS circuit is in reality an open squelch detector and only works as a COS if you leave the radio's squelch circuit in carrier squelch mode.   We are deriving our COS signal from the audio mute transistor and for it to be valid we have to leave the RX in carrier squelch mode.   The PL board mod above brings the PL decode indicator to the outside world, but leaves it hooked to the main board audio mute circuit (I wanted to avoid as many radio mods as possible - to make the radio as "serviceable / testable" on a standard-wired test bench as possible).   This means that if you place the receiver into PL mode the audio mute transistor will be controlled by the combined COS and the PL decode signals (remember the "AND" and "OR" jumpers on the PL board?), which we don't want.   It is very desirable to be able to deliver separate COS and PL decode lines to your repeater controller, and depending on the capabilities of your controller you can run the system in carrier only mode, PL only mode, carrier AND PL mode, or in Carrier OR PL mode.   As a bonus some controllers (one is the Scom 7K) allow you to run the repeater in carrier mode while requiring all commands (touchtone) to have a PL tone - and if you want, you can have a command or two that doesn't require PL...

So leaving your Mitrek local speaker in carrier mode is a "So what?" type of a problem...   You are going to leave the radio volume control turned down to zero, and at a mountaintop repeater site nobody will be there to hear it anyway.   Forcing the receiver into permanent carrier squelch mode is easily done in several ways:

• leave the control head "Monitor" switch (a push-in / push-out button) in carrier mode
• unplug control cable lead #16 from the back of the control head and tape it off (this disables the monitor switch)
• leave pin 16 on the RX control connector floating (i.e. don't solder a wire to it when you build your custom control cable).   Or leave the pin open AND put a short length of heat shrink on it (a trick I learned at NASA / JPL in the 1970s - it tells you and the next guy to check the documentation, i.e. "No, I didn't forget this pin, it's not connected for a reason!").
• cut JU14 / L16 on the interconnect board.   No matter WHAT you do to pin 16 on the outside of the radio, it's going to stay in carrier squelch mode.

Personally, I do not like option 1 - it's not idiot proof from someone opening the rack and accidentally bumping a switch while doing something else.   I've used option 2 when I'm using stock cables, or option 3 when I'm building custom cables.   I leave JU14 / L16 alone so that the radio still works properly on the test bench.

If you use the RLC-MOT board to create your repeat audio out the discusion above is irrelevant as the Mitrek stock carrier squelch and RX audio circuits are completely bypassed.

Alternative Carrier Operated Relay circuits and points
Quite a few of the other Mitrek modification web pages tell you to use test point 'E' in the radio, or to take a COS signal from U401, pin 4, or from the PL board.
I'll describe those here...

• Point 'E' is a test point on the RX schematic and is the collector of Q406 (I used the emitter of the same transistor).   Physically it can be found near the pin 8 corner of U403, between CR404 and L401.   Motorola uses this point in their consolette (tabletop base station) carrier indicator option board.   Point 'E' sits at about +2.8 volts when squelched and drops to about +0.1 to 0.2v with the squelch open.   One reason that this point is so attractive is that it's the "official Motorola" way to do it, especially when turning a consolette Mitrek into a tabletop repeater.   When I tried using it, however, the carrier squelch action changed with the circuit connected and reverted back to normal with it removed.   I didn't like that "feature" - the squelch action should be consistent with or without the open collector transistor driver connected.   Maybe I screwed up in the breadboarding, (I probably did), I don't know.



• You can also take COS from U401, pin 4.   This point is low with the squelch closed, rising to about +6 volts when the squelch opens.   This spot is nice because it is very simple to get to: it's one of the speaker leads, right off of the speaker driver IC, however as a COS output it fails if you turn up the volume... the speaker audio rides on top of the DC voltage and the low going half of the audio sinewave can dip below the COS threshold causing the COS function to get very ragged and very unusable very quickly...



• You can also take COS from pin 5 (the audio mute signal) on the PL board connector (this is accessible at a jumper, and the jumper number varies with which PL board you have, so check the schematic of your PL or DPL board).   Since this is not a true COS, but just the audio mute signal, this again only works if you leave the radio in carrier squelch.   The COS voltage will swing from about 1.5 volts (with the squelch open) to somewhere between 6v and 12v with the squelch closed (the value depends on which of the three Moto PL / DPL boards you have installed, if any).   This is a high impedance signal, so you can't put too much load on it or you will start to affect the squelch action.

All in all, if you are going to use the internal carrier squelch simply taking the DC voltage off the emitter lead of Q406 with a transistor inverter works the best for me.   It's simple, reliable, is guaranteed to work with any of the three PL / DPL boards, and the rest of the radio has no idea it's there.

After this web page went up I received an email from a friend that basically stated "Point 'E' works just fine for me.   Just connect a 15k ohm resistor to point E and use it like this:"   (his schematic below, you can add the extra transistor as above to generate a low going output signal):

 Original       !  Added
 circuitry      !  circuitry
----------------------------------
                !
         +9.5v  !
           !    !
           +-----------------+
           !    !            !
      R430 !    !        10K R
      82K  R    !            !          COS signal to 
           !    !            +--------- controller (9.5v high 
Motorola's !    !            !          with squelch open / signal present... 
point 'E'  !    !            C          You can leave out the 10k resistor 
    -----> +---------R----- B           for an open collector)
           !    !   15K      E
     Q406  C    !            !
     ----B      !            !    Transistor is almost any 
           E    !            !    NPN Silicon as listed above
           !    !           gnd
           !    !
           !
           !
           ! <------I picked my COS signal above from here
           !
     R414  !
     150   R
     ohms  ! 
           ! 
          gnd

Repeat Audio Out
I do NOT like methods that take audio out of the repeater receiver in such a way that the repeat audio level varies as the receiver's local volume control is adjusted.   It is my personal opinion that the repeater receiver volume control should be used for the local speaker volume, period.   This rule adds one more level of "idiot-proofness".   The method described below does just that.

Repeat audio out is actually pretty easy - just use a Link-Comm RLC-MOT board (described in the squelch / COS section).   But if you decide to "roll your own", it is a simple process as long as your repeater controller can mute the receive audio.   Just add a 10uf cap inside the radio between the discriminator to pin 9 of the control connector.   Note: do not use a tantalum cap here, they are bad news when used as audio coupling capacitors.   If your controller does not do the audio muting for you, like the ACC RC-85 and RC-96 models, you will have to add a series FET audio switch controlled by the COS, or use Link's RLC-MOT (which has it's own audio muting).

Here's the quick way - all of it is on the interconnect board:

From audio             +!!        choke     Pin 11    (Buffered audio out
circuitry -----------+--!!--------()()()-----< <----- to control head volume 
inside radio         !  !!      JU10 / L11            and squelch controls)
                     !  C3 
          (see text) ! 
                     ! Existing radio circuitry (C3 is located on the interconnect board)
      - - - - - - - -!- - - - - - - - - -
                     !
                     !  Added circuitry
                     !
                    +!
               10uf ---  <-- add this cap inside the radio
              at at ---
          least 15v  !
                     !
                     !     choke        Pin 9
lift this side ----> +----()()()---------< <------ audio to the
of the choke             JU8 / L9       Freq 2     controller 
from the radio                          select
circuitry

Here's the right way (which bypasses the main-board-to-interconnect-board connector):   Take the audio from pin 1 of U403A (which is an 8-pin opamp).   If you want to use shielded cable (I would) you can find ground on pin 4 of the same chip.   Note that this pin has about 600mv of audio on top of about 5.5v of DC on it.

>From U403 Pin 1 ----+
                     !  Existing radio circuitry (main board)
      - - - - - - - -!- - - - - - - - - -
                     !
                     !  Added circuitry
                     !
                    +!
               10uf --- <--- add this cap inside the radio
              at at ---
          least 15v  !
                     !
                     !     choke        Pin 9
lift this side ----> +----()()()---------< <------ audio to the
of the choke             JU8 / L9       Freq 2    controller 
from the radio                          select
circuitry

Note that in the two diagrams above the audio to the controller IS NOT de-emphasized, but in most cases it is not a problem as many controllers offer de-emphasis internally.   If yours does not, we can fix that:

                   !
                   !  Existing radio circuitry as above
                   !
      - - - - - - -!- - - - - - - - - -
                   !
             10uf --- <--- add this cap inside the radio
            at at ---
        least 15v  !
                   !
                   \
                   /
                   \  15K ohms, a magic value, do not substitute
                   /
                   \
                   !
                   !     choke         Pin 9
                   +----()()()----------< <------ audio to controller 
                   !   JU8 / L9        Freq 2     (must be a high-
                   !                   select      impedance input)
                   !
                  ---  0.1uf, a magic value, do not substitute
                  ---  and do not use a tantalum cap
                   !
                   !
                  gnd  <---  pin 2 of the control connector (audio ground)
                             or pins 6 or 17 some other internal ground

Comments on the Mitrek Carrier Squelch
Modern FM radios utilize a noise operated squelch which works by measuring the amplitude of the discriminator white noise at the audio frequelcies above the frequency range used by the human voice.   Speech lives in the range from 300hz to a certain point, above that is all noise.   The cutoff frequency between what is considered voice and what is considered squelch noise is critical: too low and the squelch will chop on higher pitched voices, too high and there won't be enough high frequency noise to work with (the top frequency of the available noise is controlled by the IF bandwidth).

Imagine a RF generator connected to the RX, the squelch is wide open, and an AC voltmeter connected to an audio dummy load hooked across the the speaker leads is showing a volt or two of noise.   As you slowly increase the level of an on-channel dead carrier, the level of squelch noise quickly lowers with just a little bit of signal.   If you reduce the carrier level the noise comes back up.   Receiver sensitivity is measured this way, look for the "20db quieting" measurment in a radio specification sheet (and some point-to-point links - especially voting RX links - need 30, 35 or even 40db of quieting).   The noise operated squelch circuit takes this high frequency AC noise and amplifies it and rectifies it into a DC voltage that varies with the noise level.   When this DC voltage drops below a specific threshold (set by the squelch control) the speaker unmutes, and as it rises above the threshold the speaker mutes.   Most modern radios use two different thresholds (the difference is called the squelch hysteresis amount), one level for squelch open, and a slightly lower one for squelch close.   This improves how the user preceives the squelch action: once the squelch opens the signal can drop back past the squelch-open threshold level and then drop a a little more before it closes.

Many people are dissatisfied with the stock Mitrek squelch, especially if thy have ever been exposed to a Micor or a GE Mastr-II.   The typical complaint is that that the squelch threshold level (to stop the white noise in the speaker) is much lower than the level required to eliminate the random "cracklies" caused by on-channel noise spikes.   In other words to get a totally quiet receiver the Mitrek squelch control has to be set much higher than the initial squelch-closing threshold - in some cases turning the squelch control all the way to the top isn't enough.

The reason for the poor performance of the Mitrek carrier squelch circuit is inherent in it's overall design.   The entire circuit uses only only five transistors.   The Mitrek came into being in a era where the use of PL or DPL was universal, and carrier squelch had gone by the wayside.   Motorola installed a simple noise operated squelch, and it almost seems to have been included as an after thought.   The designer didn't put much time or thought into it, knowing that it wouldn't be used much...

In contrast, Motorola put a great deal of effort into the Micor's noise squelch circuit since the use of PL and DPL had not become universal at the time of the Micor's design.   In short, the Micor was the high water mark for carrier squelch circuit development - the heart of the Micor carrier squelch circuit is a single custom IC chip which contains over a hundred transistors.   Unfortunately this part is obtainable only as a spare part from Motorola or by carefully scavenging it off of a Micor audio-squelch board (and it was not socketed).   The Micor squelch chip is a complete bi-level or switched hysteresis squelch system on a single chip.   Simply, the Micor bi-level squelch works better than anything else on the air.   Note that the circuit could be added to any model radio - even the Metrum-II.   In fact, it's not hard to hook an inexpensive Micor mobile Audio-Squelch card (available for $5 or so) to almost any radio that has raw discriminator audio and positive 9.6v DC voltage available.   A 25k squelch pot completes the circuit.

Naturally, when a need shows up, somebody fills it for a price.   Link-Comm makes a line of repeater controllers and they have an accessory board called the "RLC-MOT" that will add a Micor squelch into any receiver that provides raw discriminator audio.   The board will fit inside any mobile radio and is much smaller than the actual Micor audio-squelch board, works great and makes the squelch action a lot more pleasing, and eliminating the squelch tail the stock Mitrek creates.

The Link-Comm RLC-MOT module is small, only 2.5 inches by 1 inch

The RLC-MOT interfaces with a single six pin connector (pin 6 is nearest the corner of the board):

RLC-MOT pinout
Pin	Function
1	+11 to 15vDC at about 35ma
2	Discriminator audio in
3	Buffered and muted audio out
4	Active low COS
5	Active high COS
6	Ground

The RLC-MOT uses a trimpot for an on-board squelch control (a 20 turn!), has a LED that lights when the squelch is open, and and has an audio level pot on board.

Duplexing / Dual Radios / Interconnections
Duplexing a UHF Mitrek can be tricky, and in many cases is frequency dependent (i.e. it will work on some frequencies and not on others).   The 406-420mhz models will spur if duplexed (please read the entire Cactus Radio document mentioned at the top of this writeup) and since I had two Mitreks and four Mitrek Plus radios on hand for the project I elected to just use two of the Plus radios for my full duplex point-to-point link.   Using two radios also meant that I didn't have to do the standard "duplex mod" (see below).   As my long-term plans were to replace the two 440mhz Mitreks with 420mhz ones, I had to use two radios anyway.

One additional mod I made to the control head that when first described makes people wonder why: I shorted out the double pole power switch on the control head volume control with two individual jumpers and added a label to the front of the control head that says "Control head power switch bypassed - Use power switch on Astron to turn link radios on and off".   Why?   I made the change and added the label after I turned the volume down a little too far as I was leaving the site on Sunday night and killed the link... it was dead for a week until I could get back to the site and turn the switch back on.   It was very frustrating - a one and one/half hour drive just to get to the start of the 4-wheel-drive dirt road, then another hour to get to the radio site.   Out of the vehicle for long enough to open the building door, disable the alarm, flip a switch, test the link, re-enable the alarm, lock the door, then 2 and 1/2 more hours to get back home.   On my next visit I jumpered the switch out and added the label.... adding one more layer of "idiot-proofness".   Phil Lefever, KBØNES made the point quite well in an email when he said: "I hate driving up the hill to fix something I could have done better the first time around".

Here's a crude diagram of my dual radio cabling - it makes two Mitreks look like one single full duplex radio:

                            big wire                 big wire
                 +--------------------Big Fuse------------------------+
                 !                                                    !
                 !          big wire                                  !
                 !  +------------------------+                        !
                 !  !                        !                        !
 +--------+      !  !     +--------+         !   +----------------+   !
 ! Pin 19-!------+  !     ! Pin 19-! open    !   !     Astron     !   !
 ! Pin 17-!---------+     ! Pin 17-!---------+---!Neg  power   Pos!---+
 !        !               !        !             !     supply     !   !
 !        !               !        !             +--------------- +   ! 
 !   TX   !               !   RX   !                                  !
 !  plug  !               !  plug  !                               7a fuse
 !        !               !        !                                  !
 !        !               !        !                                  +---+
 !  see   !               !  see   !                                  !   !
 !  note  !               !  note  !                  +----------------------+
 !  #10   !               !  #10   !                  !          pin 19   20 !
 ! below  !===============!  below !==================!                      !
 +--------+   control     +--------+  control         !  Mitrek single-freq  !
              harness                 harness         !     control head     !
                                                      ! with bypassed power  !
            to                                        ! switch and LEDs for  !
         repeater ------>>----------------------------!    COS, PL decode,   !
         controller    DB15      Ribbon cable         !    power and PTT     !
                                  (see text)          +----------------------+
                                                        !         !
                                                        +-- mic   +-- speaker

To connect the two radios to a single control head I took two old Motrac cables (Mitrek, Motrac, Mocom 70 and some of the earlier all-tube vintage radios all used the same connector but with different wiring).   The control harness in the drawing above was built up by starting with a control cable that had been cut during removal (C. W. Wolfe sells these for a very reasonable price).   I took the control head end with the numbered pin connectors which gave me some nice long leads and then scavenged the radio end connectors off of both cables.   This gave me two connectors, two sets of connector shells and a full set of pinned and numbered leads about 5 feet long.

The leads to the radio connectors were cut to length as I added each wire to the harness.   I wired everything up using the recycled numbered leads so I had a RX radio and a TX radio on one single frequency control head (i.e. no frequency switch)... see the detailed diagram below.

To connect the dual radio assembly to the repeater controller I cut a slot in the back of the control head to allow a piece of 15-conductor flat ribbon cable to exit out of the head.   The actual DB15 connector and ribbon cable I used came off of a game port connector plate from an old combination I/O card from an ISA vintage PC.   I cut off the 16-pin header connector and separated the individual leads for a few inches and stripped the insulation off.   To get the ribbon cable out of the head I drilled two holes with a 1/8" drill bit just far apart to clear the cable and used an old soldering iron with a long thin concial tip that just fit into the 1/8"hole and dragged it sideways to melt the plastic between the two holes.   Moto used good thick plastic, it took a while to do it and do it with good ventillation.   I don't have access to a milling machine but if you do then you could duplicate that slot much faster or even mount a DB-15F into the top or front of the head itself... above the area where the frequency switch would go is one good place....

The overall radio-control head harness consisted of all the pinned cable leads that were required plus the two large power cables, all tied together with a piece of 3/8 or 1/2" (I forget which) spiral wrap (from the local auto parts store) that started with the power wiring and the TX connector on one end, the RX connector in the middle, and the control head and controller DB15 connector at the other. If you are not using a control head then you could mount a pair of miniature volume and squelch pots and a couple of LEDs in the connector shell itself.

The table below is the pinout of the DB15 that goes from my dual Mitrek lashup and the controller.   The first 7 pins are the same as a Link Comm controller, pins 8, 9 and 10 are unused, the last 6 pins are my own wiring.   I relocated the audio grounds to pins adjacent to the audio pins since the the overall construction was much cleaner.
1	Logic Gnd	DB-15 connector pinout 8 7 6 5 4 3 2 1 ----------------------------------- \ O O O O O O O O / \ O O O O O O O / ----------------------------- 15 14 13 12 11 10 9 View from the surface of the female connector that accepts the male plug, or from the wiring (back) side of the male connector.
2	PL decode output
3	TX PTT in
4	TX audio in (shield to pin 11, Link-Comm has shield to pin 8 of a DB-9)
5	RX audio out (shield to pin 12, Link-Comm has shield to pin 9 of a DB-9)
6	Logic Gnd (see note 9 below)
7	RX COS/COR out
8	Reserved for future use
9	Reserved for future use
10	Audio Gnd (TX audio shield)
11	Audio Gnd (RX audio shield)
12	Link RX power monitor (see note 6 below)
13	Link TX power monitor (see note 5 below)
14	PL encoder disable
15	Reserved for future use

Here's a pin-by-pin detail on the wiring I used - but refer to the manual so you completely understand it before trying to duplicate this:

                  TX         RX       Control
             radio pin    radio pin   head pin
                  1 ------------------- 1 see note 1
     +----------- 2 --------- 2 ------- 2 see note 1 and note 12
     !<--note 14A
     !       +--- 3           3 ------- 3 speaker hot   8.2k  4.7k
     !   +-->R    4 --------- 4 ------- 4 --------------R--+--R------gnd   see note 6
     !   !   +--- 5           5 ------- 5 speaker hot      !                               
     !   !        6 --------- 6 ------- 6 see note 10      +- DB15 pin 13 (link power on)
     !   note     7 ------------------- 7 see note 7 -------- DB15 pin 15 (PL encode disable)
     !   2 and                8 ------- 8 see note 7 -------- DB15 pin 7 (COS out)
     !   14C                  9 ------- 9 see note 7 -------- DB15 pin 5 (RX audio out) note 9
     !           10 ------------------ 10 see note 7 -------- DB15 pin 4 (TX audio in) note 9
     !  note +-- 11          11 ------ 11 see note 3
     !   14B !   12 ------------------ 12
     !    -->!   13 ------------------ 13 ------------------- DB15 pin 3 (TX PTT)
     !       +-- 14          14 ------ 14 see note 3
     +---------- 15          15 ------ 15 see note 3
                             16        16 see note 4
Ground --------- 17 -------- 17 ------ 17 ------------------- DB15 pin 1 and 6 (Logic Ground)
                             18 ------ 18 see note 8 -------- DB15 pin 2 (PL decode out)
+13.8vDC --+                       +-- 19 +12 from Astron
           !                       !
           +-- 7A fuse ------------+-- 20 +12 from Astron
           !                           21 see note 3 -------- DB15 pin 11 and 12 (audio ground)
          Big             +------------22 see note 5 -------- DB15 pin 14 (link TX power on)
          fuse            !
       (note 11)          !         +--24 \
           !              !         !      See note 15
           !              !         +--27 /
           !              !
           !      TX      !     +----- 29 or 31               DB15 pins 8, 9, and 10
           +----- 19      !  RX spkr                          are reserved for future use
           !              !     +----- 32
          8.2k  Note 5    !
           !              !
           +--------------+     NOTE - all unreferenced or not shown pins on the RX, 
           !                           TX and Control head connectors are not used 
          4.7k  Note 5                  and are left floating.
           !
          gnd

1.  TX pin 1 and 2 is a shielded cable with the shield connected to pin 2.   Pin 1 is mic audio, 2 is audio ground.
2.  TX pins 3 and 5 have a 47 ohm 1 to 2 watt load resistor connected to them (a dummy speaker load).
3.  RX pins 11, 14 and 15 are shielded with the shield hooked to pin 2 on the radio end and pin 21 on the control head end (audio ground).
4.  RX pin 16 normally connected to control head pin 16 to allow the control head "Monitor" switch to set the RX into PL or carrier mode. By leaving the pin 16 unhooked the PL decoder can never accidentally be enabled.
   I personally do not solder a wire to the pin and in fact put a piece of heatshrink over the RX connector pin 16.   This heat shrink on the bare pin is a memory-jog trick I learned at NASA in the 1970s - it tells you (and the next guy) to check the documentation before doing anything with that connector pin.
5.  These two resistors are optional... You could put them on the back of the TX connector from pins 19 to 17 and connect the center point to a lead plugged into pin 22 of the control head and from there to a DB15 pin tied to a spare digital input on the controller.   Pin 22 of the control head is normally hooked to pin 6 of the PL switch (the one labeled "Monitor") and you can cut the trace behind the switch and use the adjacent plated-through hole to connect the DB15 cable lead.   The two resistors divide the power supply voltage to a logic high, i.e. about 4.5 to 5v. If the big fuse blows the controller digital input will go low and the controller can announce "No power to link radio TX" when you switch on the link...   (if you have a leftover digital input, you might as well use it for something interesting until you need it for something useful).
6.  If you have a second spare digital input just duplicate the two resistors but put them either inside the RX radio connector shell from pin 4 to ground (pin 6 or 17) or put them inside the control head from pin 4 to ground (pin 23) and either way run the center point to the the controller digital input, and program the controller to say "No power on link radio RX".
7.  Pins 7, 8, 9, 10 are normally hooked to the control head freqency switch. If you use a single freq control head (without a switch) the virgin solder pads for the switch can be used as tie points for the leads from the DB-15 cable.   Or you can take a multifreq head and remove the switch.
8.  Pin 18 is normally hooked to pin 4 of the PL switch and you can free it up by cutting the trace behind the switch and use the adjacent plated-through hole to connect the DB15 cable pin.
9.  Both DB15 pins 5 and 6 are hooked to audio coaxial cable (I used RG174 simply becuse I had a spool of it on the shelf - it's lousy UHF coax but great as audio cable) with the shields grounded to control head connector pin 2 on the head end and to DB15 pins 8 and 9 on the other.
10.  Pins 17 and 19 of the TX connector are the TX high current pins and should connect to the Astron with heavier gague wire.   For a low power radio (T2xJJA, T3xJJA or T4xJJA model numbers) you want to use at least #12 wire, and #10 for any higher power radio (the T5xJJA, T6xJJA, T7xJJA, T8xJJA series). Use the next larger size wire for any run over 10-12 feet.   Pin 17 of the RX connector is tied to pin 17 of the TX connector and to pin 17 of the control head but that can be done with smaller gague wire to the RX-only radio.   Make sure that you jumper pin 6 of the TX and RX connectors to pin 17 at each connector.
11.  The size (amperage) of the "big fuse" depends on both the TX power level and the frequency band.   See the Motorola manual for your radio for the proper fuse rating.
12.  DB15 pin 11 and 12 (audio ground) can be connected to either pins 2 or 21 in the control head, or pin 2 on the control connector on either radio - whichever is easier for you.
13.  The above drawing does not show the internal mods to the radios (like installing JU611 forcing the frequency 1 channel elements on).
14. Inside the TX radio connector:
    1. The volume control is replaced with a jumper from the wiper pin (pin 15) to ground (pin 2) thereby setting a virtual volume control at zero.
    2. The squelch control is replaced with a jumper from the squelch wiper (pin 14) to the hot side (pin 11) thereby setting a virtual squelch pot at maximum.
    3. Connect an 47 ohm 1w or 2w resistor across the speaker leads (pins 3 and 5) and don't ground either of the speaker pins!!! The Mitrek runs both speaker leads hot and you will let the magic smoke out of your radio if either speaker lead is grounded.   If you are going to recycle an older cable or control head just assume that they ran a gounded speaker and trace both speaker leads from one end to the other and find those grounding jumpers and remove them. Better yet, strip both control cable connectors of all wiring and start over.
15. This jumper is required if you are using a factory Mitrek PL board and want to use the PL decoder in the receiver.

Note that some control cables have a wire on pin 7 but do not have any on pins 6, 8, 9 and 10 (i.e. they are the "single frequency" cables) so chose the cables that you intend to chop up carefully.   I first found this out the hard way, back when I had a 4-freq 6m Motrac, a 6-freq 2m Motrac, a 4-freq 440mhz Motrac and a 4-freq U43MSN Motran modified for 220mhz in my 1971 Ford Falcon station wagon... the 2m Motrac cable ended up with a length of 10-conductor stranded intercom cable taped to it... the conductors made up for the missing ones in the control cable plus provided for frequency 5 and frequency 6 plus a couple of spares for TX PL tone selection.

Duplex Mod For The Mitrek
If you chose to use one radio and duplex it, you'll need to do these steps (do not duplex a 406-420mhz radio, especially if it will be going to a hilltop!!! For the reason read the Cactus Radio writeup mentioned above):

1. Jumper the F1 elements to run full time using JU611 as mentioned above.
2. Clip one side of CR403 (in front of L208).   This prevents the receiver's audio amplifier (i.e. the speaker) from muting when PTT is keyed.
3. Clip one side of CR1 (near Q1004, in front of L702).   This prevents the RX channel element from shutting down when PTT is keyed.
4. Give the RX and TX their own RF ports.
   • The mechanically simplest method I've seen is to disconnect the antenna relay coil wires from the motherboard and leave them floating, thereby using the front panel antenna port as the permanent receive antenna connection.   If you want optimum performance you can replace the front antenna connection with a female-to-female type N bulkhead connector (which will require enlarging the hole in the front panel) and then using double-shielded cable to the RCA connector on the RX chassis (scavenge the old RCA plug for reuse unless you have some of the short-center-pin plugs in your junk box).   I've also seen a female-N-to-female-SMA bulkhead connector mounted in the front of the radio with a male-SMA-to-male-RCA cable used to connect to the RX.   Either way, you will want to use high grade double shielded coax on the RX port (I prefer RG-400).   I've even seen a scrap of small diameter Superflex used here.
   • Next, you have to give the TX it's own port.   One way is to mount a double-female-BNC bulkhead connector in the sidewall of the radio.   Then cut the transmit coax at the back side of the antenna relay and solder or crimp a BNC male to it, then plug it into the new connector.
   • Probably the prettiest dual antenna jack mod I've seen is shown below this paragraph.   Not visible is the back side of the plate that replaces the stock antenna connector.   This plate mounts two female-to-female SMA bulkhead connectors, and the two new cables that run from the RF decks to the front panel have SMA male plugs to connect to that plate.   This makes for the best shielding and minimum RF leakage.   Unfortuntely this method requires enlarging the hole in the front panel to clear the two SMA connectors, and milling machines are not in the workshops of most hams.
     
   • Motorola made some Mitrek-based high band mobile extenders by leaving the final transistor out of the PA deck, and below are four photos of how Moto modified the PA deck and how they did the TX antenna jack... they mounted a SO-239 into the low power heat sink (looking at the heat sink end of the radio it's on the right side).   This mod would not be hard to duplicate, but again, milling machines are not in the workshops of most hams.
     
     Inside the PA deck.   You can see the driver transistor under the red capacitor, and that the final transistor to the right of the driver has been replaced with a metal strap jumper.
     
     
     The right side of the back of the full duplex Mitrek radio showing the TX antenna jack
     
     
     A long shot of the inside of the PA deck showing the back of the TX antenna jack
     
     
     A closeup of the inside of the PA deck showing the back of the TX antenna jack
     

Status Indicators
I used a single freq control head, and for status indication I mounted LEDs in the top of the head for COS and PL decode.   I replaced the two incandescent bulbs in the head with red and green LEDs just for consistency.   This gives me 3 LEDs in the top of the head: green for power, red for TX PTT, and a three-wire red/green for COS and PL decode.   Someday I may add a blue LED for PL encoder disable (that feature was added to the radio after the control head work was complete).   On an incoming signal without PL the dual LED glows green, and if PL is present it momentarily glows green showing the presence of the carrier, and when the PL decodes the red comes on with the green making amber to make it evident that both are present.   There is no reason I used a 3-wire 2-color LED except that I had a few in the junk box.   Two individual LEDs for COS and PL decode are just as easy.   The PL decode signal from the one-jumper mod above (JU6 to JU1) is a logic level as opposed to an open collector, I added an open collector driver transistor in the head to drive the PL decode LED.   I showed the open collector transistor in the PL decode section above as that's how the most recent radio was done.

The resistance rule of thumb for a 10ma led is 100 ohms per volt, so a 1.2k resistor in series with each LED is just about right for 12-13v, but the value is not critical - a 1k or 1.5k will work just as well.   For 20ma leds (the most common) use 50 ohms per volt, so 560 or 620 ohms is appropriate.   For 2ma LEDs I use 4.7k or 5.1K...   Whatever value you pick, use that value on all the LEDs so the light intensity is the same - except on the blue LED.   One characteristic of blue LEDs is that most of them take a little more current for the same intensity (they are not as efficient) so if you use blue you may have to use a little less resistance to get it to look right.   This is one place where an old Heathkit resistance substitution box comes in handy.

Mounting
To mount the radios in the system rack cabinet I bolted together a "sandwich" of two Mitrek mobile mounting trays and a thick steel rack shelf we found in surplus.   On the top of the shelf is the TX radio, under the shelf is the RX radio upside down. An Astron SS-25 is mounted adjacent to the TX Mitrek, and a control head bracket is bolted below to complete the assembly.   The sizing is perfect, you'd think that Astron designed the SS-25 specifically to fill the leftover space in the rack shelf.   When lifting the assembly by the ends the shelf started to "bow" in the middle due to the weight, so I added two pieces of right-angle steel stock, one at the front and the other at the rear of the shelf, both running the full width of the shelf.   The spacers in the drawing below are 1" long pieces of half-inch EMT conduit (only because I had a short length left over from a remodeling project) - lengths of a dowel with a hole drilled in the center or even stacks of washers would work just as well and are there only to allow the front of the lower radio to clear the front right-angle steel bracing when inserting it into or removing it from the mounting tray, and to allow access to reach the control head knobs and to see the LEDs.

The Astron is mounted to the shelf with long ty-wraps that extend through two holes drilled in the tray (at the "X" in the diagram below), across the bottom of the shelf, around and up the right side and back across the top of the supply.   Yes, I could have opened the Astron and drilled a couple of holes in the bottom to mount it, but I didn't want to affect the warranty, and I wanted to be able to swap it out without having to pull the rack shelf out of the cabinet.   The rack shelf I used is the exact height of the Astron, and the long ty-wraps (I used a couple of leftover riot handcuffs) work just fine.   A speaker bracket is mounted to the bottom of the rack shelf behind the control head.   The speaker lies flat against the bottom of the shelf with the speaker facing downwards.

  +------ rack mount shelf ears and mounting holes -------------+
  !                                                             !
+---+ +------------------------------------+ +--------------+ +---+
! O ! !                                    ! !   Astron     ! ! O !
!   ! !       TX Mitrek                    ! !    SS-25     ! !   !
!   ! !                                    ! ! power supply ! !   !
! O ! +----+--------------------------+----+ +--------------+ ! O !
+---+------!--------------------------!-----X---!----------!--+---+
        spacer - see text          spacer     spacer    spacer
      +----+--------------------------+----+ +--+----------+--+
      !                                    ! !     Mitrek     !
      !    RX Mitrek                       ! !  Control head  !
      !    (mounted upside down)           ! +----------------+
      +------------------------------------+    with speaker 
                                               mounted behind 
                                              the control head 
                                              facing downward

Second Receiver Concerns
There may be times when you want to put a second Mitrek-based link into your system, or perhaps you already have an existing radio with a 10.7mhz IF in your rack. Well, Motorola has come to the rescue with the MSR-2000 repeater.   The Motorola MSR2000 and the Mitrek share a common history.   In fact you could really stretch and say that the MSR is a base station version of a cross between the Mitrek and the Micor.   While the Mitrek is a single-board "unified chassis" design, the MSR uses separate RX and TX boards, a control shelf, and other non-mobile features.   Despite this there are many common components.   One of these is the receiver IF crystal filters.   The normal Mitrek and MSR-2000 IF frequency is 10.7mhz.   When there is a second receiver in the same cabinet the second receiver IF needs to be moved to a different frequency to avoid in-cabinet interference.   If you put a second Mitrek based link in a system cabinet and have an interference problem you can move the IF of the second RX by re-ordering the channel element crystal for a 10.8mhz IF, installing the 10.8mhz IF filters and realigning the RX.   The part numbers for the 10.8mhz filters are (thanks to WA6KLA for the information): Y201 is a 48-84396K07 and Y202, Y203, Y204, Y205, Y206 are all 48-84396K06.   If you do convert a mobile to a 10.8mhz IF PLEASE leave a note inside the radio and on the front of the radio - maybe use a big Sharpie pen on the sides of every IF can and write "10.8 IF".   The next guy who gets to recrystal that radio will appreciate it!

Closing notes
The dual Mitrek assembly described above is currently used as a single channel full duplex UHF point-to-point link in our UHF repeater system.   The main repeater is on controller channel 1 and the dual Mitrek is on channel 2.   The reason for the use of dual radios is that the initial implementation used 450 Mitreks tuned to the regular user input and output frequencies of the far-end repeater, later replaced by 420 Mitreks on real link frequencies.

Setting the receiver frequency can be a bit tricky as the Mitrek uses a quadrature detector rather than a ratio detector or a crystal discriminator, therefore there is no "Motorola standard Metering Point #4" zero-center test point.   The Mitrek designers added a circuit to help tune with a zero-center meter, but I've found that using a SinAdder gives me better results.   If you don't have one, sometimes the simplest way is to use a frequency counter with the hot side of the test probe hooked to pin two of the channel element socket.   Just remember to compensate for the 10.7mhz IF frequency.   Then start looking for a Sinadder - maybe on eBay - they aren't that expensive and are a valuable addition to your test equipment collection.

Remember that the Mitrek mobile transmitter was not designed to be a 100% duty cycle transmitter.   In fact, Motorola states in the manual that the Mitrek is a 20-25% duty cycle radio.   Mobile radios are built small and inherently have undersized heat sinks, so to keep the heat rise down make sure you keep the output power at no more than half of the rated output (the lower the better, but they get dirty and squirrely if run too low).   Obviously the lower the power on the same sized heat sink means the low power ones are preferred for repeat / link service - even a low power radio at half power is enough to drive an external amplifier to whatever power level you can afford.   And pont-to-point links can use directional antennas... 15w into a 10db yagi gives the same performance as 150w into an omni with a lot less duplexer required...

Consider doing the TX tuning with a eye on a spectrum analyzer - remember that a dummy load accepts all manner of frequencies without complaint, being non-resonant by design, while a tuned circuit (like an antenna or duplexer cavity) will complain about harmonic content and spurious output.   To quote WB6VYX's comment on spectrum analyzers "a picture tube is worth at least 20db of clean..."   A transmitter that is not producing clean output will not behave on a hill and sooner or later WILL cost you your site and give amateur radio yet another black eye.

Cooling of the RF power amp is critical if you want it to last.   One trick is to remove the silver shield covering the PA and use a hole saw to cut the top cover of the radio so that a cooling fan can blow cool air directly onto the PA output transistors:

                              ===finger guard===
                              +----------------+
                              !      fan       !
                              +----------------+                ===finger guard===
  +------------------------------!          !---------------------!            !-------+
  !  Top cover plate             ! fan hole !                     ! air outlet !       !
  +------------------------------!     !    !---------------------!            !-------+
  !!                                   !                                ^             !!
  !!                                   !                                !             !!
  !!                         air flow  +-------->----->----->----->--->-+             !!
  !!                                              PA transistors                      !!
  !+----------------------------------------------------------------------------------+!
  +------------------------------------------------------------------------------------+

Use one hole to mount the fan and finger guard and another hole (with a second guard) for the air outlet.   Finger guards in a variety of sizes are available at your local PC hardware shop.   Cheap fans use brass or bronze bushings, good fans use ball bearings and are worth the extra money.   Fans with needle bearings are even better but not too common.

Design your rack mounting assembly with an eye to allowing air flow to and from the PA fans mentioned above and to keeping the rear heatsink cool... a rear fan and good air flow is mandatory.   Personally I use two thermostat-controlled fans on the heat sink, one set 10 to 15 degrees higher than the other, and if the hotter one turns on it also turns on a digital input on the controller, which announces "over temp on south link transmitter" and changes the pitch on the system MCW ID.   If the users miss the voice announcement then the change in IDer pitch gets people's attention within the first 10 minutes of dialing up the channel.

The hookup:                                 Hook this to +5 or +12, depending on 
                                                 your controller's pull up voltage
+12v ----------+-------+----+--------+           + V ---+---------+
               !       !    !        !                  !         !
               !       !    !        !                  \         \
               !     first  !     second                / 1.2k    / 10k 
               !     rear   !      rear                 \         \ (this resistor is just for
            PA deck  heat   !      heat                red        /  for insurance in case the
              fan    sink   !      sink            overheating    \  LED opens up. It may not 
             motor    fan   !       fan             indication    /  even be necessary if the
               !     motor  !      motor               LED        \  controller has it's own 
          /    !       !    !        !                  !         /  pull-up resistors... Some
         /     !       !    !        !                  !         \  do, some don't, some pull 
        /!     !       !    !        !                  !         /  up to +5v, some to +12v...
     +-O ! O---+       !    !        !                  !         !
     !   !             !    \        !                  +---------+------- to controller
    gnd  !             !    /        !  200v 1a         !                  logic input
         !             !    \       --- diode or       --- 50v 100ma       (active low)
    -----)             !    /       \ / at least       \ / is overkill
         )             !    !        V  twice as        V  here
  reed   )             !    !       --- large as       ---
  relay  )             !  green      !  necessary       ! 
         )             !   LED       !  to carry        !
   ------)             !    !        !  fan motor       !
                       !    !        !  current         !
 Connect               +----+        +------------------+
 reed relay            !             !
 in place              !             !
 of antenna       thermostatic     thermostatic switch #2 
 relay so            switch #1     (10-20 degrees higher)
 fan runs              !             !
 while                 !             !
 transmitting          +------+------+
 (a low current               !
 fan can be run               !
 directly)                  ground

Onther method of fan control is to use a CMOS 555 timer chip trigerred by the PTT, and staying on for a time period after unkey.   This idea can be extended by adding a thermistor on the heat sink and incorporating it into the 555's timing capacitor charging circuit so that as the heat sink gets hotter the timer takes longer to shut off the fan.

An alternate way of controlling the PA deck fan is provided by some repeater controllers: a digital output can be programmed to turn on withthe first keyup and stay on for a programmed time period after the last transmission.   In fact, the Scom 7K programming manual gives this fan control function as one of their programming examples.