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Converting and Interfacing the Motorola MSR2000 Station to an External Repeater
By Henry Wingate K4HAL (callsign at arrl dot net)
HTML'ized and edited by Mike Morris WA6ILQ
Notes from WA6ILQ are in italics...
This writeup by K4HAL is oriented towards moving a high band MSR-2000 to 146mhz. Most of this writeup is devoted to the process of interfacing the MSR to an external controller. MSRs on other frequency bands will interface identically. In the text below "station" is the radio industry term for a base or repeater (i.e. a "station" is not a mobile or a handheld).
The MSR2000 is the station version of the Mitrek series of radios. It is like a Mitrek mobile and a Micor station shelf combined. The model we used for our repeater was a C73GSB-3106BT, a 100 watt 136-150MHz high-band repeater with an intermittent duty amplifier. This repeater will tune to the two meter amateur band with no modification. We wanted to incorporate a ham-type controller and chose the CAT200B from Computer Automation Technology, although most any ham-type controller would work. Below we have listed the steps we used for the conversion.
First, connect a dummy load to the MSR and power it up. Make
sure it works on its original frequency first !
The unit we purchased was set up for a wire line voting type of arrangement, but we could key the transmitter to test it using both the local microphone and the “xmit key” switch on the station control module. If you have the test box that comes with the station plug the speaker in to test the receiver. If you do not have the test set, connect the cable from an external speaker onto the speaker terminals on the control chassis backplane. The control chassis may be accessed by removing the upper screws and tilting the whole chassis forward.
Note from WA6ILQ:
Before you do anything to the radio, test the receiver and the transmitter on the original frequencies. Note the transmitter power output, the total DC current draw, the receiver sensitivity, etc. Record baseline numbers for every position of the test / metering set. If your unit has any additional options, like a multi-tone PL decoder or encoder, make sure all the options work. If it has a second receiver, test it as well. Map out all the jumper options (xerox the backplane and option card layouts and draw in the jumpers on the copies with a colored pen). Take lots of pictures! Digital photos cost you nothing but the time spent taking them and some bytes on a hard drive, and it's better to have them and not need them than vice versa. Note all of this info in a system documentation notebook that will stay with the station. After the frequency conversion is done you will want to compare the receiver and transmitter performance, and having baseline numbers is necessary to find out why any of the numbers are different on the new frequencies. The transmitter DC current draw at several different power levels before and after conversion will help you determine the transmitter efficiency on old and new frequencies.
Remove the channel elements
Once you are satisfied that the station is working properly on the original frequencies, shut off the power, remove the shields covering the exciter and receiver, and unplug the exciter and receiver modules. Remove the channel elements and send them to a crystal manufacturer to be re-crystalled for your frequency. We used Bomar, but there are a number of other manufacturers that are just as good. We recommend doing it this way since we have had very poor luck with replacing the crystals in Mitrek style elements. When you receive the elements back from the crystal company then you can continue as outlined below. This article may be of interest.
Remove the duplexer and re-cable R.F. connections
The version we converted used a TLD2502A duplexer. This is a wide frequency spacing duplexer and must be removed for use on the 2 meter amateur band. If your station contains such a duplexer, remove it and use a barrel connector to connect the coax going from the amplifier to the coax going to the connector on the cabinet. Label this connector "TRANSMITTER". Now mount a new connector on the cabinet and connect a piece of double shielded coax to it and to a PL259 connector. Now use a UHF barrel connector to connect the receiver coax to the cable. (I used a coax pigtail from an old Micor or Mocom 70 mobile here.) Label this new connector "RECEIVER".
Editor's note - The factory high band duplexer, as K4HAL states, is designed for a wide (multi-megahertz) recever-to-transmitter spacing and simply will not work with the 600 KHz spacing used on the amateur 144-148 MHz frequencies. On the other hand, a UHF MSR will have a 5 MHz duplexer in place that will probably tune right down to amateur channels. Note that you really want to use silver-plated "N" connectors at 100 MHz and higher frequencies. I prefer double shielded RG-400 cable for all repeater RF connections at 100 watts or less and here's why.
Install channel elements and tune
Follow the procedure in the manual. A manual is a MUST !! By presetting the slugs to the proper number of turns initially as outlined in the manual, the tuning is fairly straightforward. There are two manuals for an MSR, one is the RF manual and the other is the manual on the card cage backplane and on the cards. You will need both. Install the channel elements that are back from the crystal company and tune the station to the new frequency. Even though our power amplifier was rated for 110 watts, we set ours to 60 watts because of the high duty cycle in amateur repeaters. Once you are satisfied that the MSR is operating normally, on frequency and with a clean signal, continue below with the controller interfacing.
Remove power and remove all cards from the card cage except the following:
On the receiver: (these jumpers may already be in place)
On the R1 Audio board: (these will probably be missing)
On the backplane:
Editor's note - You also can replace the jumper from 17 to 18 with a
switch, with the switch wiper on the audio amp input and one position on the
receive audio, and another position on the exciter audio. This lets you use
the local monitor speaker to listen to what goes out over the air, even if
the transmitter is disabled.
Another useful mod is to add DC Power and PTT LEDs on the Station Control card as there is no indication on the front of the station of power on or transmit status. After all, it was intended to be in a closed cabinet in the back of a business or on top of a mountain.
Adn you can skip the jumper on the “F1-tone board” card connector pins 18-22 as there is a jumper “JU-1” on the back-plane which was put there for that specific purpose (thanks to Tony Lelieveld VE3DWI for that reminder).
Install the cable that goes to the controller.
Now make the following connections to the backplane using a piece of 6 conductor SHIELDED wire. Simply solder the wire to the correct backplane pin. Try to solder the cable to the solder pin at the base of the pin. This way if you need to troubleshoot a card and don’t have an extender, just plug the board into the rear of the backplane. This is the cable that will connect to your controller (the CAT has a DB-25 connector on it).
Editor's note - K4HAL simply soldered a cable to the backplane pins
and on the other end of the cable was a DB-25 that mated with the
controller. One additional trick is to mount a small metal bracket
on the MSR chassis next to the backplane and mount a DB-9 or DB-15 connector
into the bracket. Then run leads from this connector to the appropriate
backplane pins. This chassis mounted connector can be used as a test
connector during the initial conversion process. A cable plugged into
this connector can connect to the repeater controller. The pinout
established by Link Communications years ago for a DB9 radio connector
has become a semi-standard for controllers:
pin 1 gnd
pin 2 PL decode logic level
pin 3 PTT
Pin 4 Transmitter Audio in (gnd goes to pin 8)
pin 5 Receiver Audio out (gnd to pin 9)
pin 6 gnd
pin 7 COR (Receiver carrier detect)
pin 8 and 9 gnd
The Scom 7330 3-port uses a different pinout:
pin 1 Audio from receiver to controller (gnd goes to pin 6)
pin 2 COR (Receiver carrier detect) logic level from receiver to controller
pin 3 CTCSS (PL) Decoder logic level from receiver to controller
Pin 4 PTT from controller to transmitter
pin 5 Transmitter Audio in (gnd to pin 9) from controller to transmitter
pin 6 Receiver audio gnd
pin 7 CTCSS Encoder audio gnd
pin 8 Transmitter CTCSS Audio Encoder or Logic Level Out (jumperable) from controller to transmitter
pin 9 Transmitter audio gnd
The ground referred to on the Link connector pins 4 and 5 or Scom pins pins 6 and 7 refers to the outer shield of the audio cable - many folks use RG-174 coax as audio cable in a repeater cabinet. You can also use a DB15 with the first 9 pins wired as to one of the lists above and the rest as you like - perhaps with the speaker on two of them.
The connections from the MSR backplane to the CAT controller we used are:
Before connecting the controller, make sure that the transmitter still works by keying it by using the Xmit switch on the station control card. Now connect an audio oscillator set to 1 kHz to pin 12 of the exciter. While transmitting, observe the deviation on a service monitor. Slowly advance the audio level from zero until you see limiting starting to occur. Set the deviation to 5 kHz by adjusting the “F1 IDC” control on top of the exciter channel element.
In the CAT-200B we set the 8-position DIP switch as follows:
|1||OFF||COR is active high|
|2||OFF||CTCSS is active high|
|3||ON||Link COR is active low - see below|
|4||ON||Link CTCSS is active low|
|5||OFF||Mode select - Repeater controller (off) or Remote Weather Station controller (on)|
|6||OFF||CAT CI-200 option board present or not present|
|7||OFF||Initialization select - see CAT-200B manual|
|8||OFF||set password or RS-232 mode - see CAT-200B manual|
Now connect the controller and apply a full quieting signal with a 1 kHz tone at a 3 kHz deviation to the receiver. Set the RX1 level pot inside the controller so that 220 mv is measured on TP1 inside the controller. Now adjust the TX1 level control in the controller for 4 kHz of deviation. This produces the slightly hotter audio preferred on amateur repeaters.
Editor's note - My personal opinion is that repeater audio should be "transparent" - the repeater should not "color" the audio except for (optionally) cutting off the lower frequencies (the PL tone range), and the in-to-out ratio ideally should be 1:1 up to about 4.5khz - i.e. 1khz in gives 1khz out, 2khz in = 2khz out, 3khz in = 3khz out, 4khz in = 4khz out, 4.5khz in = 4.5khz out, 5khz in = 4.5khz out, 5.1khz in = 4.5khz out etc. up to the point that the receiver IF bandwidth limits the audio recovery. The 1:1 ratio is the ideal but you won't see perfectly linear relationship due to the audio processing in the audio path from the receiver through the repeater controler and the transmitter modulator - it might be 1:1 at one frequency, 1:1.1 at another and 1:0.95 at a third, and that's OK. In an ideal world, if you were to switch a receiver between the repeater input and output you should not be able to tell which channel you were listening to - except by the absence of the low frequencies (the PL range) and the presence of the repeater ID.
At this point the conversion is complete except for programming the controller with the ID message, timeout timer value, carrier delay timer value, etc.
Since we were not going to use the second channel of the controller, we
decided to use it as audio in for the local microphone.
We used the following connections:
Connect pin 5 of the CAT to “Local PTT” on the backplane. (COR2 on the CAT)
Connect pin 12 of the Cat to “un-notched mic hi” on the backplane (RX audio2 on the CAT).
Set switch 3 on the CAT to on (COR2 low enable).
Now configure the CAT for no link timeout, and enable the link. The local mic will now function and you can set the microphone audio level by using RX2 pot in the CAT.
Notes from WA6ILQ:
1) One common mod to the R1 audio board is replacing the local speaker volume pot (screwdriver adjust) with one that has a shaft and a knob. The squelch pot is also a screwdriver adjust, but I leave that one alone... it keeps people from adjusting it. Besides, the cabinet has a lock to keep out the folks that like to turn random knobs...
2) You don't really need to sacrifice a controller audio channel to have a local microphone. If you look at the schematic of the MSR card cage and the cards you you will see that there are multiple audio paths that mix and feed the transmitter. I prefer to leave the local microphone connector hot and feed repeat audio in a different path so that I can walk up to the repeater, turn up the volume control (local speaker volume), pick up the local microphone and talk on the repeater transmitter mixing with the repeat audio. If I need local takeover I have a clearly marked two-position mini-toggle switch with one pole in line with the repeater controller audio output and the other in line with the controller PTT. One flip of the repeat disable switch kills the repeater without killing the local receiver or the local microphone. Note that the design of the regular Motorola mobile microphone has a two-pole PTT button, with one pole as the actual PTT switch and the other pole in series with the microphone audio - you can be assured that you will not be mixing in-cabinet audio on the transmitted signal - the microphone element is not active until the PTT button is pressed. And hanging a mobile microphone clip into the cabinet is easy.
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Editor's note from WA6ILQ -
Comments, critiques, attaboys and complaints are all welcome! 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 can be reached at (callsign /at/ repeater-builder /dot/ com) - my permanent email address - or my snail mail address available at www.qrz.com.
Text © copyright by Henry Wingate K4HAL (callsign at
arrl dot net) 2003
HTML and notes © copyright by Mike Morris WA6ILQ
Original article posted 23-Aug-2003
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All rights are reserved, including that of paper and web publication elsewhere.