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.

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. 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. 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. 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 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 ! 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. 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...) 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. 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). 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. 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 Pin Function 1 +11 to 15vDC at about 35ma 2 Discriminator audio in 3 Buffered and muted audio out 4 Active low COS 5 Active high COS 6 Ground 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:  http://www.repeater-builder.com/micor/micorsquelch.html   Another writeup on the Micor squelch by Kevin Custer W3KKC  http://www.link-comm.com/squelch.html   The LinkComm RLC-MOT info page (from their web site)  Link-Comm RLC-MOT board layout and schematic   39kb   (in PDF format, courtesy of Link-Comm, used with permission)  Link-Comm RLC-MOT Tech Specifications and notes   36kb   (in PDF format, courtesy of Link-Comm, used with permission) 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