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  Connecting a CAT-250 to an MSR2000
By Nate Bargmann, NØNB
HTML'd from Nate's PDF by Mike Morris WA6ILQ
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This article describes the conversion process as used on
the WØDOD/R, 147.285+, Marysville, Kansas repeater.


The radio used for this repeater is the Motorola MSR2000.   The radio itself can be packaged in several different configurations in several different cabinet sizes or in a rack mount enclosure for installation in a customer's equipment rack.   This particular radio is housed in a "Compa Station" cabinet and stands about two feet tall.   The radio takes up most of the space, but a CAT-250 rack mount controller can be fitted in the area just above the receiver/transmitter shelf in front of the power supply.   This particular radio has the "Intermittent Duty" power amplifier which will tune down to 146 MHz without modification as will the rest of the RF modules.   Depending on the duty cycle of your MSR2000 you may want to add a fan or two.

Connections between the CAT-250 and the MSR2000 are made on the Remote Control backplane (the card cage backplane).   Configuration is straight forward with a minimum of custom connections and only the R1 Audio/Squelch card, which provides squelch setting and a source of constant level emphasized audio.   Configuration details follow.

Setting up the MSR2000

First, get the Motorola manuals (one for the RF sections and the other for the Remote Control chassis).   The manual numbers are on the MSR page at this web site.   Note that the stations have two books, one for the control shelf, and one for the RF section.   Motorola unfortunately chose to put the power supply schematics in the VHF book, so if you have a UHF station you need three books.   Once you have the RF book for your station you should follow its alignment procedure carefully, and do it before you have the crystals replaced in your channel elements.   Why?   Just to be sure the radio is working to original specs (it's difficult enough re-aligning a working radio let alone a broken one).   Then ship the elements off to the crystal manufacturer.   Once the elements are returned to you rebuilt on your frequency, re-align the radio to it's new frequency and be sure it meets the performance specs you measured earlier.

Some caveats:   The Intermintent Duty PA needs to be carefully aligned along with the transmitter driver alignment per the manual or it can go into oscillation or generate broadband noise.   The IDC potentiometer in the transmitter has a tendency to get dirty and cause erratic transmitted audio over time.   The MSR2000 can be optioned in almost an infinite number of ways.   The philosophy I used is to keep modifications and the use of uneeded modules to a minimum.   Consequently, this document may differ greatly from other approaches.

Option Strapping Summary

The MSR2000 is designed to fulfill a variety of uses by the use of jumpers on the various boards.   The particular radio I used had been a wireline control simplex base station with with a T/R relay incorporating one antenna connection.

Transmitter / Receiver RF connections

Of course, this being a repeater, there needs to be a separate connector for the transmitter and the receiver.   In this case I disabled the T/R relay (mounted to the cabinet wall with an integral SO-239 connector) by unplugging the relay coil wires at their wiring harness socket connection.   This leaves the original antenna connector functioning as the receive antenna.   The cabinet of this radio has the holes for two additional panel mount connectors already drilled (bonus!).   I was able to procure at a hamfest a couple of surplus cables made of RG-400 with a N type panel mount female connector at one end and an N type male at the other.   Since the output of the Intermintent Duty PA features an SO-239 connector, it was a simple matter to crimp on a PL-259 (RF Industries RFU-505-ST from RF Parts, Inc.) after cutting the RG-400 to length (bonus, as I still have a nice length with a male N connector on it for a future project) and mounting the panel connector into the cutout with small pop rivets.

The final result is that the RF connections to the MSR2000 are an SO-239 for receive and type N for transmit and the connections are handy on the outside of the cabinet.   They do require water proofing.

Module Option strapping

Below is the strapping used by the company I work for in interconnect/repeater installations with remote control.   The CAT-250 accomplishes nearly the same thing and these jumper options work well with it.

CAT-250 connections

Connections from the CAT-250 to the backplane are made to the Squelch Gate and Exciter connector pins on the backside of the Remote Control chassis.   I used a shielded cable and terminated the other end into a DB-25 connector per the CAT-250 manual.

CAT250 DB25 Connections
Function Remote Control
Chassis Connection
6 COR Squelch Gate slot pin 5
13 Receiver audio Squelch Gate slot pin 17
11 Transmitter audio Squelch Gate slot pin 11
10 PTT Exciter pin 7
17 Ground Backplane ground

Remote Control Chassis
(card cage backplane)
Straps In Straps Out
Exciter pin 5
to ground (see note)
Note: Strapping the Exciter pin 5 to ground selects F1 in the exciter and eliminates the need for the the Station Control module.

This particular repeater doesn't use CTCSS. If it did it would require a separate "coded squelch" decoder module. If your application requires the repeater receiver to detect tone or the repeater transmitter to send a PL tone, then you would need the decoder-encoder module.

This completes the hardware configuration of the MSR-2000.

Interface notes

The COR voltage of the MSR2000 is rising positive on squelch opening, so on the CAT-250 dipswitch 1 is set OFF.   The PTT requires ground to key the transmitter at Exciter pin 7, which the CAT-250 provides.

Level setting

Setting levels on a repeater is important.   Ideally, the transmitted audio should be a faithful reproduction of the station being repeated.   The MSR2000 does this job well.

The R1 Audio and Squelch module provides a constant amplitude audio signal to pin 17 of the Squelch Gate connector.   This audio is not de-emphasized so J8 on the CAT-250 must be in place.

Follow the level setting procedure in the CAT-250 manual.   I was fortunate to have a pair of communications service monitors handy.   By generating an on-frequency signal with a 1 kHz tone @ 3 kHz deviation @ 1000 microvolts, I was able to adjust the transmitted audio to match.   Some interaction between the CAT-250 controls and the MSR2000 IDC control occurs so you will need to experiment to strike a balance between linear audio levels and not exceeding 5 kHz deviation of the transmitted audio.

Finally, I set the squelch control of the R1 Audio and Squelch to where it just opens when a signal strength of 12 dB SINAD is received.   This should allow a mobile unit to work near the edge of the repeater's range without kerchunks due to extremely weak signals.


Again, this is just one more way of interfacing the CAT-250 and the MSR2000 to each other.   Each repeater has different requirements so your installation may differ slightly.   Feedback is always welcome.   You may email the author at n0nb /at/ arrl /dot/ net.

73, de Nate, NØNB

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Article text © Copyright 2007 and date of last update by Nate Bargmann, NØNB
Artistic layout and hand-coded HTML © Copyright 2007 and date of last update by Mike Morris WA6ILQ for repeater-builder.com.

This page originally posted on Monday 13-Aug-2007

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 Reserved, including that of paper and web publication elsewhere.