Secrets of the Mitrek
HLN-4181 PL Board

How it and the TRN4224 tone element work together
with a few notes on the HLN-4020 dual-reed board


Comments and additional material are welcome
(even "Hey - you've got a typo at..." messages)
Look for the authors email info at the end...

There was a discussion on the Mitrek HLN4181 reedless board and the associated TLN4224 tone elements on the repeater-builder mailing list a long time ago, and this writeup is a enhancement (in reality almost a total rewrite) of my posting on that topic...   basically I deleted some irrelevant information and added some additional information I had since generated.   The second major revision happened when I added the photos.

Note: When you print this file make sure that the diagrams below come out in a monospaced font (like Courier) if you want to see the diagrams below properly.   I used an exclamation point as a vertical line since the actual vertical line character is one of the first ones replaced by a local language character in non-USA keyboards... I wanted this writeup to be as international-compatible as possible.

This writeup was generated using a 40w UHF Mitrek and one each of the HLN-4020 reed board and the HLN4181 reedless board as test beds, but since the PL boards are universal to all the Mitrek models the fact that I used a UHF radio shouldn't matter.

If you need a Mitrek manual read this web page.   The manuals are about US$30 each and the manual part numbers are on this web page.

The Mitrek Plus supplement is 68-81046E05 and is definitely worth getting if you can get one...   the last time I called they were out of them and not going to reprint it.   As long as you have Motorola parts ordering dept. on the phone you will want to order the Mitrek tuning tool, part number 66-82977K01.   The real Mitrek tool is (in May of 2004) only $2.92 and breaking a slug with the wrong tool is not worth it.   Note also that the Mitrek is an all-metric radio - don't casually mix the leftover screws into your parts bin.

Another note... the UHF book that I have (68-81045E70-A) is the only Mitrek manual I have found that has documentation on all three PL/DPL boards - the HLN-4020 reed-type PL board, the HLN-4181 element-type PL board and the HLN-4011 DPL board.   These three boards are completely interchangeable - the most you might have to do if you swap a reed PL board for an element PL board or for a DPL or vice versa is to adjust the encoder level.   The highband book I looked at seems to be incomplete - it was missing the HLN-4020 reed-type board schematic.   The low-band book I have in my file cabinet (a -O original version) has the HLN4020 reed board and the DPL 2-code adapter, but no DPL board or the HLN4181 reedless tone PL board.

Note also that Moto uses a -O for the Original release of a manual, a -A is the first revision, and they work their way up the alphabet, skipping the letter O when they get to it.   After -Z they continue with -AA, -AB, -AC, and so forth to AZ then BA...etc.

This document started in late 1998 / early 1999 with a request for a TRN-4224 element for the HLN-4181 PL board that was posted on the mailing list...

My response started with:
> I bought a tone element for 127.3hz from Telepath in San Jose (south of
> San Francisco in California) last week. They are at 800-292-1700, they
> take credit cards over the phone, and will ship parcel post, ground UPS,
> FedEx and next-day UPS.
> Obligatory disclaimer - I have no relation to them except as a satisfied,
> in fact, very satisfied, repeat customer.

For what it's worth, Telepath has since put up a web site at

The rest of my response to the inquiry was how to roll your own element from a few resistors, and was about half of the material that is below.   Kevin W3KKC later emailed me and asked if I would write it up for his web site, so I did, and the first version was posted.   That was over a dozen revisions ago.   Happy reading!

Photos of the two types of Mitrek tone PL boards (The HLN4011 DPL board is not shown).   All connections to the board are made through the 9-pin connector header which looks like a vertical brown line at the right side of the photos (adjacent to a silver mounting screw).   The strange shaped holes in the board provide access to tuning adjustments in the receiver.

The reed-type Mitrek PL board HLN-4020
The HLN 4040 reed-type board.

The two reed sockets are at lower center, with the black plastic hold down arm above them.   The KLN-6210 encode reed is on the left, the KLN-6209 decode reed on the right.

The reedless Mitrek PL board HLN-4181
The HLN4181 reedless board.

The horizontal white component at the top right is a TRN4224 tone element that is plugged into the black socket under it.

The pinout of the single-row 9-pin connector at the edge of the PL or DPL board is:

Pin   Function
1 = External tone in or out (depending on option jumpering)
2 = Squelch disable (via JU5)
3 = Tone encoder output to TX modulator
4 = PTT in
5 = Audio mute (via JU4)
6 = discriminator audio in to PL decoder
7 = PL filter
8 = B+ (pos 12v)
9 = gnd
For reference, pin 9 is at the top, nearest the only 8-pin IC on the board.

The HLN-4020 reed-type board uses one each KLN6209 and KLN6210 reeds and in perusing the board's schematic I did notice one nice feature that I've not seen in any other reed-based PL board: it can be jumpered to use one reed for a common encode/decode tone or two reeds for split tones.   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 in revision F).   An early board can be jumpered for a single reed using the info from the later schmatic.

If you can't find the reeds you need on the frequency you need, the HLN-4020 board seems to be just as happy with any of the following reeds (I've tried all of them at one point or another):

You do have to remove the hold-down bracket (one screw) to create enough clearance to use the fat decode reeds in the HLN4020, so don't use one in a mobile as the reed will fall out into the receive circuitry (if you haven't been inside a Mitrek the normal mounting position has the PL board upside down over the receiver), but in a pinch the fat decode reed will work just fine as either as a decode or an as an encode reed...

If you are looking for a HLN-4181 board, at the time of this writing they were in stock for $20 each at C. W. Wolfe Electronics, a used 2-way radio dealer in Billings Montana at 406-252-9220.   They also have the HLN-4011 DPL board for $10.   Note that this is not an endorsement by, I'm just mentioning it as a service.   I have to say this - like I said in the above email I have no relation to C. W. Wolfe company except as a very satisfied repeat customer.

Note that the HLN-4181 reedless board cannot do split tones due to it's inherent design - the circuitry can only decode the same tone as it encodes due to the fact that the encoder is used as the frequency reference for the decoder.   On the web page that I created describing how to interface a Mitrek to a repeater controller I've included a schematic of how to connect a separate encoder.

HOWEVER... If all you need is a different encode tone, and can live with the decoder not decoding it's intended tone while transmitting, well... looking at the HLN-4181 schematic it appears that the custom chip that is used on the board was designed to use two tone element sockets and switch between them (look at pins 12, 13, 14, 15 and 16 of schematic 68-80200W-06)... Just tack a tone element socket (ordered from Moto as a replacement part or scavenged from a dead board) onto the foil side of the board with a dab of acid-free clear RTV, add some jumpers (I'd use wire-wrap wire) to parallel all the pins except pins 1 and 2 to the existing socket, clone the choke and resistor combination that is on pins 12 and 15 onto pins 13 and 14, and slave the "code select" pin to PTT (in the stock form the pin is grounded, just unplug the chip, bend pin 16 to clear the socket, and tack a piece of wirewrap wire to it and run it to a 22k pullup resistor and a .05uf cap to ground, and to your PTT) ....   Look at the schematic and trace what I have written above...   I've not tested this but it looks like it should work... just remember that your decoder will follow your encoder tone.

The HLN-4181 reedless board can be set up in many unusual ways...   The normal jumpers are arranged like this...

HLN 4181 Reedless Mitrek PL Board normal jumpers
1 OUT Used to couple internal PL tone encoder to outside options
2 OUT Disables the internal encoder when installed
3 IN In to couple internal encoder to the TX 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 (RX carrier squelch only mode)
7 OUT In to connect external encoder to TX modulator (and remove JU3)
8 OUT In to disable reverse burst
9 IN When in couples RX audio into the PL decoder
JU4 and 5 are redefined if the radio has the "Busy Light"
option – see the footnotes on the schematics in the manual.
If your 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 in.
If you want your PL encoder to work then JU3 must be in and JU2 must be out.

Until I found the tone element I needed at Telepath, a friend of mine and I were reverse engineering the HLN-4181 PL board and the associated TRN4224 tone element because we couldn't find a 127.3hz element in the junk boxes of any of the usual suspects, and in our application we absolutely had to have 127.3hz and we had to have reverse burst (which precluded a Com-Spec TS-32).   We were 90%-95% done when another friend found out about our project and called and told me about Telepath, gave me their phone number, said he had done a lot of business with them and was very happy with them.   I called them, they had what I needed at a reasonable price, they had a couple of other items I wanted and hadn't been able to find anywhere else, so I gave them my credit card number...   Yes, at that point I knew enough about the board's design to make a 127.3hz element, but this was for a radio that was going to a repeater site that I can't get to very often.   We were also in a bit of a rush, we had to get this unit in service in a couple of weeks - not much time for a burn-in.   All in all, for the cost of the element involved (under $15) it wasn't worth risking a future middle-of-winter failure with a homebrew element...   and other parts of the project needed the time we had left.

That caveat said, if you need to you can "roll your own" element with nothing more than a bunch of 1/4 watt, 1/8 watt or 1/10 watt 100k resistors (the smaller the physical size the better), some solder and some electrical tape.   The TLN4224 element is simply a variable number of resistors (from 1 to 10 depending on the tone) that are used to program a counter.

In the process of reverse engineering the HLN-4181 board my friend and I discovered that the tone generator is based on a 10-bit binary counter that is preloaded with a number that is held by the element (see the schematic below) as two 5-bit nibbles.   The nibbles are selected by alternately strobing pin 1 or pin 2 of the element socket.   Once the 10-bit number gets into the counter it counts up to overflow then reloads the starting number and does it again...   The higher the starting number the more frequent the overflow resulting in a higher tone frequency generated.

The encoder audio output is smoothed and filtered before injection into the transmitter.   When the incoming PTT line goes inactive the encoder generates the reverse burst, then drops the outgoing PTT line.   This takes care of the encoder function.

The decode function is accomplished by passing the RX audio through a low-pass filter that passes only the PL frequencies and feeding it to another part of the custom chip which uses the encoder as a frequency reference and checks for a matching frequency from the receiver audio.   If there is a match it unmutes the receiver by way of JU-4 or JU-5.

There is also an audio notch filter on the board to remove the received PL tone from the received audio.   However you don't want to try to use it in repeater service - just plan on using an external high-pass filter betweek the receiver and your repeater controller.

The HLN4181 board will encode and decode simulataneously (like in a duplex radio) as long as you encode and decode the same tone.   As I described in the "However..." paragraph above you can modify the HLN4181 to do split tones (one tone on TX the other on RX) if you want to.   If you want simultaneous split tones just use an HLN-4020 dual reed board with the appropriate pair of reeds, or use a separate encoder.   There is detailed info on how to set up split tones using the HLN4181 and a separate encoder on my Mitrek Interfacing page.

Three images of the TRN4224 Mitrek reedless PL tone element,
I don't have a digital camera, so I used a flatbed scanner.
Front view of the TRN4224 tone element This scan is of the front of the TRN4224 element with a US ten-cent coin for size comparison.   Pin 1 is on the right.   The "114.8" is my own marking (done with an ultra-fine-tip Sharpie pen) on the ceramic.   Every element, no matter what the frequency, has the same part number.   If you find an element with a part number of TRN6005 you have a DPL code plug, which is used in the HLN4011 digital PL board.
Rear view of the TRN4224 tone element This is the rear view of the same element, just rotated 90 degrees and less the coin.

In this photo pin one is at the top, and pin thirteen at the bottom.
Magnified rear view of the TRN4224 tone element This enlarged image shows how Motorola marks the PL tone elements with the frequency and tone code.   If you look very carefully at the top dark stripe you can see the vertical numbers 114(dot)8 in grey ink on a black background.   The short dark stripe at the bottom says (vertically) "2A".   I had to play with the brightness, contrast and gamma adjustments of the scanner software to get the printing in this image to show up at all, and even then it's not too visible.   On some of the elements the grey color of the ink blends into the dark epoxy stripe on the back of the element.   It takes better eyes than I have to read them without a magnifying glass, which is why once I know what the frequency is I label the front side of my elements with an ultra-fine-tip Sharpie pen or in a pinch, a pencil.

Note that on the TRN4224 tone element that there is no physical pin in location 4.   In fact the socket has a plastic block there - it's used as a key to prevent people from inserting the element backwards.   And while there is a pin in location 3 it is not actively used in the TRN4224 tone element (but it's grounded on the PL board).   I assume that it's connected to the substrate inside the tone element.

Internally the element and it's socket is set up like this (Each "Rx" is a position for either an open or a 100k resistor):

Here's an ASCII attempt at a schematic.
You will need a monospaced font (like Courier) to view it properly.

  !                                 !  !    !    !    !
  !   +-----------+---+---+---+-+   !  !    !    !    !
  !   !           !   !   !   ! !   !  !    !    !    !
  !   !          RA  RB  RC  RD RE  RF RG   RH   RI   RJ <-- 100k resistors
  !   !           !   !   !   ! !   !  !    !    !    !
  !   !           !   !   !   ! +-+-+  !    !    !    !
  !   !           !   !   !   !   !    !    !    !    !
! 1 ! 2 ! 3 ! X ! 5 ! 6 ! 7 ! 8 ! 9 ! 10 ! 11 ! 12 ! 13 ! <-- Mitrek socket pins
  !   !   !   !   !   !   !   !   !    !    !    !    ! <-- Mitrek PC board
  !   !  GND  !   !   !   !   !   W    !    !    !    !
  !   !       !   !   !   !   !        !    !    !    ! W=data bits 0 and 1, chip pin 22
  ! strobe B  !   !   !   !   +--------+-X  !    !    ! X=data bits 8 and 9, chip pin 23
  !           !   !   !   +-----------------+-Y  !    ! Y=data bits 4 and 5, chip pin 24
strobe A      !   !   +--------------------------+-Z  ! Z=data bits 2 and 3, chip pin 21
              !   +-----------------------------------!-- data bits 6 and 7, chip pin 18
              +---- blocked pin in socket

The RA-RJ references are not in the Moto manual, I assigned them just for simplifying this writeup.

As you can see you can make your own element by inserting resistor leads into the socket and bussing the top leads of the two sets of resistors together and inserting the busses into pin 1 and pin 2.

My friend and I started with elements for the three different frequencies we had in stock, and determined the resistor pattern inside each element with a VOM (every resistor was about 100k, ranging from 85k to 115k), and created a table of tone frequencies versus data bits.   We also determined that with no element inserted the encoder produced 273.3hz when the TX is keyed.

As another source of information in the reverse-engineering attempt we also used a single 100k resistor and inserted it in the socket pins - i.e. from pin 2 to pin 5 for resistor A, and noted the frequency that was generated when PTT was keyed, then from 2 to 6 for resistor B and remeasure, then from 2 to 7 for resistor C, all the way to resistor J from pins 1 to 13.   We noted the frequency change that each pin produced by itself:

  Change in Frequency vs. data bit position  
0 0.3
1 0.6
2 1.3
3 2.2
4 4.3
5 8.6
6 17.1
7 34.1
8 68.3
9 136.52

So pick a frequency, figure out what the difference between it and 273.3 is, then figure out the bits you need to activate to get that difference, and program it in (or you can cheat and look at the table at the end of this writeup). Just plug the resistors into pins 5 to 13 and buss the appropriate top ends to pins 1 or 2. A flap of electrical tape over the strobe busses takes care of any insulation requirements. For pin 9 (resistors E and F) solder the two bottom leads together and clip one lead short – do not try to plug both leads into the pin in the socket.   If you want some physical rigidity slop a big glob of some acid-free clear RTV onto the resistors.   If you want some shielding (usually not required) just fold a flap of copper foil (available at model airplane hobby shops) over the hardened RTV and solder a trimmed piece of a resistor lead (or a short piece of #24 wire) to it and plug the wire tail into pin 3 (ground).   If you chose to use RTV, make sure you get the acid-free clear RTV – check the label! – most of the common brands give off acetic acid fumes as they cure and that will eat up your solder connections - and the damage will be hidden inside the hardened RTV! (This is the voice of experience with Permatex #66...)

One advantage to the HLN4181 - you can have a Mitrek on each end with NO tone element installed (i.e. set to 273.3hz) and they can talk to each other.   The higher frequency tone also means minimum pickup delay.   This is ideal on point-to-point links (and it saves having to make up any elements).   Yes, the 273hz tone is harder to filter out than a lower frequency, but it can be done with an active filter.

The HLN-4181 PL board also allows you to pick weird or non-standard tones in 0.3hz steps from 273.3 on down, but to make them workable you need these PL boards on both ends...   this trick can be handy on point-to-point links that you want to keep the riff-raff out of (if you don't mind the PL pick-up delay - a lot of links use carrier squelch to minimize the delay that comes from cascaded links). Maybe 199.2 hz anybody?   Or 235.0 hz?   However, you have to remember that PL (or Digital PL) doesn't solve interference issues, it just covers them up.   Sure, the RX won't unsquelch if it is requiring a weird tone and the idiot only has the standard tones but the idiot can still sit there and transmit on the channel causing a heterodyne or capturing / blocking the receiver, so what real security (or increase in usability) have you really gained?   And what prevents a determined and resourceful idiot from listening to you on the input with an OptoElectronics tone grabber and finding your weird magic tone ?   (it only takes maybe 5 seconds of signal).   But if you feel like playing around with this concept you can also build up a test jig by taking ten resistors, a ten-position DIP switch and some wire and hooking them up as follows:

!                                   !  !   !   !   !        !
!   +-----------+---+---+---+--+    !  !   !   !   !        O
!   !           !   !   !   !  !    !  !   !   !   !         \
!   !           O   O   O   O  O    O  O   O   O   O        O  is one pole
!   !            \   \   \   \  \    \  \   \   \   \       !  of a DIP switch
!   !           O   O   O   O  O    O  O   O   O   O
!   !           !   !   !   !  !    !  !   !   !   !
!   !          RA  RB  RC  RD  RE   RF RG  RH  RI  RJ <-Tone element
!   !           !   !   !   !  !    !  !   !   !   !    resistors
!   !           !   !   !   !  +-+--+  !   !   !   !
!   !           !   !   !   !    !     !   !   !   !
1 ! 2 ! 3 ! X ! 5 ! 6 ! 7 ! 8 !  9  ! 10! 11! 12! 13! <--PL tone element socket pins

Here's a table / drawing that might make it easier.   Verify the tone generated after you plug the resistors into the socket.   You can juggle the tone frequency in 3-tenths- of-a-cycle steps... (see the frequency difference table above).

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

!                                   !   !   !   !   !
!   +-----------+---+---+---+--+    !   !   !   !   !
!   !           !   !   !   !  !    !   !   !   !   !
!   !          RA  RB  RC  RD  RE   RF  RG  RH  RI  RJ <-Tone element
!   !           !   !   !   !  !    !   !   !   !   !    resistors
!   !           !   !   !   !  +-+--+   !   !   !   !
!   !           !   !   !   !    !      !   !   !   !
1 ! 2 ! 3 ! X ! 5 ! 6 ! 7 ! 8 !  9  !  10 !11! 12! 13! <--PL socket pins
67.0    XZ      -   X   -   X   -   X   X   -   -   -
71.9    XA      X   -   X   -   X   X   X   X   -   X
74.4    WA      X   -   -   -   -   X   X   X   X   X
77.0    XB      X   -   -   -   -   -   X   X   -   X
79.7    WB      X   X   X   -   -   X   X   -   -   X
82.5    YZ      X   X   -   -   -   -   X   -   X   X
85.4    YA      X   -   -   -   X   -   X   -   -   X
88.5    YB      -   X   X   -   X   -   X   X   -   X
91.5    ZZ      -   -   -   -   -   X   X   X   X   X
94.8    ZA      -   X   X   -   X   -   X   -   X   X
97.4    ZB      -   X   X   -   -   -   X   -   -   X
100.0   1Z      -   -   -   -   -   X   X   -   X   X
103.5   1A      X   X   X   -   X   -   X   X   X   -
107.2   1B      X   X   -   -   X   X   X   X   X   -
110.9   2Z      X   -   -   -   X   -   X   X   -   -
114.8   2A      X   -   X   -   -   X   X   -   -   -
118.8   2B      X   -   -   -   X   X   X   -   -   -
123.0   3Z      -   X   X   -   -   -   X   X   -   -
127.3   3A      -   X   -   -   -   -   X   X   -   -
131.8   3B      -   -   X   -   X   X   X   -   -   -
136.5   4Z      -   -   -   -   X   -   X   -   -   -
141.3   4A      X   X   -   X   X   X   -   X   X   X
146.2   4B      X   X   X   X   X   -   -   -   X   X
151.4   5Z      X   -   -   X   X   -   -   -   X   X
156.7   5A      -   X   X   X   X   -   -   X   -   X
162.2   5B      -   -   -   X   X   -   -   X   -   X
167.9   6Z      -   -   -   X   X   X   -   -   X   X
173.8   6A      X   X   X   X   X   -   -   X   -   -
179.9   6B      X   X   X   X   -   X   -   -   X   -
186.2   7Z      X   X   -   X   X   X   -   -   -   -
192.8   7A      -   X   -   X   -   X   -   X   X   -
203.5   M1      -   X   -   X   -   X   -   -   -   -
206.5   8Z      X   -   X   -   -   X   -   X   X   X
210.7   M2      X   -   -   -   X   X   -   X   X   X
218.1   M3      X   X   -   -   X   X   -   -   X   X
225.7   M4      -   -   X   -   -   X   -   X   -   X
229.1   9Z      X   -   -   -   -   -   -   -   -   -
229.1   9Z      X   -   -   -   -   -   -   -   -   -

   Socket Pin   5   6   7   8   9   9  10  11  12  13
       Strobe   1   1   1   1   1   2   2   2   2   2
    Resistors  RA  RB  RC  RD  RE  RF  RG  RH  RI  RJ

233.6   M5   ???     If anybody figures out these last four 
241.8   M6   ???     tones please let the author know.  I 
250.3   M7   ???     dismantled my test bench in preparation 
254.1   0Z*  ???     for my first household move since 1965.

0Z*= the "zero-Zee" tone.

Good luck and after you get your Mitrek going please drop me an email and let me know how things went.   I've spent a lot of time on this web page and the revisions and updates (over a dozen of them), and sometimes I'm wondering if anybody reads these missives to the masses...

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If you see something above that is vague, missing (or outright wrong), please let me know!   It's input from the readers that make these writeups better - I've probably either totally missed or shortchanged topics and /or subtopics that really need to be covered.   I can be reached at or my snail mail address available at

(I've included the QRZ link above instead of my real snail mail address since my first household move in over 35 years is coming up, and modifying all the web pages at my leisure saves having to update a lot of web pages at once right in the middle of moving).

Copyright © Michael R. Morris WA6ILQ 2002, 2003, 2004
Last modified 29 October 2004 (fixed a couple of typos)