Mitrek Conversion Instructions
Pinout for 16 pin IDC connector on Mitrek board:
1) /Receive PL Detect 2) /Xmit PL
3) /Receive Squelch 4) /Push to Talk
5) Receive Audio 6) + 2.5V Reference
7) Transmit Audio 8) Raw Discriminator Audio
9) + 13.8 10) Gnd
11) + 8 12) Gnd
13) + 5 14) + 5
15) Gnd 16) Negative 5
Pinout for 19 pin connector on Mitrek:
(Note: 1st 15 pins match the pins on the D connector on the Mitrek board.
Pins with an asterisk (*) are Not what is normally on that pin.
1) Mic Hi - Input for Tx audio. Level is about 1 V p-p.
3) Speaker +
4) +12 (power on for unit)
5) Speaker - Note: There is a 2 pin .1" header for the speaker
7) F1 - No connect, decks are strapped for 1 frequency operation,
which typically puts this pin at Gnd.
8) F3 - Rec PL Detect. Pulls to ground when receiving PL *
9) F2 - Squelch Detect. Pulls to ground when receiving RF *
10) F4 - Xmit PL. When pulled to ground, PL is added to the transmitted
signal. Has passive pull up so can be left open, which means No
transmit PL. *
11) Detector Audio. Raw, pre-deemp audio which also feeds the volume
and squelch potentiometers.
12) (was PTT hi, at +12) Demod Audio. Receiver audio out. *
13) Push to Talk "low". Pull to ground to xmit. Passive pull up to
+13.8 in RF deck
14) Squelch in from squelch potentiometer
15) Volume in from volume potentiometer
16) Audio Mute (not used)
17) Gnd. Serious ground for PA - connected via dedicated wire
18) Spare (not used)
19) + 13.8. Serious + for PA - connected via dedicated wire
Modifications to the Mitrek deck:
1) Connect "PTT Hi" to +12 internally, and then connect demodulator
audio from the internal adapter board to J1 (the main external
a) Cut run from J10-P25 to J1-P12. J10 is the 25 pin .1"
spacing connector at the bottom of the internal PCB that
connects the external connector to the main Mitrek PCB.
b) Connect J10-P25 to J10-P1. This connects +12 to the PTT
Hi line. Note that the PCB trace from J10-P1 goes right by
c) Demodulator audio out from the internal adapter board
("A OUT") will go directly to J1-P12 at the connector J1 a
2) Frequency selection area modifications:
In the area around the receive channel elements, configure
Jumpers IN: JU611
Jumpers OUT: JU606, JU607, JU609, JU610
It does not matter if JU608 is in or out.
In the area around the transmit channel elements, configure
Jumpers OUT: JU601, 2, 4, 5
It does not matter if JU603 is in or out.
Both receive and transmit are wired for one frequency only,
in the F1 channel element position.
Before you do the Audio/Squelch/PL modifications.
After you have done all of the above you can test the RF deck
and align it (see "Alignment and Standards" section). Note that
the changes to the PCB are very minimal, with just one PCB cut
and a few jumpers (you can leave the connections for the internal
adapter board and J10 off until basic operation is verified and
it is time to do the those changes). As a summary, all you should
have done by now is:
a) One cut on the J1/J10 PCB and one jumper (Pin 1 to 25)
b) Modified jumpers around the channel elements
Align the unit and be sure it works before proceeding any further.
The unit is now a basic receive OR transmit (not both) radio,
and all the RF Deck levels can be set, adjusted or tuned. PTT
works, squelch works, but there are no digital or analog outputs
The next few sections work on connecting the internal
adapter board. This board is mounted over the receive F2, F3,
and F4 channel elements position, components down, on a 90 degree
bracket. On the transmit side of the RF deck, the same screw
that holds in the 90 degree bracket also holds the heater
resistor. Be sure to mount the bracket with the threaded hole
facing up, so that the internal adapter board can be easily
fastened. Also, place a ground solder lug on the screw holding
the resistor and connect this end of the resistor to the lug
(all on the transmit side). The resistor can be added after
basic operation and alignment are done when you add the 90 degree
3) Demodulator Audio Out connections:
The demodulated audio out from the main PCB receiver section
connects to the internal adapter board at the square pad labeled
'G'. It comes from the test/access pin on the main PCB by U401
(5 leaded TO-220 style part) labeled 'G'. This is a very low
level signal (about 39 mV), and there is an amplifier on the
internal adapter board that brings the level up to about 1 V p-p.
Note that the level at point G is effected by the volume control
on the external Mitrek conversion PCB. This means that when you
change the speaker volume, you also change the level of the audio
out of the machine. The gain on the internal adapter board is
set such that a reasonable monitoring level produces about 1
V p-p out. You want to use the volume control on the external
Mitrek conversion PCB to set the demodulator audio level out, and
then DON'T MESS WITH IT ever again (put some RTV on it). After
you have the correct audio level, you simply accept whatever the
speaker volume is. The only reason to use the speaker output
would be if you connected it to a switch which would allow you to
listen to the receive audio. You can run the radio with no
speaker attached, and the audio levels stay the same.
Then, you can connect from the round "A OUT" pad on the
internal adapter board to J1-P12 at the connector. You may need
to temporary remove this when doing some of the other connections
to the internal adapter board, so if you want you can wait until
after the PL modifications are done to solder this wire on.
Power for the internal adapter board is best obtained from
J10. J10-P1 is + 12, and J10-P3 is Gnd. It is strongly suggested
that these be the only Red and Black wires in the group. Connect
these wires to P10 now. They will connect to the adapter board
4) PL Modifications:
The PL board plugs in to J3 on the J1/J10 board in the Mitrek.
These modifications are for the Model HLN4181 series PL boards, which
have the 40 pin chip doing most of the work. Be sure the RF deck
works OK (without the PL board plugged in) before doing the PL
modifications. The 1st step is to change the jumpers on the PL board
Jumpers IN: JU3, JU9
Jumpers OUT: JU1,2, 4, 5, 6, 7, 8
Then connect the signals from the internal adapter board to pins
on J10 on the J1/J10 board in the Mitrek. This should be done after
basic functionality of the RF deck has been obtained. The connection
should be made from the square pad on the internal adapter board to the
pin on J10. There are three new signals that connect as follows:
a) J1-P8/J10-P20 (formerly F3) is now /Receiver PL Detect
b) J1-P9/J10-P19 (formerly F2) is now /Squelch Detect (NRxSQ).
c) J1-p10/J10-P21 (formerly F4) is now /Xmit PL (NXMPL).
You will probably want to connect the 3 wires to J10 (pins 19,
20, and 21)
first, then install the internal adapter board and solder onto
the pads. Then connect your +12 and ground that were left from
step 3 above.
Next, install the PL board (but don't screw it down yet) and
connect from the PL board to the two square holes on the internal
adapter board as follows:
a) From "JU5" on the internal adapter board to JU5 on the
PL board, on the R21 side.
b) From "JU2" on the internal adapter board to JU2 on the
PL board, on the R23 (Not Gnd side).
Once the entire system is working, if you want you can add
about .056 uF to C40 (i.e. parallel on to C40) to move the PL notch
filter frequency down from around 180 Hz to 140 Hz. This will also
pick up some bass response. For PL's in the 107.2 and 100 range, the
combination of the attenuation obtained by being near the 140 Hz
notch and the roll off of the audio in general removes all traces of
the PL tone. In essence, all PL's up to about 160 Hz are removed, and
the "low end" or base response of the unit begins at about 190 Hz,
roughly 40 Hz lower than it was before.
5) Receiver Squelch Modifications:
While the squelch is working internally, we need to bring
out a digital "pull to ground" version of the squelch. Move the PL
board off to the side for a moment. Then, take a wire from the main
PCB off of R414 (Not the Gnd side: the side closest to Q405) to the
square pad labeled "R414" on the internal adapter board. The digital
out for the receiver squelch was connected to J10 when you did the PL
mods. You may now tighten down the PL board. If you did not connect
the "AOUT" to J1-P12, do so now.
6) Heater Resistor:
On the transmit side of the RF deck, there is a 10W 100 ohm
resistor. It is held in place with the same screw that holds
the internal adapter board. It dissipates about 2 watts of
heat if the temperature inside the RF deck falls below 65
degrees Farenheight. In addition the holding the bracket for
the internal adapter board and the power resistor, the screw
should have a solder lug on the resistor side and that should
be connected to one side of the resistor. The other side of
the resistor connect to the big square pin "H" by the TO-220
transistor center pin.
At this point, all the connections to the internal adapter
board are done. You should have 11 wires going to the board.
7) TR Relay Modifications:
These modifications are done for repeater units only. Remote
base / repeater link machines leave the TR relay and diodes
The Transmit/Receive relay (UHF in from front panel, two coax
cables out) will be left in the receive position. The cable that was
the transmitter out is replaced with a section of RG-174 and routed
through a hole in the chassis right next to the input helical filters
and is then connected to a BNC connector. Connected int RG-174 to the
LPF under the PA deck is not for the soldering challenged. You need
to disconnect the Red wire from the TR relay. Note that when you are
first bringing up the unit, you may want to leave the TR relay
connected for easy tune up and "does it work" verification, and then
cut and replace the coax after proper operation has been obtained. If
full duplex operation is not desired (i.e. remote base on a fixed
frequency), the TR relay is left intact. Finally, it is possible that
over the course of time the receive could get bad, that this relay
will need to be cycled to make sure it has a good contact. While this
is not known for sure, in normal operation this relay gets a lot of
use, and in this application it will get no use, so this should be
kept in mind.
Normally, the Mitrek turns off the receiver and mutes the audio
when you are transmitting. For repeater units where the TR relay has
been modified, you need to remove or lift one side of the following
components in order to turn off this feature: Xmit side: CR1
(by VR10). Rec. side: CR403.
8) Digital connections on the external adapter board:
The four digital pull to ground signals on the external adapter
board can be probed at the protection diodes. Receiver squelch and PTT
are on phone jacks. The Receive PL detect and Xmit PL are only
available on the 16 pin interboard connector and off of the protection
diodes. With the phone jacks facing you, as you go from left to
right the signals on the cathode of the diodes are: PTT, XM-PL,
x-PL, SQL. There is a four pin .1" center header just above the
iodes which, when the center two pins are shorted, connects Rx-PL
to XM-PL. This has the effect of "if the signal comes in with PL,
transmit PL when it goes out". This also means that if the controller
is transmitting on it's own with no input (such as a squelch tail
or ID sequence), that no PL is transmitted. The outside pins are
9) Front panel board LEDs:
The front panel board, which connects in parallel with the
16 pin IDC connector on the external adapter board, has four leds
on it which show the state of the four digital signals. As you
stare at the LEDs, the leds are for: Rec-SQL, PTT, Rec-PL,
Xmit-PL. Hopefully, the PTT is the yellow led and the rest are
green. While all the logic levels are ground true, the front panel
board is wired such that when a logical input is at ground
(asserted), the led is ON. In other words, when the PTT is pulled
to ground to transmit, the PTT led is ON.
10) Phono jacks on the external adapter board:
There are four phono jacks on the external adapter board.
As you face them, from left to right they are: Rec-Audio,
Xmit-Audio, Rec-SQL, PTT.These are primarily for diagnostic
and tune up use. Note that if you simply connect the audio's
and SQL/PTT, you have a really dumb repeater (which of course
you would only do for diagnostics).
11) SCOM 5K connector:
There is a 25 pin female connector which can be connected
directly to the 25 pin connector on a SCOM 5K controller. It
is wired pin for pin. The Receive PL detect is connected to pin
4 "CTCSS Decoder", and the Transmit PL is connected to pin 7,
"Logic Output 1". Remember that all the logic inputs and outputs
are ground true, so you need to program your 5K accordingly.
Alignment and Standards
There are no shortages of opinions on how to setup and
align radios. The Motorola Mitrek manual recommends a 4.8 kHz
deviation, which results in an occupied bandwidth of 11.8 kHz.
The channel elements from international Crystal were setup for
this peak deviation when they arrived from the factory. Azden,
Icom, and Yaseu all have in their specifications a peak deviation
of 5 kHz, which would result in an occupied bandwidth of 12 kHz.
The cellular industry sets up all of it's gear on a 6.1 kHz
deviation, resulting in a 14 kHz occupied bandwidth. The goal of
a repeater is to repeat the incoming signal with minimal distortion.
These units will be aligned for a 5.9 kHz deviation, resulting in
an occupied bandwidth of 13.85 kHz. That way, when the PL is added
in (at 950 Hz occupied bandwidth when adjust all by itself),
the total occupied bandwidth is about 14 kHz, which is the same as
maximum cellular bandwidth. All these measurements are done with
a 1 V p-p 1 kHz test tone, and an PL of 107.2 Hz. Most receivers
have a IF pass band of 12 to 15 KHz. Since the Mitrek's have an
instantaneous deviation control, they will not deviate much wider
than 14.25 kHz, even if over driven. High end radios with good
receive frequency accuracy also tend to have sharp IF strips, and
should receive the 14 kHz fairly well. Lower end radios with bad
receive frequencies tend to have broader 6 dB points on the IF's, and
so even though they may be off by as much as 4 kHz (10 PPM accuracy),
their wider effective IF frequencies should pass the signal OK.
Radios running the "correct Ham" deviation of 12 kHz will see receive
signals very close to the original one, even when slight difference
in receive and xmit frequencies exist.
The other factor figuring into the 14 kHz maximum deviation
is that a standard 455 kHz ceramic IF filter has a -6 dB bandwidth
of 12 kHz. At 14 kHz, the filter shows a -7 dB bandwidth. At 18 kHz,
it shows a -8 dB bandwidth. At 100 kHz, they are down 30 dB. The
Mitrek's can recover deviations up to 6.5 kHz deviation (14.8 kHz
occupied bandwidth, -100 dBm level) with their 10.7 MHz crystal IF
filters. Since the noise floor on the Mitrek's is at -120 dBm, it is
hard to measure any smaller levels.
This should allow the unit to receive a signal and then pass
it at the same bandwidth. It should also be noted that many HT's have
been observed to deviate in excess of the 5 kHz, so the system should
be forgiving to these units.
The alignment proceedure follows:
1) Set signal generator to 1 kHz internal modulation, with
the FM deviation set at 5.9 kHz.
2) Adjust the volume control on the external adapter board
such that the signal level coming from the RCA jack is 1
3) Remember- this will also control the speaker volume.
Normally the speaker is NOT connected. You may put a
switch in to connect the speaker for monitoring purposes.
If you do this, do NOT adjust the volume control to set
the speaker level. Just take what you get on the
speaker- after all, it is just for maintenance monitoring.
Adding a speaker (or turning it on to monitor) distorts
the audio slightly, so it is recommended that the speaker
normally be left disconnected.
1) Input a 1 V p-p 1 kHz signal into the RCA jack.
2) Adjust the potentiometer by the op amp on the external
adapter board so that you see a 1 V p-p signal on pin
7 of the op amp.
3) Adjust the deviation control on the channel element
so that you see 13.85 kHz occupied bandwidth.
4) Remove the input signal from the RCA jack.
5) Enable the xmit PL by grounding the cathode of the 2nd
protection diode in from the left.
6) Adjust the PL xmit level on the PL board for 1.8 kHz
7) re-connect the 1 V p-p signal.
8) If the occupied bandwidth is > 14.25 KHz, go back to
step (3) and adjust the no-pl occupied bandwidth
to 13.75 kHz and continue from (3)
9) remove the 1 V p-p signal.
10) Re-check the PL- it should be > 1.7 kHz occupied
Note: If you have to go back to step 3, and things still are not
right, there is a problem with the RF deck.
Pinout for 25 pin D connector that can go directly to an SCOM
Pin 5K signal Name Function .
3 Logic Input 3 Goes to pad on PCB, normally a No
4 CTCSS Decode Goes to ground when rec. PL is
6 Repeater Rx COR Goes to ground when receiving a
7 Logic Output 1 Pull to ground to xmit PL
9 Logic Ouput 3 Goes to pad on PCB, Normally a No
10 Transmitter PTT Pull to ground to transmit
11 Transmit Audio xmited audio. 1 V p-p max 3.5 K in
13 Repeater Rx Audio Received audio. 1 V p-p max 100 Ohm
14, 15, 17, 19-25 Ground