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  Interfacing the Motorola Mitrek Mobile Radio To A Repeater Controller
(of any type, even an IRLP computer interfacing board)
By Mike Morris WA6ILQ
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Date printed: 14-October-2008

Comments and additional material are welcome
(even "Hey - you've got a typo at..." messages...

Note: If you decide to print this file to put in your Mitrek manual make sure that the circuit diagrams below show up in a monospaced font (like Courier) if you want to see them properly.   I used an exclamation point as a vertical line since the actual vertical line character (upper-case backslash on many keyboards) is one of the first ones replaced by a local language character in non-USA keyboards and as a rule I try to make my writeups as international-compatible as possible.

This writeup discusses how to modify any common Motorola Mitrek mobile two-way radio to add connections for a generic repeater controller - it does not matter if it's an NHRC, an Advanced Computer Communications (ACC), an S-Com, an ICS, a Link/RLC, a homebrew controller, or an IRLP computer interface board.   These radio modifications are completely frequency independent - all they affect are receiver and transmitter audio connections and the control signals (push-to-talk, COR, etc).

This modification set was originally developed by necessity for a dual UHF radio configuration (one radio for receive, the other for transmit) that implemented one end of a full duplex point-to-point link, but will work on any dual Mitrek configuration, a regular single Mitrek mobile radio, a duplexed Mitrek mobile, a tabletop base (Motorola marketing dubbed it the "Super Consolette"), a tabletop base converted into an intermittent duty low power repeater or a Mitrek used in mobile extender usage.

And here's the disclaimer. The circuits and techniques described in this writeup worked for us and scratched our itch. As with everything in the hardware side of amateur radio (i..e experimental electronics), someone else always has a better way of doing it. If you do, please cut and paste the appropriate section of this article text into an email, fix it, and email it to us so we can update this web page with the better method.

I started this mod writeup as a set of handwritten notes on ruled notebook paper in the back of my Mitrek 3-ring binder as I read through the Mitrek manual.   Then I got my hands on some surplus Mitreks, and Googled every mention of "Mitrek" and "modify" or "modification" or "repeater" or "remote base" on the internet and printed every article.   Then using the schematics from the manual I reverse engineered every published mod to see what it did, and on some I tried them out - for example audio mods were tested using an Hewett Packard 200CD generator swept from 100hz to 5khz and a Heathkit distortion analyzer.   This writeup is the result of my experiments and experiences.

All of the work described here was done in a negative ground environment.   The Mitrek mobile radio itself is designed for operation in either a negative or positive ground environment, but if you are using this info to help you build in a positve ground environment please realize that you are plowing a new field and are on your own.   In all my years of working on commercial two-way radios I can count the number of positive ground installations I've been involved with on one hand and have multiple fingers left over.

The reason I used Mitreks in my system was that the Mitrek is an excellent performer, readily available, inexpensive at US$10 to $30 (because everybody wants synthesized radios like Maxtracs where the Mitrek uses crystals - in fact it was the last crystal controlled mobile that Motorola made), and it is easy to work on (it uses through-hole components instead of surface mount). And if you do blow one up you are not out a lot of money.

I used dual radios in my point-to-point link since you can't duplex 406‑420 MHz Mitreks and keep them clean, so my initial implementation used two 450‑470 MHz identical Mitreks tuned to the regular user input and output frequencies of the far-end repeater (443 Mhz and 448 Mhz), later replaced by Mitreks on real link frequencies (420.xxx one way and 439.xxx the other way...   The 450 radios will make 439 easily, but not much below 435 (varies with the radio) so the 420 radios are required for the low end frequency of the link pair).

This is a "cafeteria collection" of modifications - you can pick and chose which ones you want to implement and no one mod is dependent on any other mod.   If you are going to use two identical radios to implement a repeater or a duplex link as we did in the beginning, I suggest you make the mods to both radios leaving half of each radio idle so that if something dies and you are in a bind you can simply find out what killed the radio, swap the two radio chassis and be back on the air, then at your leisure swap out the half-dead radio with a good spare.   Note that this means that to implement your first one-way point-to-point link (with any radio - not just the Mitrek) you need to buy at least four radios...   one for transmit from site #1, one for receive at site #2, one for a shelf spare, and one to blow up while getting familiar with the radio (and then to use for spare parts to keep the other three running).   You can also make the same four radios a duplex link, as long as they aren't 406‑420 Mhz Mitreks.   If you are implementing your first Mitrek-based 420 Mhz full duplex link you will need six...   (a pair at the near end, a pair at the far end, a shelf spare and the sacrificial lamb...).

This writeup assumes a certain level of electronics and mechanical competence.   It does not walk you through basic circuit construction techniques, how to add a 1:1 audio transfomer to prevent hum from a ground loop, how to solder, etc.   The target audience for this writeup is a mid-level or higher repeater builder or maintenance person who is unfamiliar with the Mitrek radio and is adding one to his/her system, or exploring doing so.

There is some deliberate repetition in this text - please excuse that as you read it, but since I wrote this document as a "cafeteria collection" a few things need to be mentioned in each relevant section.   Even with this separation into individual modifications I recommend that you read this entire document all the way through at least once before you heat up the soldering iron.

Make sure that you have a correctly working radio, set up on the frequency of interest before you start the mods.   Examine the radios carefully looking for leaky (physically) capacitors, etc.   As Neil McKie WA6KLA once told me about repairs to remote base stations or repeaters, "better on the bench than at the site".   You don't want to start modifying a radio, and when it's done discover it's not working, and you don't know if the problem is in the radio or the modifications.   And if you are going to do multiple mods, test all functions of the radio after each mod is completed.

I guarantee that you will also find typos, and at no extra charge.   A lot of these 50+ pages were done late at night after everyone else had gone to bed.   I don't mind emails pointing them out, either.   And to update one of my late fathers's old jokes, the new PC keyboard can't spell any better than the old one (he was licensed in Colorado at age 13 or 14, let it lapse when he moved to Toronto, then relicensed in the late 1970s in Los Angeles as WB6SOX (my mom was WA6SOX)...   he was an old time two-finger-typing newspaperman and a professional photographer for over 50 years...   the original joke referred to when he upgraded his office from an Underwood mechanical typewriter to a Smith-Corona portable electric typewriter (complete with a power carrige return!) in the late 1960s and then again to an IBM Selectric in the 70s. He used to joke that the new typewriter couldn't spell any better than the old one.

The modifications described in this document are generic and nothing in the repeater controller needs to be adapted to the radio, with one exception: if you plan on using the the stock Mitrek carrier squelch to drive the COR / COS / carrier detect line (the signal to the repeater controller) then the controller has to be able to mute the audio coming in from the receiver (the repeat audio)... the old ACC brand controllers couldn't do that - they relied on the receiver squelch circuit to do the audio muting or using the not-too-well-documented mod of installing an audio delay board and strapping its reset line to the COR signal. Both the Scom (now ICS) and Arcom audio delay boards were specifically designed to do the same trick in almost any controller, even an ACC.   This article covers that situation.   Yes, either the Arcom board or the Scom board (now sold by ICS) will replace the ACC delay board just fine.   If you use the Link-Comm RLC-MOT board to add a Micor squelch to the radio (highly recommended) then even this concern is eliminated.

The interfacing modifications and methods described here have been personally tested by me on three different controllers. One of the test bed controllers was a home-brew Z80‑microprocessor-based unit, the second was a home-brew PIC processor, and the third was an Scom 7K.   Emails have reported that this information has been used on several single simplex radios (used as remote bases), several duplexed radios (used as repeaters or as duplex point-to-point links), several dual radio combinations (used as point-to-point links) including the one described in this article, and at least two full duplex tabletop base stations (used as tabletop repeaters), and with controllers by Arcom, Creative Control, Computer Automation ("CAT"), Link (RLC-series), Pacific Research and NHRC, so I am confident that there are no hidden problems in these procedures.

Note that everyone who plans on using a Mitrek (especially UHF) in repeater or link service should read (and then re-read) the excellent Cactus Radio writeup on these radios located at http:// www.cactus-intertie.org/LA/tec_not5.pdf.   In fact, click on it now and go read it and save a local copy.

I keep all of my Mitrek manuals in a 4-inch 3-ring binder and also included are documentation sheets on my mods plus printouts of various Mitrek related web pages.   You should print the Cactus Radio writeup, read it, 3-hole punch it, and put it into your Mitrek binder - it is a very well done writeup and is full of excellent technical info.   There is one conflict between Cactus's writeup and what I have done here, they bring repeat audio out on the connector pin that I use for the PL decode logic level, but please read both sets of info and make your own choice on signal routing.   If you have or are working on a UHF Mitrek I especially call your attention to page four of the Cactus writeup concerning the transmitter tuneup precedure and capacitor C‑800L (that's C-eight hundred-L).   I lost two UHF transmitters before I discovered that quirk... So unplug the PA deck from the exciter before you start the tuneup. More on this later.

I recommend that you read this entire writeup completely (at least once).   If you have already read through this and are returning to reference a certain section you can jump ahead to any section using these jump links:

These mods were developed using a pair of T34JJA 30w (low power) UHF Mitrek radios as test beds.   The radios started out life as mobiles on a 464 Mhz and 469 Mhz commercial community repeater frequency pair (chassis numbers HUE1074C and HUE1082C).   You can look up a chassis number or model number and see what it translates to on the model and chassis number lookup list.   If you have a Mitrek chasssis number that is not listed there please send me full information so I can add it.   Please don't email me asking about any chassis number not listed there - all the info I have is already up on the web page.   Note that many of the Mitrek chassis are very similar - for example, the same high power chassis can have any of eight different chassis numbers depending on if the receiver preamp was factory installed or not, and if the output power control variable resistor was set to 60, 75, 90 or 100 / 110 watts during produciton line final test.

When I started this writeup I had low band, UHF and consolette (tabletop base station) manuals in the file cabinet, and before web-ifying this writeup I compared them to a friend's high band manual, mainly to make sure the component designators for the resistors, capacitors, jumpers, etcetera that I mention here did not change between bands.   Since these mods only affect the audio and squelch circuits they should transfer over to other bands (excluding 800 Mhz as I have no manual) with no problems, but they have not been tried on actual low band or high band hardware (yet)   (June 2003 update: I have since received emails from four different folks that have converted high band radios using this information and they state that everything matches).   And courtesy of a reader who had two, there is now a highband manual in my file cabinet.

Get a manual! I repeat - Get a manual!
This document is complete as I can make it as far as how to connect a Mitrek to a repeater controller, but is not a substitute for a real manual - and you will need one anyway to tune up the radio.   See the
Mitrek Index web page for a list of manuals.   You can call Motorola Parts at 800-422-4210 and they will be happy to take your credit card or ATM card info and ship you a manual.   You can also watch eBay and the other auction web sites for Mitrek manuals... Click here for an eBay search on "Mitrek".   Some are out of print, some are still available from Motorola, at about US$30 each.

The Mitrek "Super Consolette" tabletop base radio has it's own supplementary manual: 68‑81040E80.   You don't need this manual to do any of the mods listed here, but if you end up with a tabletop Mitrek base it's very, very nice to have, especially if your unit has any options like DC (or AC) wireline remote control, VU meter, battery back up or channel scan (yes, Moto offered a scanning base station).   Note that you also need the appropriate mobile manual (low band, high band, UHF, 800 Mhz) to go along with it as the Mitrek tabletop base is simply a pretty black box that contains the power supply, the volume and squelch controls, the frequency switch, a speaker, a 1:1 audio transformer (between the Mitrek chassis and the speaker) and a mobile radio chassis.   And if you end up with the high power (i.e. the larger) power supply then you can run a low power tabletop repeater.   The chassis even has a hole for a second antenna jack pre-punched (on some it's hidden behind a black label).   This web page has more details on the "Super Consolette".

As long as your are ordering manuals, you will want to order the Mitrek tuning tool set, part number 66‑82977K01 (unless you have a huge collection of diddle sticks - I have the standard 20-year collection that fills a wooden cigar box....   even a piano tuner's wand for doing the old solenoid-tuned wide-spaced UHF Motracs).   The real Mitrek tool is only US$2.92 (as of May 2004) and cracking or breaking a slug with the wrong tool is not worth three dollars.
(Update: January 2006... It's now $4.20, still cheap compared to a cracked slug - and tuning tools break at the most inopportune times - I suggest that you order two, and keep the second in the wrapper in your kit. When you break #1, unwrap #2 and stuff the wrapper in your pocket, then use #2 to finish the job. When you get home and empty your pockets you will find the wrapper and that will remind you to order #3.)

The Mitrek Plus supplement is 68‑81046E05 and is definitely worth getting if you can get one...   it includes the mods for using a Motrac / Motran / Mocom 70 control group with the Mitrek.   Unfortunately the last time I called they were out of stock and not planning to reprint them.

For more information on manuals and how to order them, see the "How to order manuals or parts from Motorola" web page that can be reached from the main Motorola page.   The "Figuring out what you have" page can also be useful.

Another note...   the manuals are not consistent on the option boards: the UHF book that I have (68‑81045E75‑A) is the only Mitrek manual I have in my collection that has documentation on all three PL and DPL boards - the HLN4020 single or dual reed-type PL board, the HLN4181 "reedless" (also known as the "tone-element") PL board and the HLN4011 Digital PL (DPL) board.   And the late UHF book - the 68‑81045E75‑B does not have the later HLN4181 (reedless) board in it! (at least my copy didn't). The 4181 schematic is downloadable from this web site.

The ‑A revision highband book I looked at does not have the HLN4020 reed-type board schematic.   There may be a newer revision out there.

The ‑O original low-band book I have does not have the HLN4181 reedless PL board or the DPL board documentation.   There may be a newer revision out there.

Note also that the Mitrek is an all-metric radio - don't casually mix the leftover screws into your parts bin.


Now I'm going to define some terms that I'll use in the rest of this writeup:

Lastly, be aware that the same jumper number can appear in multiple places - there can be a JU3, for example, on the main board, another JU3 on the interconnect board, a third JU3 on the PL board and maybe a fourth JU3 inside the control head.   I've been very careful in this writeup to specify where a specific jumper is when I reference it, but keep this in mind, especially if you are reading someone elses writeup.

CAUTION:     DON'T LET THE SMOKE OUT !
The radios that preceeded the Mitrek in the Motorola mobile product line mostly used push-pull audio PA decks and audio output transformers, and drove the speakers as a hot wire to ground.   The Mitreks, and many later radios eliminated the audio transformer and they run the the speaker as the push-pull load directly ... both sides floating off of ground.   This quirk is significant on your workbench: any audio test equipment that connects to the speaker (such as you would use to make a quieting measurement) must be on the far side of a 3.2 ohm or 8 ohm 1:1 audio transformer.   Motorola has a low-power one as part number SLN6435 in their test equipment catalog (being "test equipment", it's high priced at about $80), and they also include a cheaper but higher audio power one in every tabletop base station (part number 25‑80188B01, about $35 in late 2006) since common wireline remote controls expect ground-referenced audio to drive the remote sepakers.   You can order either one as a spare part, or you can get one from a web-based car stereo seller or at a local high-end car stereo shop (many dash-mount radios expect floating speakers, and many trunk mount audio amplifiers have gounded inputs).   In a pinch you can put a 2 watt resistor of any value from 8 to 16 ohms on the radio as a load, and use a 600 ohm to 600 ohm line transformer between the radio and the test equipment.   Note that the audio bandpass characteristics of that isolation transformer will have to be wider than the audio frequencies you hope to pass through the radio.

In short, if you ground one of the Mitrek speaker leads, the odds are better than 75-25 that you will let some of the magic smoke out of your radio.   So use an audio isolation transformer to prevent this - just put the transformer between the radio and any load that isn't an ungrounded stand-alone speaker.   If you are looking to buy an audio isolation transformer, sometimes they are marketed as "audio ground loop eliminators", or as "balanced to unbalanced adapters" - but some of them are at 600 ohm impedance levels, ideally you want one that is at 8 ohm levels at a watt or two of audio... (the audio level alone precludes using a transformer salvaged from a old modem or PC sound card).   There are also ones designed to work at millivolt levels - they are a collection of resistors and capacitors.   No matter what, check the schematic - you want a transformer-based one that has no DC path from either input lead to either of the output leads.   If your junk box includes two identical audio transformers that have an 8 ohm secondary (no matter what the primary is, as long as it is the same) then you can use the pair to make a single substitute 8 ohm to 8 ohm audio isolation transformer.   Just tie the primaries together and use the secondaries of the two transformers as if they were one 8 ohm to 8 ohm transformer.   And BTW the most flexible audio isolation transformer I ever found was an aircraft radio power transformer from the 1940s - the two separate 6.3 volt windings worked fine as 8 ohm windings, the 110/120 volt winding worked fine as a line level connection, and the 400hz design worked just fine from 250hz to 3500hz.


Since a repeater or a point‑to‑point link is a single frequency beast the four frequency select leads plus the "SP" (spare) lead in the cable (pin 18 - the middle big pin) are available for reuse as input and output pins.   Note that there is a 0.01uf bypass capacitor between the F1 lead (pin 7) and ground (at C602).   If you are going to reuse that pin, you might want to remove C602, depending on how you plan to use it.   If you decide to yank it, look for it right next to the Frequency #1 transmit channel element.   It's interesting that the F2, F3 and F4 lines do not have similar bypass caps.

There are two versions of the Mitrek: the early "Mitrek" and the later "Mitrek Plus", however you can't tell which one you have without looking at the radio's interior or looking at the chassis number.   The differences are fully documented in the Mitrek Plus supplement manual I mentioned above and among other things the "Plus" radios have a better front end physical design, extra IF filtering and some filtering chokes on the control cable pins.   The Mitrek Plus really is the preferred radio when you go hunting for Mitreks.   Not that the original Mitrek isn't a fine radio, just if you have the opportunity to paw through a pile of radios it makes no sense not to pick the better / nicer ones...
GENERALLY, the "A" series is the "Mitrek", the "C" and later are "Plus", and somewhere in the "B" series the transition took place. But look for the RCA plugs and the crystal filter first (see later in this text).

To determine which version you have you will have to do one of the following:

  1.  Look at the chassis number (the Mitrek Chassis and Model Numbers web page lists all the ones that I know of)


    Photo 1: In this photo you can see the HUE1082CPR chassis number stamped on the front lip of the chassis.   On this particular radio the "U" looks strange in the photo because it has a screwdriver scratch through it.

  2.  This is the easiest an d simplest thing to look for: the RCA connectors on the front end / exciter filter assembly.   Trace the two coaxial cables from the antenna relay.   One of them will go to the middle of the tuned lines right in front of the PA deck.   The early radios had the cable hard-wired into the front end, the later ones had RCA plugs.   You want the ones with the RCA plugs.

    Photo 2: This is the early casting.   The unjacketed silver braid cable on the left in this picture comes from the antenna relay and connects to the receiver front end - note that it is wired in to the tuned circuit casting.   The white teflon cable on the right couples the transmitter exciter output to the PA deck input.
    Photo 3: In a Mitrek Plus the same coax has RCA plugs.   You really, really want to see this casting.   The big screw on the left (the out-of-focus "L1" at the bottom left) is the receiver first helical resonator, the smaller 4 screws on the right ("L12", "L11", "L10" and "L9") are the exciter helical filter.

  3.  Look for Y204 - the extra receiver IF crystal filter:

    Photo 4: This is an early Mitrek - the pen points to the jumper that replaces the filter (this is a carrier squelch radio - no PL board - note the red connector at the bottom left - that is where the PL board plugs in.   And as mentioned elsewhere in this writeup, the red connector indicates the later plating.)

    Photo 5: This is a later Mitrek Plus - the black pen points to the extra crystal filter - You want the ones with the extra filter.   If you need to add Y204 to a radio it's Moto part number 48‑84396K02.

  4.  Look the back side of the interconnect board around the back of control connector for several RF chokes.   In a number of places where the interconnect board silk screen has JU (for jumper) the Mitrek has a wire jumper and the Mitrek Plus has a choke - but not always.   The later interconnect boards had the "L" designation in the silk-screened labeling, the earlier boards had "JU".   There were Mitrek Pluses made with L-designations and jumpers in place.   Finding a radio with chokes is nice, but a later board (with L-designations) is better (chokes are easy to make and can be added).

    Photo 6: This is the back side of a later interconnect board. Note the "L" markings in white ink at the top.   Early boards had "JU" markings.
    Photo 7: This is the front side of the same interconnect board - but it has jumpers (the wires with white sleeving) in place of the chokes.
    Photo 7A: This is the front side of a later interconnect board - when Moto switched to automated assembly the hand-installed wire jumpers were replaced with zero ohm resistors which could be inserted by the component insertion machines.   These jumper replacements can be identified by a single black band in the middle of the resistor body (single black band = zero ohms).

  5.  Another artifact to look for the two large tubular electrolytic capacitors on the left side of the interconnect board - they are 500uf in the Mitrek, and are 1,000uf in the Mitrek Plus - but I have found 1,000uf capacitors in the radios that don't have Y204...   almost if the parts list used to stuff the interconnect boards during manufacture was updated and a large number of boards produced before Y204 was added to the main board.   Note that you may have to remove the interconnect board from the radio to verify the value of the capacitors.   If you can't, note the body length and lead bending angle.   But realize that the larger caps only indicate a later production run of the interconnect board and in our use (repeater or link service) are totally irrelevant (they only affect the local speaker audio).

    Photo 8: This is the interconnect board in a Mitrek - the pen points to the upper 500uf capacitor (there are two identical caps, one above the other)
    Photo 9: The capacitors in a later Mitrek Plus are 1000uf.
    The small board visible behind the interconnect board is the optional timeout timer.   If you accidentally sit on the microphone it drops the PTT after 3-4 minutes and generates a really loud and obnoxious tone through the radio speaker.

The Bottom Line:   If you have a choice when you select a radio, pick a Plus radio by the chassis numbers that are on the Mitrek Chassis and Model Numbers web page, but if you don't have that list with you (I have it in my Palm Pilot) by all means pick the radios first with the the RCA connectors on the front end / exciter filter assembly.   The second item to look for is the Y204 extra IF crystal filter - but if it's not there is can be added easily.

Channel Elelements: Get at least one set of channel elements with each radio - and really, really try and get the original elements - and then test it using a dummy load and a calibrated signal generator before you make any modifications.   You don't want to spend a lot of time converting a radio then find out it's dead, and you can't tell if the problem is in the original radio or the conversion.   If you can't get the original elements, then try and get ones that are close in frequency for testing and can later be recrystaled to your needs.   Make sure that the elements you do get are the proper ones...   one of my six meter radios came with UHF elements in it - and I suspect the eBay seller obviously knew they were the wrong ones because he had carefully removed the element model number / frequency stickers.   Note that GMRS service requires 0.0002% tolerance.   If you end up acquiring an ex-GMRS radio you might want to put the high-accuracy elements aside for recrystalling on your repeater or link frequencies. And note that the crystals are cut differently between 0.0002% and 0.0005% elements - they will be off frequency if you use the wrong elements.

The Mitrek and MSR-2000 use the same elements, as described on the Mitrek / MSR-2000 channel element page.   You can change the crystals in the elements yourself, but it's better if you have the rock chippers do it.   I suggest you read the article titled "Why should you really spend $50 to re-crystal a channel element or ICOM" on the Tech-Info page at this web site.   While on the topic of crystals, the transmitter uses 12 Mhz rocks (with a x3 multiplier on low band, a x12 multiplier on high band/VHF, and a x36 multiplier on UHF), and the receiver uses 48 Mhz rocks and low injection (on VHF the calculation is (channel frequency-10.7 Mhz)/3, on UHF replace the /3 with /9).   Remember that like most Motorola radios in a Mitrek the first tripler is inside the element.   I've used 30-year-old scanner crystals tack-soldered into a receive element for a simple bench receiver tuneup, but never for permanent use. I've also used an element as a buffer for an RF signal generator to get a head-start on tuning a receiver from 460MHz down to 439MHz before the recrystalled channel elements arrived.


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 Motorola mobile radio 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.
Foam rubber gasket

Photo 11: Don't be surprised if some radios have a foam gasket on the control connector. This was very common in dusty and humid areas.

If you are picking radios and heads out of a pile to build a repeater or a single frequency link pick a single-frequency head with the monitor switch and be sure to pick out a multifrequency cable.   Moto supplied two different cables with the Mitrek, one without pins 6, 8, 9 and 10 and the other with them.   Look for white, grey, blue and black wires (labeled on the control head end with the numbers).   Also make sure that you have a pin 18 - it is a black-grey wire.

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.

Mitrek Control Cable Pinout
The view below from front of the radio or back (wire side) of control cable plug.
+---------------------------------------+
!                                       !
! 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 !-------------
!                                       !
+---------------------------------------+
The "(T)" in the drawing above represents the large thumbscrew
that secures the control cable connector into the radio.
Control
connector
pin
Cable
lead
color
Early
Interconnect
Board
Jumper
Later
Interconnect
Board
Jumper or
Inductor
Function of the pin in the radio
1 Brown JU5 L1 Microphone audio hot (center conductor of a shielded cable)
2 Shield     Microphone audio ground (shield of pin 1 cable)... This is the control head audio ground
3 Orange (see note 1) (trace cut) Speaker (floating !)
4 Yellow     Switched and fused +12vDC from the control head to the radio (receiver power)
5 Green (see note 1) (trace cut) Speaker (floating !)
6 Blue     Frequency select switch wiper - If operating without a control head jumper this pin to pin 17.
7 Violet JU6 L7 Frequency 1 select
In this collection of mods this is used for PL encoder control (from the repeater controller to the PL board inside the radio).
8 Grey JU7 L8 Frequency 3 select
In this collection of mods this is used to send the COR / COS / Channel busy signal from the radio to the repeater controller
9 White JU8 L9 Frequency 2 select
In this collection of mods this is used for repeat audio from the receiver to the repeater controller
10 Black JU9 L10 Frequency 4 select
In this collection of mods this is used for repeat audio into the transmitter from the repeater controller
11 Blk/Brn JU10 L11 Buffered discriminator audio to the control head Volume and Squelch pots
12 Blk/Red     Switched and fused +12 from control head to the radio (transmit enable) (jumper this pin to pin 4 if you aren't using a control head)
13 Blk/Orn JU11 L13 PTT in (ground to transmit). You will want to put a disable switch in series with this pin (see the schematic later on)
14 Blk/Yel JU12 L14 Squelch pot wiper (from the control head to the radio)
15 Blk/Grn JU13 L15 Volume pot wiper (from the control head to the radio)
16 Blk/Blu JU14 L16 Ground to put the receiver into PL mode - this lead is normally hooked through the control head "Monitor" switch to the microphone hang-up box, then back to the head to connect to audio ground.   More details on this signal are in the text.
17 Big Black JU1 JU1 (big pin) Ground - DO NOT CUT JU1
18 Blk/Gry JU15 L18 (big pin) See the note below.
This is the so-called "SP" lead in the control cable, short for "Spare". It connects through JU15 and then to one side of both JU3A and JU3B on the interconnect board.   In these mods it's used to carry the PL decoder logic level output (PL sense) from the radio to the repeater controller.
19 Big Red JU16 L19 (big pin) +12v (This pin supplies power to the PA deck directly, and via JU16 / L19 to other sections in the radio)
Note 1: There is an unlabeled jumper on the early interconnect boards - if you want to use it you'll have to trace it to find it.   The later boards (with silk-screened "L" designators) have traces that can be cut to accomplish the same functionality.
Note 2: In a low band radio pin 18 is normally used to enable or disable the "Extender" circuit (a noise blanker). It's wired to a slide switch that is on the back of the control head. If you are modifying a low band radio for repeat or link duty and you chose to use the Extender you will need to do some re-engineering to handle both the it and the PL decode line. One way would be to wire the Extender to a toggle switch mounted in the front of the radio and leave Pin 18 to the PL decoder (you will discover that 99% of the time you will want the Extender operating - but you can't use an extender equipped receiver on a pass-notch duplexer).

A note on control cables...
A lot of Mitreks were installed in 18-wheel trucks.   Some of those control cables end up on the surplus market wired for positive gound.   Because manufacturers (Mack, Kenworth, Peterbilt, etc.) had not standardized the polarity of their tractors when the Mitrek was introduced, it was common for Mitrek cables to be modified in the field from negative to positive ground use.   As a result, part numbers stamped or printed on cable assemblies may not correctly reflect the polarity for which they have been wired.   It is best to refer to the factory negative ground versus positive ground cable diagram in the manual BEFORE you use that cable. (thanks to KI4BQQ for the reminder)

If you are going to use a Mitrek that does not have the chokes on the interconnect board in a repeater system please consider replacing the jumpers with chokes in the 5 or 6 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 one layer of 24 or 26 gauge enameled wire (three layers if you feel energetic) 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 (see the trick below), and a drop of varnish, hot-melt glue, or even clear fingernail polish on the starting end of each layer 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 over the entire resistor will finish the job.   As long as the resistor value is 1K ohms or more the actual value doesn't matter as the choke will short it out.   Personally, I use whatever value I can get cheap at the surplus store.   At the time I was interfacing the link radios to the controller (i.e. doing the mods that resulted in this writeup) there were 1/4w 10K and 1K resistors at 25 cents a dozen, and 1/2w 2.2K resistors at a penny a piece.   A free source of enameled wire is the deflection yoke from an old TV set.   Stretch it by clamping one end in a bench vise (or wrapping a few turns around a fence post or your car's trailer hitch) and string off 10 to 12 feet of wire and lean back with your whole body weight...   you will feel the wire stretch a little - that's all it takes.   Then wind the choke with the stretched wire and it will lie flat as you do so.   My dad (WB6SOX) taught me this trick - he claimed to have learned it from a Western Union telegraph lineman in the 1920s or 1930s as they ran the telegraph lines along the road in front of the family farm outside Cripple Creek, Colorado.

If you decide to improve the RF filtering of a Mitrek and you intend to do all of the mods listed here you will need to replace the jumpers with chokes at pin 1, 7, 8, 9, 10, and 18.   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 one leg of the choke into the hole that connects to the connector pin, but install it on 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 one of the large speaker coupling capacitors - you will need to temporarily lift one end of one cap to access the jumpers.

As long as you are inside the radio lift off the PA deck cover and do a visual check of the tantalum electrolytic bypass capacitors on the positive 13.8 volt supply rails - and use a good magnifying glass and good lighting.   Replace any that look suspicious.   The failure mode is that they go down in value or open up, and either can allow spurs and grunge...   A second problem is that some radios didn't have enough driver stage bypassing.   If you still have spurs (rare) add a large value cap on the driver stage +12v lead.   Another Mitrek failure mode is intermittent connections in the preselectors, and the Cactus writeup mentioned above has the cure for it, as well as WB4HFN's web page...   see http://www.wb4hfn.com/Modifications/MT2311.


Power Connections:
In a normal mobile installation pin 19 is hooked to the positive side of the vehicle battery through a large diameter cable and a big fuse - and the fuse is usually placed under the hood near the battery.   Both constant and ignition switched power is run to the control head via relatively small wires (#20 ga).   In a normal mobile installation the receiver can be run with the ignition key off, and the key must be on for the transmitter to be enabled.

Normal mobile installation:
                                     thick wire
Pos --- Big Fuse ----------------------------------------------------- Pin 19 of 
12v     Amperage depends on                                            the control 
        the transmitter power                                          connector 
        level of the radio.                                             (big pin)
        See the manual for                                             (PA deck +V)
        the value.

Battery                               thick wire
Gnd ------------------------------------------------------------------ Pin 17 (big pin)

                                       inside the control head
                                       !---------------------!
                                       !                     !
                                    control                  !
Local                                head                    !
Gnd --------------------------------Pin 17                   !
(local to the                          !                     !
control head)                          !                     !
                                       !           /         !
constant +12 from                   control       /         control     control 
underdash fuse box                  head         /!         head        connector
(e.g. the battery  ---- 7A fuse ----pin 19 -----O !  O---+--Pin 4 ------Pin 4
side of the clock                      !          !      !   !          (receiver power) 
fuse)                                  !          !    J O   !
                                       !          !    U !   !
                                       !          !    1 !   !
                                       !          !/   0 !   !
switched +12 from                   control       /    1 O  control     control 
underdash fuse box                  head         /       !  head        connector
(e.g. the battery  ---- 7A fuse ----pin 20 -----O    O---+--Pin 12 -----Pin 12
side of the broadcast                  !    double pole      !          (transmit enable)
radio fuse)                            !    power switch     !
                                       !   (part of the      !
                                       !   volume control)   !

If you choose to use a control head there is an already-existing jumper (JU101) option inside the head (left open by default) that shorts pin 4 to pin 12.   If you use JU101 you can ignore control head pin 20.

If you are not going to use a control head (for example, mounting the radio on a rack panel, with the controls mounted to the rack panel or in in the lid of the radio), or if you want very simple wiring, we can consolidate and simplify the power wiring as per the schematic below (using the power switch on the power supply as the on/off switch):

                        thick wire       Control connector
Pos --+-- Big Fuse ----------------------Pin 19 (PA deck and transmitter) 
12v   !
      !                 thin wire
      +-- 5A fuse ---+-------------------Pin 4 (receiver power)
                     !
        thin wire--> !
                     !
                  \  O  This transmit enable switch can be installed in the  
                   \    body of the control head if you like. Find the pads 
                    \   labeled JU101 on the control head PCB and use them
                     O   for the switch.
                     !
                     !  However keep reading - there is a better way to do
                     !  it: short JU101 (or jumper pin 12 to pin 4) and 
                     !  relocate this switch in series with pin 13 
                     !  (PTT) - this is shown later on in the PTT section.
                     !
                     +-------------------Pin 12 (transmit enable)

                                      +--Pin 6 (frequency select logic)
                                      !
                         thin wire--> !
     thick power wire                 !
Gnd ----------------------------------+--Pin 17 (radio ground)

The size (amperage) of the big fuse depends on the band and power level of the radio and is covered in the Mitrek manual. By the way, the receiver and transmitter enable pins (pin 4 and pin 12) draw very little current.

The size of the wire you will need for the "thick wire" connections above is dependent on the current flow and some basic electrical theory.   Large diameter cables have less resistance, and E=IR.   Low current means the voltage dropped across the power supply cables is minimal.   Now when you keep the same power supply cable in the circuit (R remains constant), and you attempt to transmit (draw lots of current through the battery cable) the E (the voltage) dropped across the cable goes up and the voltage at the transmitter goes down.   The transmitter (not just the Mitrek - any mobile radio) does not like low voltage at all.   It gets squirreley if it works at all.   Basically, for a low power radio (T2xJJA, T3xJJA or T4xJJA model numbers) the smallest you want to use is 12 gauge wire, and 10 gauge for any higher power radio (the T5xJJA, T6xJJA, T7xJJA, T8xJJA series).   Use the next larger size wire for any run over 10-12 feet.   You want a minimum of 13 volts DC across pins 17 and 19 of the radio while under full load.

Some folk run the hot side of the 7a fuse to pin 19...   I don't like to do that because if the big fuse blows for some reason it will kill the receiver as well.   You can run the 7a fuse either to the positive 12v directly or through the big fuse, whatever works easier for you.   In my case I was dedicating an Astron power supply to the link double-radio assembly plus I was using a control head so I added a large in-line fuseholder for the PA deck power (radio pin 19) plus a small fuseholder for everything else.   I put both of the fuse holders in the cable harness that coupled both radios to the control head and the power supply.   A diagram of the power wiring is later on in this writeup.

Control Head Connections, and how to make it work without one...

If you are not using a control head you can connect your own speaker, volume and squelch controls as follows (don't forget to ground the Frequency 1 channel element pin inside the radio - read further on for notes on JU611):

        interconnect board and ! control !
        control connector      !  cable  ! control head
       !-----------------------!---------+----------------+
       !                       !         !                !
        +!!C1                 
radio ---!!----------Pin 3 ----------------pin 3---Pin 29----
main     !!                                (in)      or       \ 
board                          !         !         Pin 31    speaker - note that
                               !         !          (out)    both sides are hot!
        +!!C2                                                 /
radio ---!!----------Pin 5 ----------------Pin 5---Pin 32----  
main     !!                                 (in)    (out)
board                          !         !
                                          
radio---------JU13---Pin 15 ---------------Pin 15-------------------+
main          L15    Volume                on head                  !
board                wiper     !         !                          !
                               !         !                          !
        +!!C3                                                       !
radio----!!---JU10---Pin 11 ---------------Pin 11---+-----+         !
main     !!   L11    Buffered              on head  !     !         !
board                audio out !         !          !     !         !
                  (unsquelched)!         !          \     \         !
                               !         !    3.3k  /     / 10k     !
                               !         !    1/4w  \     \ 1/4w    !
                               !         !     or   /     /  or     !
                               !         !    1/2w  !     ! 1/2w    !
                               !         !          !     !         !
                               !         !          !     !         !
                               !         ! 25k pot  \     \ 25k pot !
                               !         ! Squelch  /     / Volume  !
                                                    \     \         !
radio---------JU12---Pin 14 ---------------Pin 14-> /     / <-------+
main          L14    Squelch               on head  \     \
board                wiper     !         !          !     !
                               !         !          !     \ For this 
                               !         !          !     / resistor 
                               !         !          !     \ see text
                               !         !          !     !
radio----------------Pin 2-----------------Pin 21---+-----+----------Pin 27--+
main                 Audio                 on head                  on head  !
board                ground    !         !                                   O  /
                               !         !                        Hangup box   /
                               !         !                        (mic clip)  /
                               !         !                /                  O
         1.8k                                            /                   !
radio--+--R---JU14---Pin 16 ---------------Pin 16-------O  O---------Pin 24--+
main   !      L16    PL/Carrier            on head   "Monitor"      on head
board  !             Squelch   !         !          switch inside
       !             select    !         !         the control head
PL     !            (gnd for   !         !     (PL / Carrier squelch select)
board--+             PL)

The actual Monitor switch mounted in the Mitrek control head is DPDT, and the second pole is used with other options like channel scan, 2-tone decode, or PAC-RT remote extender (mobile repeater).   All six switch connections are brought out to pads on the control head PC board.

Capacitors C1, C2 and C3 are located on the interconnect board.   C3 is either 4.7uf or 10uf depending on the vintage of the radio.   If you are going to run your Mitrek as a mobile and you find a 4.7uf cap in your radio as C3 I suggest you change it (or just parallel another one across it) as the higher value improves the receiver audio at really minimal cost.   C1 and C2 are 500uf in the Mitrek, 1,000uf in the Mitrek Plus (if you have 500uf caps don't bother changing them unless you are putting the radio in mobile service and need lots and lots of speaker audio - like in a fire truck...   In a repeater, in link service or as a mobile in a regular passenger vehicle, don't bother).

The lead for pin 16 is not required in 95% of the amateur repeater or link applications - most repeater controllers use separate carrier and PL decode connections rather than one line and a carrier squelch / PL select line.   Most radios in repeater or link service usually sit in carrier squelch mode forever, and the repeater controller does all the work in selecting PL or Carrier squelch mode by selecting either the carrier squelch or PL decode line, or ANDing them together.   If you end up needing an extra wire in the control connector you can cut the JU14 / L16 jumper (disabling the carrier / PL switch on the control head) and solder to the pad on the interconnect board that connects to pin 16.   Then tie into pin 16 by the head.

The resistor in the bottom lead of the volume control is in most control heads - it is there so that the regular commercial 2-way radio user cannot turn the receiver volume down to zero.   In my case, I shorted it out as I was going to be installing the Mitrek link radio at a site where there are many racks of radios, and I did not want anybody in the room to be listening to the conversations on the system.   Make your own choice, but if you are going to be inside the head for some other reason, why not ?

If you are trying to build up a minimal system you could mount two trimpots for the volume and squelch pots inside the radio and tie them to the radio side of JU10 / L11, JU12 / L14, JU13 / L15 and to ground.   Doing so, however would free up pins 14 and 15 which could be used for other things, at the possible expense of not being able to bench test the radio with a standard cable and head.   Or you can mount mini-pots (the ones with 1/8" diameter shafts) inside the control plug shell.   If you are going to mount the volume and squelch controls any distance from the radio remember that Moto uses a braided shield around the conductors connected to pins 11, 14 and 15 on the control cable connector, with the shield hooked to pin 2 on the radio end (audio ground) and pin 21 on the control head end.

Don't even try to try and make it work without some kind of a squelch pot - the squelch circuit uses high frequency noise (usually above 6khz) from the discriminator to close the squelch when no signal is present. The squelch pot supplies descriminator audio to the squelch circuitry. When a signal is present, (up to full-quieting...) the high frequency noise goes down in level to the point of going away completely, and the squelch opens. Without the pot, no audio arrives at the squelch circuit, and without any high frequency squelch noise audio the squelch opens assuming a signal is present.

The stock Mitrek volume and squelch pots are audio taper 25k ohms which is not a common value in the electronic component world (especially in the surplus world) - the nearby common ones are 10K and 50K (personally I've used both 10K and 50k pots in several receivers and not noticed any difference).   The standard grey Mitrek "clamshell" control head and white Micor control head uses PC board mount pots with shafts and knobs, and the "ACM" (short for "Alternate Control Module") head uses PC board mount thumbwheel pots.   If you are going to mount the controls on a rack panel and you want to be precisely correct in value then you need to locate an old Motrac, Motran or Mocom-70 mobile control head to scavenge the panel-mount 25K pots from.   DO NOT try to use the earlier radio's head directly on the Mitrek without modification as the older radios had slightly different control head wiring that included a grounded speaker connection, where the Mitrek runs both sides of the speaker hot to ground.   If your old head came with the palm microphone you can scavenge it and the mic jack and use it on the rack panel as well.


In the rest of this writeup I am going to assume that you are either modifying a Mitrek Plus radio (with the factory inductors on the interconnect board) or you have replaced the above mentioned JU6 / L7, JU7 / L8, JU8 / L9, JU9 / L10, JU15 / L18 and maybe JU5 / L1 jumpers with homebrew inductors...

I'm also going to assume that if you are going to use PL, and that you have a factory PL board in the radio.   Unless you need simulataneous decode and encode a used HLN4181 is much cheaper than a TS-64 and besides it was made for the Mitrek.   These days it makes zero sense to not have a PL decoder, even if you are going to run a carrier squelch repeater.   Besides, you can program the repeater controller to require PL to do any commanding.   In addition, there are too many times where it would be nice to have the ability to select the other mode.   If you don't have a HLN4181 then buy one from a used equipment vendor like Telepath in San Jose, Air-Comm in Phoenix, or C. W. Wolfe in Montana.   I have another web page here at www.repeater-builder.com that shows how to make your own TLN6824 PL tone elements for the HLN4181.   If you do dig a Com-Spec TS‑32 out of the junk box this web page on connecting the TS‑32 may help.   If you use a TS‑64 here's the Com-Spec TS‑64 instructions for the Mitrek.

Don't forget to jumper the select line for the Frequency 1 channel elements to ground or you will be scratching your head while you look at a very dead receiver and transmitter.   The easiest way to do this is to cut or remove (if present) JU601 and 610 and insert JU611 (it's near the receiver channel elements, photo below, and when inserted enables both the F1 receive and transmit channel element).   Note that JU611 grounds the F1 lead on the front panel connector (pin 7) that we will be using for something else in the next step, but we can fix that very easily.


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 HLN4181 or JU3 of the HLN4020 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 function 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 rapid deadline - from concept to installation of fully operational hardware in a few weeks, and on this particular project I did as much as possible to minimize any debugging time.   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 receiver COS signal without worries of a worn-out relay.   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... Several manufacturers make FET-output optoisolators, so there is second sourcing.   In other words, don't use single-manufacturer-sourced parts unless you really have to and have sufficient spares on hand.


PL Decode
Photos of the three types of Mitrek PL / DPL boards.

All connections between the radio and the PL board are made through the 9-pin single-in-line 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 reed-type Mitrek PL 
board HLN4020
Photo 11: The HLN4020 reed-type board.
The two reed sockets are at lower center, with the black plastic hold-down arm above them.   The KLN6210 encode reed (the grey one in the photo) is on the left, the KLN6209 decode reed (the tan one) on the right.
The reedless Mitrek PL 
board HLN4181
Photo 12: The HLN4181 reedless board.
The long thin horizontal component at the bottom left is a TRN4224 tone element that is plugged into the black socket under it. note that it hangs over the socket on both ends.
The Mitrek DPL board 
HLN4011
Photo 13: The HLN4011 DPL board.
The wide white horizontal component in the lower middle of the board is a TRN6005 DPL code element.   Note that the element is almost flush with the socket on the right and is too short to occupy the left-most pins of the socket.
(anybody have a photo ?)
The YLN4011 DPL board.
The YLN4011 is a variant on the HLN4011 and uses a small PROM chip to allow selection of 4 different DPL codes using the 4 frequency select lines. If anyone has a manual on it we'd appreciate a scan or PDF of it.

While perusing the schematic of the HLN4020 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.

For more information on the HLN4020 dual-reed board, the HLN4181 reedless board and the tricks they can do, plus information on how to build your own TRN4224 tone elements please see my HLN4181 web page.

The rest of this section is focused on the HLN4181 "reedless" tone PL board since I had seven radios with five HLN4181s, and only one of the HLN4020 reed board and the HLN4011 DPL board.   Working for NASA for six 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.   Service trips should be limited to swapping entire units out, not fixing them on site (unless that is the absolute last resort).   Especially if it takes you a couple of driving hours to get to the start of the twelve mile long dirt road, and another hour or two in 4-wheel-drive to get up to the radio site...   and that's in good weather.   Naturally Murphy's Law says that things will fail (locking every transmitter on - even the links) during a snow storm the week after the road washes out in the biggest rainstorm in twenty years....

The HLN4181 reedless board can be set up in several ways...   The normal configuration of jumpers (and the most useful one) is listed in the table below...   The board can NOT run simultaneous encode and decode - the encode portion generates the PL tone continuously and the PL decode output goes active when the PTT line goes active.   I initially wrote this writeup in 2002, and at time thought that the board would work as a encoder-decoder, but never tested it as I always used two radios, one for transmit and the other for receive.   It wasn't until 2006 that someone told me that a duplexed radio would not run in PL when using the HLN4181 board as both the encoder and decoder.
I haven't tried this, but looking at the schematic suggests that you could cut the trace from pin 4 to E2 to lock the board in receive mode, and use an external encoder.   Or if you don't have a HLN4181 just use a Com-Spec TS-64 (which does do simultaneous encode and decode).

HLN4181 Reedless Mitrek PL Board Factory Jumpers
Jumper
JU-
Normal
setup
Function
1 OUT Normally used to couple the internal PL tone encoder to equipment outside the radio
2 OUT Disables the internal encoder when in place
3 IN In to couple internal encoder to the transmitter modulator (and remove JU7)
4 IN Install for "AND" squelch (and remove JU5)
5 OUT Install for "OR" squelch (and remove JU4)
6 OUT Install to disable PL decode (receiver carrier squelch only mode)
7 OUT In to connect external encoder (fed from control connector pin 18)
to transmitter modulator (and remove JU3)
8 OUT In to disable the encoder's reverse burst
9 IN When installed couples receiver discriminator audio into the PL decoder. Note that some early revision boards do not have a JU9.
If your HLN4181 board has JU1, JU6 and / or JU7 in place, remove all of them.   If you want your PL decoder to work then JU9 must be installed.   If you want the PL encoder to work then JU3 must be installed and JU2 must be removed.

If you are in a situation where the local speaker will be used and in a PL environment then in most cases you want JU4 in place and JU5 out...   AND squelch is much more pleasant to listen to than OR squelch.

JU4 and JU5 are redefined if the radio has the HLN4119 "Busy Light" option (very rare)...   for more information see the footnotes on the receiver and control head schematics in the manual.   The radios that do have the busy light option must be paired with matching control heads (the "busy light" heads have an 8-pin IC installed into the only IC position on the circuit board inside the head).

A PL decode signal is available at the collector of Q1 of the HLN4181 PL board, which is available at one end of the JU6 jumper.   This transistor takes the PL decode signal from the custom chip and inverts it before sending it to the audio mute transistor (located on the main board).   I used a length of insulated wire to run that signal over to the end of the JU1 jumper that feeds pin 1 of the 9-pin connector that connects the PL board to the interconnect board of the Mitrek.   The connections are pretty obvious when examining the schematic, and are easily traceable when looking at the actual circuit board.

You will need a monospaced font (like Courier) to view this diagram properly.

    +12v
     !           add this jumper
     R 94k   +----------------------------+
     !       !                            !
     +-------O -JU6- O--        --O -JU1- O---+------+ 
     !         (out)                (out)     !      !
 Q1  C                                        !      !
 --B                  to other  --O -JU7- O---+      !
     E                circuitry     (out)            !
     !                                               !
    gnd                                              !
               HLN4181 PL board                      !
                                                     !
                                                     !
+--------------------<<------------------------------+
!                 9-pin PL Board
!               connector P3 pin 1
!
!                        L18
+---O --Ju3A-- O------O -JU15- O---Pin 18 of the control connector (to
      (added)            (in)      repeater controller CTCSS decode in)     

             Interconnect board

PL 
decode jumper
Photo 14: This is what the JU1 to JU6 jumper looks like.
You can also install it on the component side of the board.

Note: On the interconnect board make sure that jumper JU3A is installed, and that JU3B and JU3C are out.   This connects the PL decode indication on pin 1 of the PL board connector to pin 18 of the control connector - requiring nothing more than a funny jumper on the PL board and a short jumper on the interconnect board.   Normally pin 18 is used to bring the tone from an external PL tone encoder (mounted in the control head) into the PL board - here we're using it to route the PL decoder logic level output to the controller by the same path.   I've seen other Mitrek modification web pages that use pin 18 for other things such as transmit audio or COR, but I much prefer using it for the PL decode logic level -- it's just a "cleaner" mod (and besides there are four more frequency select pins available for the other signals).

WRONG!
WRONG !
RIGHT!
Photo 15: Above is a picture of JU3A and how the Mitrek designer at Moto expected you to jumper it.   It's a cute, inventive and resourceful idea but those 1/4 inch tab connectors fall off in mobile service, even if you squeeze them very tightly and push them on with pliers. Photo 16: Personally, I bend up a piece of bare copper wire into a hairpin and solder one end to one side of the board and the other end to the other side.   I leave an air gap under each loop so I can clip an EZ-Hook™-style test lead to it.   I install a wire on the left side simply as a handy ground point (one side of JU3C is ground) for a VOM or an oscilloscope probe (as shown), the right side wire in this picture is the actual JU3A tone decode jumper.

Note that this voltage from the collector of Q1 is not a true high or low logic level - it will drive the PL decode input of most repeater controllers just fine, but if you need a signal that goes all the way to ground - for example to drive a LED (perhaps in a control head) you'll need to add a driver transistor.   The version below provides an open collector output for the PL decode signal.   If the upper transistor has enough gain the lower transistor may not be needed (see above for the rest of the details).

    +12v
     !                     --O -JU1- O--------->>--- 
     !                         (out) !      P3 pin 1
     !                               !
     !                          +----+
     !                          !    !
     !               100k       C    !
     R 94k    +--------R----- B      !
     !        !                 E    !
     +--------O -JU6- O--       !    !
     !         (out)            !    C
 Q1  C                          +--B    two 2N4401, 2N3904, 2N2222, etc
 --B                                 E
     E                               !
     !                              gnd
    gnd              (available at several locations, one of which 
                      is one side of JU2, another is test point E3, 
                      a third is pin 9 of the PL board connector)

              HLN4181 PL board

My first version of this setup used the plain JU-1 to JU-6 jumper described above, and drove an Scom 7K directly (which has about a 2v threshold between the inactive and active states).   I added the transistor driver for the PL decode LED inside the control head.   My second version (above with the two transistors) was used on a home-brew controller that needed a PL decode signal that went all the way to ground.   The two transistors drove the controller and a LED just fine.

    +12v (junction of L3, C43 and R45)
     !
     +-----R-------+                --O -JU1- O--------->>----
     !    see      !                    (out) !      P3 pin 1
     !    text     !                          !
     !         +---------------------------------+
     !         !                                 !  6N135
     !         ! LED                     photo-  !  Opto-isolator
     !         ! emmitter             transistor !  with open 
     !         ! side                     side   !  collector
     !         !                                 !  output
     R 94k     +---------------------------------+
     !             !                          !
     !             !                          !
     +-------------O -JU6- O--               gnd
     !               (out)
 Q1  C
 --B 
     E
     !
    gnd              HLN4181 PL board

If I were to do this over today I'd use the circuit above - I'd use the existing transistor on the PL board to drive the LED side of an open-collector optoisolator such as the 6N135 (effectively shunting the 94k resistor to the point that it pretty-much drops out of the circuit).   The resistor in series with the LED side (the one shown without a value) would have to be selected to pass enough current to turn on the opto-isolator's LED fully on without going into overcurrent (check the spec sheet for the optoisolators LED).   The use of the optoisolator makes this circuit pretty universal.

If your target environment is IRLP, then note that the IRLP interfacing board does not have separate PL and COS inputs - it has only one input that is default active low.   In this case, you would hook the Mitrek COR signal (actually the audio mute line) to the IRLP COS input and use one of the three AUX outputs (a 6 amp FET set up for active low) to drive the PL / Carrier select line.   Or driving a reed relay whose contacts are hooked to the PL / Carrier line.   Then modify the custom decode script file so that you can use touchtone commands to switch the appropriate AUX lead on and off, selecting PL or Carrier squelch. I suggest using AUX3 as AUX1 is frequently used as an auxiliary PTT control in the IRLP system.


Using a separate PL encoder with the HLN4181
The design of the reedless board uses the encoder as the frequency reference for the decoder function, and as such it can't run split tones (i.e. different encode and decode tones).   If you need to run simultaneous split tones in a single duplexed radio you will have to add a second device, either a PL encoder or decoder.   Having done both, I can tell you that it's much easier to add an encoder.   I've used home-brew twin-T oscillators (which worked surprisingly well when built with metal film resistors and polystyrene caps), Cetec-Vega 188's (very small, and the tone is selected by varing a 20-turn trimpot), Com-Spec SS-32s, TS-32s, SS-64s, and TS-64s (the SS-nn is the encode-only version of the equivalent TS-nn product).   The
TS-32 series does not have reverse-burst (unless you add an RB-1) board, the TS-64 does.   There is more information on the SS32, TS32, SS64, TS64 and RB-1 on Com-Spec web page here at Repeater-Builder. The Cetec-Vega 188 is documented as well, and is simple enough that you could build it from the schematic.

The hookup of an add-on encoder is fairly simple:

Connect the audio output of
the separate encoder here ---------------------------+
                                                     !
And the associated gnd                               !
to here, ---------------------------------------+    !
or or to one side of R2                         !    ! 
or to test point E3                             !    !
or to P3 pin 9                                  V    !
                                               gnd   ! 
Control                              P3              !   R27 
connector    L18                    Pin 1            V   22K
Pin 18 ---O -JU15- O-----O -JU3A- O--<<---+--O -JU7- O----R--+
            (both jumpers are on          !    (out)         !
            the interconnect board)       !                  !
                                          !                  !
                                          !                  !
      +----------------------------+      !                  ! 
      !                            !      !                  !
      V                            V      !                  !
  ----O -JU6- O--        --O -JU1- O------+                  !
        (out)                (out)                           !
     PL decode from previous section                         !
                                                             ! 
                                                             ! 
   alternate insertion point for PL encoder ---+             ! 
                                               !             ! 
                                               !             ! 
         R25 4.3K      R25 4.3K                V             !   C24
 -----------R------+------R------------O -JU3- O-------------+----C--- to filtering 
audio from         !               (normally in but          !  1.0uf  then to 
internal PL        !               but remove it when        !         transmitter
encoder        Thermistor          using a separate          !         modulator
                   !               encoder - see text)       !
                   !                                         O <-- Test point E1
                   !
                   R  R29
                   !  6.2k
                   !
                   +-----O <-- Test point E3
                   !
                   +-----O <-- Pin 9 of PL board connector P3
                   !
                  gnd

Note that JU3A is on the interconnect board and JU3 is on the PL board.   JU7 is normally used with applications that used external PL encoders built into the control head - these fed the PL tone in via control connector pin 18 and JU3A.

Before connecting the new encoder leave JU3 on the PL board in place and make sure JU7 is out.   Then key the transmitter and measure the existing encoder's audio level (AC voltage) at test point E1 and write it down.   Now remove JU3 and connect the new encoder's audio output to the R27 side of JU7.   Ground can be acquired from one side of R2 nearby, from one side of JU2, from test point E3 or from pin 9 of P3 (the 9-pin connector to the interconnect board).   Depending on what DC supply voltage your new encoder requires you can find filtered +12v on the wide trace that connects L3, C43 and R45, or you can get regulated +9.1v from the trace that ties pins 31 and 6 of the 40-pin chip together, or from the solder pad shown in the photo below.   With the AC voltmeter back on E1 you can now set the level of the replacement encoder to match the original encoders output.   If you are running separate transmit and receive radios you can also locate the external encoder outside the radio and use the interconnect board JU3 and PL board JU1 and JU6 and JU7 jumpers in the transmitter radio like they were intended...   Note that the above implementation will not have reverse burst, as that is generated inside the HLN4181 board's custom chip.   If you need reverse burst you can use the external PL encoder that was designed for the Mitrek or Micor "System's 90" control head setup, or a Communications Specialists RB-1, or the TS-64 encoder that has it built in.   Here's Com-Spec's own writeup on the TS-64 for the Mitrek.   It's not oriented to using it as an encoder-only, but the encoder and reverse-burst hookup is quite relevant.

+9.1v DC on the HLN-4181 board
Photo 17: You can find +9.1v DC to power the external PL encoder in the empty solder-filled hole just above the pen (which slipped just as the flash fired...)


Test point E3 on the HLN4181 board
Photo 18: Test point E3, pointed to by the pen, can be used as the ground connection for your PL encoder.



Repeat Audio In
Bringing transmitter audio (i.e. repeat audio) into the radio from the outside world 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 the modulation line going to the row of transmit elements and and insert our audio.   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 (pin 4).   The difference is where we tap into that modulation line.

Transmit Audio Test Point B
Photo 19: 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--+
Transmit       JU9/L10        10uf    !     (out)
audio in        choke        -! !+    !
>-------------()()()()--------! !-----O <-- plated-through hole 
control head               ^  ! !     !      on interconnect board
connector pin 10           !          !      right below and in between 
(Frequency 4 select)       !          !      JU3B and JU3C.
                           !          !
                           !          +--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 transmitter modulator

Note: do not use a tantalum cap in the above schematic, they are bad news when used as audio coupling capacitors.

The better way to insert repeat audio into the transmitter is to run one end of that same coupling 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 without a photo (which I've provided below)...   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.   The main board to interconnect board connectors were a weak point in the Mitrek until Moto changed the metalurgy.

Milt N3LTQ pointed the following out to me in an email: On the Mitrek interconnect board take a look at the color of the plastic on the pins that connect the interconnect board to the main board in the radio.   If the PL connector plastic is WHITE, get a book and order out a new set of main board-to-interconnect board pins and sockets.   Take your time and carfully remove the old connectors and replace them.   Sacrifice the old connectors and rather than damage the board traces. New connectors will fix many intermittent problems.   Eric Lemmon WB6FLY commented that the problem was bad enough that Moto came out with Field Modification Kit, part number RPX4277A (which is no longer available).   If the PL connector plastic is RED in color you already have the new version connectors (the plating metalurgy is different).
The new PL board connector (red 9-contact J3) is part number 0980140H02, at about $3.50, and the new main-board to interconnect-board connector (white 25-contact J10) is # 0980140H03, about $6 for a pack of two (thanks to Eric for the kit number, the connector part numbers and prices).

Transmit 
Audio Test Point B
Photo 20: 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:

Transmit       JU9 / L10         10uf
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

Important!   Note that the transmitter audio injected at the modulation line by either method shown above is NOT pre-emphasized by the transmit audio circuits in the radio before modulating the transmitter.   If you need pre-emphasis (most do) you need to do it to the audio before it hits point B.   The AP‑50 board sold by Repeater-Builder does this quite nicely.

You may find that the local mic audio is a little low when the radio's deviation pot was set properly and the repeat (transmit) audio level out of the controller is adjusted properly.   This is dependent on the output impedance of the audio source (the repeater controller).   Sometimes, as in my case, it's "fixable" if the 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.   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 perfectly acceptable for test use).   If you find it a problem, just add a series resistor between the repeater controller output and the "Point B" (i.e. raise the controllers output impedance), then tweak up the audio level from the controller to compensate.

It is my personal opinion that good repeater design has the repeat audio level be independent of the receive volume control, plus having a working local speaker and permanently installed local microphone jack on every radio in the system is very, very useful - it allows a local user to simply walk up to the rack, turn up the regular volume control 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.   I always take the repeat audio from a point unaffected by the local volume control, period (even if I have to relocate the volume control to a new position in the radios audio chain).

Personally I feel that a working local speaker (in carrier squelch mode) is a firm "must-have" diagnostic tool on any good repeater, remote base or link radio.   And a working transmitter disable switch / force PTT switch is #2 on the "required" list (see the PTT section of this document), and a working local mic is #3. And even if the system is going to run in tone squelch mode all the time I include a carrier squelch function in the repeater controller programming just for the day that intermod shows up... I can put the system into carrier squelch and listen for any identifying characteristics of the signal.


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 frequencies above the frequency range used by the human voice.   Speech lives in the range from 250-300hz 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 high frequency knee of the IF bandwidth curve).

Imagine a bench setup with an RF generator connected to the receiver antenna connection, the squelch is wide open, and an AC voltmeter is connected to an audio dummy load hooked across the the speaker leads.   Set things up with the generator cranked to minimum, and the AC voltmeter 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" measurement in a radio specification sheet (and some point-to-point links - especially voting receiver links - need 30, 35 or even 40db of quieting).   The noise operated squelch circuit takes this high frequency AC noise, amplifies it and rectifies it into a DC voltage that varies with the noise level (this is the Received Signal Strength Indicator (RSSI) voltage that folks pick up and send to an analog metering input on several brands of repeater controllers).   When this DC voltage drops below a specific threshold (set by the squelch control) the speaker unmutes (i.e. squelch open), and as it rises above the threshold the speaker mutes (squelch closed).   Most modern radios use two different thresholds (the difference is called the squelch hysteresis amount), a slightly higher one for squelch open, and a slightly lower one for squelch closed.   Using two voltages improves how the user perceives 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 the overall design.   The entire circuit uses only only five transistors, one of which is the main audio mute transistor.   The Mitrek was designed in an era where the use of PL or DPL was universal, and carrier squelch had gone by the wayside.   It really looks like the Motorola designer didn't put much time or thought into it, almost seems to have been included as an after thought... probably knowing that it wouldn't be used much... it's just a very simple noise operated squelch.

In contrast, Motorola put a great deal of effort into the previous generation radio - the Micor. They had to since the use of PL and DPL had not become universal at the time of its design.   In short, the Micor was the high water mark for carrier squelch (noise squelch) circuit development... it just works better than anything else on the air.   The heart of the fabulous Micor carrier squelch circuit is a single custom IC chip which contains over a hundred transistors that implement a complete bi-level or switched hysteresis squelch system on a single chip.   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's not socketed).   Note that the circuit could be added to any model radio - even the infamous Modar Triton / 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 10k, 25k or 50k squelch pot completes the circuit (if you are interested in doing this, look at any Micor mobile manual and trace the receiver discriminator audio through the audio squelch card to the audio PA transistors). The schematic of the Micor audio-squelch card is available as a download from this web site - look on the Micor page.

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, is much smaller than the actual Micor audio-squelch board, works great, makes the squelch action a lot more pleasing, and eliminates the squelch tail the stock Mitrek creates. It's also transferrable to a another receiver later on.

The Link-Comm RLC-MOT module
Photo 21: 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:

RLC-MOT pinout
PinFunction
1+11 to 15vDC at about 35ma
2Discriminator audio in
3Buffered and muted audio out
4Active low COS
5Active high COS
6Ground
By the way, pin 1 is a square pad, the others are round pads.
Pin 6 is nearest the corner of the board, and the trimpot adjustment screw.

The RLC-MOT uses a (20 turn!) trimpot for an on-board squelch control, has a LED that lights when the squelch is open, and also has an audio level pot on board.   You can demount the trimpot and run leads to a front panel pot if you desire.   Note that the audio level pot is a single turn pot and has no stops, so adjust with care.

If you are using an RLC-MOT module and need a de-emphasized repeat audio output you can take advantage of the fact that the option of de-emphasizing the audio was designed into it from the start - just add a capacitor across R9 on the module itself.   There are solder pads for it already in place (labeled C18).   They are small and you will have to use a very small cap and a very small soldering iron, and work quickly and carefully but they are there.   Depending upon the audio response you'd like to see, the cap should be somewhere between .0075uf (breakpoint at about 200hz) to .022uf (breakpoint at about 75hz).   While you can tailor it to suit your desires of low-frequency response the lower you go in frequency the less "effective gain" you will achieve due to the modification of the buffer/amp circuit's frequency response.   A 0.01 cap will give you a breakpoint of 150hz, which is a good compromise, or if you want to use a lower PL tone you might want to go to a 0.015 cap.   The RLC-MOT is available from Link Communications at 800-610-4085 for about US$60 (at the time of this writing).   Yes, I know, you spent less on the entire Mitrek mobile radio - but if you blow up the Mitrek you can move the RLC-MOT to a follow-on radio (the author has added one to a Kenwood, to a Motrac, to a Mastr-II and, as a joke, to a 1950's Sensicon-A tube-type receiver, which even today, over 50 years after it was built, is still one outstanding performer...   and moving it to 220 Mhz is simple - just apply a short across each of the tuned lines at the right point, change the crystal and peak the front end!!).

The audio feed to the RLC-MOT must be discriminator audio, from before the de-emphasis network in the radio.   It can be derived from two places: purists can take if from pin 1 of U403A in the receiver, bypassing the main-board-to-interconnect-board junction (suggested) or from the radio side of JU10 / L11 (which feeds pin 11 of the control connector).

You can mount the RLC-MOT using screws and standoffs, or with double-sided foam tape.   Just make sure that yo it in such a way that you can get to the squelch adjust pot (labeled R3), the audio level pot (R14) and to be able to observe the squelch open / closed indicator LED (D1).

More info on the Micor squelch is at:

A quick-and-dirty hookup of the RLC-MOT to a Mitrek - NOT RECOMMENDED:

+-------------------+
!     Function  Pin !
!                   !
!          +12   1  !------ +12v
!                   !
!     Audio in   2  !------ See text
!                   !                    choke
!    Audio out   3  !------------------()()()()----< <--- repeat audio out (to controller) 
!                   !                   JU8/L9    Pin 9 - Freq 2 select
!  Low COS out   4  !------------+                
!                   !            !      choke
! High COS out   5  ! unused     +-----()()()()---< <--- Active-low COS out (to controller) 
!                   !                   JU7/L8    Pin 8 - Freq 3 select
!          GND   6  !--+
+-------------------+  !
                       gnd  <-- pin 6 or 17 of the control connector, or
                                any other internal audio ground

Here's a better way to do it:
I really, really suggest that you use the "High COS Out" signal to drive an open collector transistor as shown below, with the collector going to the outside world via JU7 / L8 / pin 8.   Doing this will protect the RLC-MOT module from any errors in hookup outside the radio - I'd much rather toast a 25 cent (or less) transistor than a US$60 module... the transistor takes the heat, so to speak.

                                          choke         pin 8
     pin 2 ! audio in                +---()()()()-----------<
           !                         !    JU7/L8        F3 select
     pin 3 ! audio out               ! 
           !                         ! <--open collector COS output...
     pin 4 ! Low COS out             !    low when squelch open / signal present
           !                         C
     pin 5 !-----------R---------- B
           !       3.3 or 4.7K       E
           !                         !
           !                         !
                                    gnd

Here's the right way to do it:
If I were to do it over today I'd use the RLC-MOT to drive an open collector optoisolator (like the 6N135), in a socket, instead of the open collector transistor.

                                                            choke         pin 8
                +12-----/\/\/\/\---+              +--------()()()()-----------<
                                   !              !         JU7/L8    F3 select
                                 +---------------------+
                                 ! LED           photo !
     pin 2 ! audio in            ! emitter  transistor !
           !                     ! side           side !
     pin 3 ! audio out           +---------------------+
           !                       !              !
     pin 4 ! Low COS out           !              !
           !                       C             gnd
     pin 5 !---------R---------- B
           !     3.3 or 4.7K       E
           !                       !
           !                       !
                                  gnd

Modifying the Mitrek so that you can hear the RLC-MOT output:

If you want you can disconnect the top end of the Mitrek volume control (as shown below) and feed audio from pin 3 of the RLC-MOT to it.   This will allow you to use the Mitreks internal audio amp and local speaker to monitor the repeat audio (which is really handy while on the workbench). Just break the connection as shown below, connect the high side of the volume pot to a spare pin on the control head connector then run an extra lead from the RLC-MOT inside the radio to that pin on the head. Note that this connection expects NON-de-emphasised audio since the de-emphasis circuitry in the Mitrek is after the volume control. In other words, if you are going to do this, do NOT add the C18 de-emphasis cap on the RLC-MOT circuit board.

                                                        Break here
                                                         /
        +!!C3                                           /
radio----!!---JU10---Pin 11 ---------------Pin 11---+--X--+ <---insert RLC-MOT 
main     !!   L11    Buffered              on head  !     !     audio here
board                audio out                      !     !
                  (unsquelched)                     \     \
                                              3.3k  /     / 10k    This resistor
                                              1/4w  \     \ 1/4w   can be jumpered
                                               or   /     /  or    if you insert the 
                                              1/2w  \     \ 1/2w   RLC-MOT audio 
                                                    /     /        at the top (above)
                                                    !     !
                                                    !     !
                                           25k pot  \     \ 25k pot
                                           Squelch  /     / Volume
                                                    \     \          to pin 15 on
radio---------JU12---Pin 14 ---------------Pin 14-> /     / <------- head then to
main          L14    Squelch               on head  \     \          pin 15 on radio
board                wiper                          !     !
                                                    !     !
                                                    !     !
                                                    !     \ This resistor can be 
                                                    !     / be jumpered - see the
                                                    !     \ text in the Control Head 
                                                    !     ! Connections section above
radio----------------Pin 2-----------------Pin 21---+-----+
main                 Audio                 on head
board                ground

Using the receive audio stage as a monitor amplifier:

Simply connect the top of the volume control to an SPDT switch, with one side to pin 3 of the RLC-MOT, and the other to the repeat audio input (i.e. the audio from the controller to the transmitter).   You might have to add a trimpot on one side to make the audio levels equal...   The ability to monitor the transmitter audio lets you hear what the users are hearing - the repeated audio complete with the controllers courtesy beep, the identifier, etc. and will reveal any mismatched audio levels (for example, between the identifier, the DVR playback level and the user audio).   As said above, that there is a de-emphasis stage downstream from the Mitrek volume control and trying to monitor additional audio sources that have already been de-emphasized will sound really bad... See the articles on pre-emphasis and de-emphasis on the Tech Articles page.

from channel element pin 4 buss----cap to block DC---+
                                                     !
                                                  insert
                                                  trimpot  
                                                 to ground
                                                   here 
                                                 if needed
                                                (to adjust
                                                audio level)
                                                     !
                                                     !
                                                     O
                                                      <---o-------- to volume control high side
                                                     O
                                                     !
                                                  insert
                                                  trimpot  
                                                 to ground
                                                   here 
                                                 if needed
                                                (to adjust
                                                audio level)
                                                     !
from RLC-MOT audio out-------cap to block DC---------+
                               (if needed)


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

So what is COR / COS /CD / CB ?

This is a DC voltage or logic output produced by the radio that changes whenever the squelch is opened.   This signal is required for repeater controllers and IRLP nodes to know when the receiver is receiving a valid signal.   Back when radios had tubes and glowed in the dark the common technique was to add another tube that drove a sensitive low-current relay, which pulled shut when the squelch opened and dropped out when the squelch closed, hence the term "Carrier Operated Relay".   One set of contacts on this relay keyed the transmitter PTT lead.   A "carrier delay" capacitor kept the relay closed for a few seconds - this prevented a fluttery or weak signal from banging the COR and PTT relays to an early death, and was sometimes treated as a seperate timer stage, and referred to in the documentation as the "hang-in timer" or the "carrier delay timer".   When switching transistors relaced tubes, the term became Carrier Operated Switch, or COS. Early mobile telephone systems ran carrier squelch, and had a light labeled "Channel Busy".   In the computer data communications world one of the standard modem status signals is CD, or "Carrier Detect".   It's all the same - a signal that indicates that the channel is in use, or the receiver squelch is open.

I seriously suggest you use the Link-Comm RLC-MOT board (as described above) to generate the COR signal and repeat audio outputs, but if you chose not to, then generating your choice of an active-low or active-high 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
     --------------------------------
              !
              !                     Positive
              !                     DC voltage 
              !                    (+5 or +12)
              !                        !
              !                        !    Leave out the 10k resistor for 
              !                        !    an open collector, or connect 
              !                      10K R  to +5 or to +12v if you want
              !                        !    that voltage as your high level.
              !                        !    +12 can be found on the control 
              !                        !    connector pin 4.  +5 can be 
              !                        !    generated by a 2-resistor 
              !                        !    voltage divider.
              !                        !    Most modern repeater controllers 
              !                        !    expect open collector.
              !                        !
              !                        !  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
Q