A Straight-Forward Modification to place an external controller into operation on a Motorola MSR 
2000 (and Micor) Repeater, connection via the Squelch Gate Module. 

This Revision Jan/Feb 2005 (previous revisions 04/98 & 07/03) www.radiowrench.com/sonic 
By: Skipp May P.O. Box 192 Elmira, CA. 95625 Email: nospam4me@juno.com 

You may use this information, free without special permission, provided you do not remove or delete 
any portion of the contents, labels or credits. Thank you. Please read this document through before 
starting your external controller modification/conversion.

The addition of an external Repeater Controller to a Commercial Repeater Station allows the users and 
operators many advantages. Features might include Telephone Interconnect (Auto-Patch), Macro Control 
functions, External Linking Ports, Voice Response and Data Telemetry Functions. There are many 
brands of Amateur Repeater Controllers made; all have their merits, features and special "bells & 
whistles". This modification procedure has the advantage of being brand generic and simple. This 
modification should also work on the Micor Repeater using the same Squelch Gate Module connections.

Starting Point, Ground Zero: 
You should have a Motorola MSR 2000 (or Micor) Repeater Station in working order, configured for 
normal duplex carrier squelch operation using the factory Squelch Gate Module jumpers. Don't be 
confused by the many possible repeater - base station card & jumper configurations; based on my 
experience with this type of Motorola equipment, there are many possible working module or card – 
jumper options. Only three or four specific back plane (card-cage) jumpers are actually required. 

You should have the Service Manuals for your Repeater: 
There are actually three basic MSR 2000 Manuals, one is the (modules and back-plane) Control and 
Audio Instruction Manual 68P81061E40-C (or similar part number), and the other two are for the RF 
portions of the hardware. There is separate VHF 68P81061E50-C (green stripe on cover) and UHF 
68P81061E55-C (blue stripe on cover) Station Manual (stripe color depending on the frequency range). 
It's worth mentioning that the power supply appears to be covered only in the VHF Station Manual only. 
The Control and Audio Manual has a burnt orange color stripe on the cover. 

I have seen and own various copies of MSR 2000 Service Manuals for exported and special production 
units, the most popular being the Canadian and Special Order "As-Built" Models. For the most part the 
major differences in the Canadian units are the smaller power supply and lower power RF Amplifier. One 
can probably use a Mitrek Consolette (Base Station Radio) or Mobile Radio Service Manual to repair the 
lower power Canadian units. Motorola also supplied micro Fiche manuals for late production units. 

Modification Concepts, Description and Scope: 
Repeater Audio, Sub-Tone Detection and Carrier Operated Squelch "COS" (also known as Carrier 
Operated Relay "COR") functions and logic are routed through the Squelch Gate Module. This is a 
straightforward modification to insert an external controller into the required function and logic paths 
through an interface cable connected to selected points on the Squelch Gate Module Card. 
The added interface cable can be hard-wired (permanent) to the Squelch Gate Module, or connected 
through a mounted jack & plug assembly such as the common DB-9 (computer style) may be used. A 
picture of a completed hard-wired Module should be included with this text package. Using the DB-9 
(actually D-9) or similar jack /plug method, an (optional) simple spare "service jumper plug" allows 
original factory card operation (revert) while doing upgrades and repairs on the external controller. 
Similar modifications found on the net use a back plane mounted switch, which could also be used in this 
Required Modules / Cards: 
Standard repeater operation can be obtained with minimal number of modules installed into the back 
plane (Module Card-cage). The minimum cards I use and recommend as required items are the Station 
Control, Time Out Timer, R1-Audio, Tone Private Line "PL" and Squelch Gate Modules. In reality, the 
Tone Private Line "PL" (not required for carrier squelch operation) and Timeout Timer Modules are not 
required, but I highly recommend you use them. Do not allow a repeater on the air without a proper time 
out timer installed, don't rely on the external controller to provide the function. It's very simple to 
include the original TOT Module set for 10 minutes, although my fixed timeout timer value is typically 1 
or 2 minutes maximum transmit time. Don't trust an external repeater controller to be failsafe. 

Getting ready: 
This modification interfaces with the only Squelch Gate Module. There are numerous jumper 
combinations on the back plane (card-cage) and the Modules. You should not remove or modify the 
jumper settings unless you clearly understand their function. Write down (record) the original jumper 
configuration and any changes you make, if possible keep the record with the equipment in a logbook. 
This modification procedure assumes you have a problem free working MSR 2000 or Micor Repeater. 

Repeater Controller Basics: The typical external interface connections include the following terms. 

Transmit "PTT" Logic: DC (voltage level) logic sourced from the external controller to the MSR 2000. 
Receive "COS/COR" Logic: DC (level) logic sourced from the MSR 2000 to the external controller. 
Sub Tone (CTCSS) Logic: DC (level) logic sourced from the MSR 2000 to the external controller.
Transmit Audio: AC voltage (normal transmitter audio) from the external controller to the MSR2000.
Receiver Audio: AC voltage (normal receiver audio) from the MSR2000 to the external controller.

Audio connections are relatively low-level AC voltages. When properly connected as described in this 
text, the receiver audio to the external controller is gated (muted) off with no received signal. Muted or 
gated (silent when off) audio keeps some external controller DTMF (touch tone) controller circuits from 
false detecting (falsing-off) or locking onto a specific receiver port during moments of rushing no-signal 
discriminator white noise. 

Logic connections are often DC voltage changes above and below some relative voltage value set in the 
external controller. A high to low ("sink" or "active low") or low to high ("source" or "active high") 
transition supplies receive signal indication and transmitter keying. Refer to your specific controller 
manual for the voltage value. The typical logic transition voltage is often fixed in the controller hardware 
somewhere from 2.5 to 9 volts dc. Most external controllers operate on +12 volts dc negative ground. 

A little secret revealed: The MSR2000 circuit designers (engineers) probably didn't want to redesign 
another Squelch Gate Module when the original Micor Module circuit works so well. So they simply used 
the same circuit, in fact almost the exact same circuit and board layout. 

The MSR 2000 and Micor Squelch Gate Modules are pretty much exactly the same. The only 
significant difference is the module edge connector, which can be changed. You can remove the white 
plastic edge connector from a spare MSR Module and place it onto a Micor Squelch Gate Module. 

Micor Squelch Gate Modules (cards) are often priced much lower (vs. MSR 2000 Squelch Gate Modules) 
from the surplus two-way radio dealers and on line auction sites (Ebay). I've saved serious money by 
removing the plastic end connectors from non-essential (unused) MSR 2000 modules for placement back 
onto a lower priced surplus Micor Squelch Gate Modules. 

Getting down to work: 
Remove the Squelch Gate Module from the card cage. Drill and file (de-burr) out to shape a hole in the 
front panel for a common DB-9 (actually D-9) jack or your choice of wire strain relief. A mounted jack 
and matching cable (with plug) provide easy removal of the external controller (for service). A bypass 
controller jumper plug can be made for controller service and firmware upgrades. I've never had a 
Squelch Gate Module fail; so most of my later conversions have the interface cable routed through a 
common Heyco Brand strain relief type grommet without the DB-9 (actually D-9) jack plug assembly. 

The interface cable I use is special sold by Motorola for about $1.10/ft under part number 3083560A02 
and can be ordered direct by calling Motorola Parts (800) 422-4210. Do not use common unshielded wire 
for the interface cable unless you are willing to deal with serious potential transmitter audio noise 
problems. The mentioned interface cable has a very well cross-shielded brown wire which servers to 
bring the receiver audio to the external controller. I use Motorola Base Station microphone wire color 
codes for matching functions as described in most Motorola Base Station, Remote Control and 
Repeater Service Manuals. The Motorola sourced multi conductor microphone wire is/was used and 
often found on older surplus non-modular plug type, hard-wired Motorola desk mics. It has a very durable 
flexible black vinyl or rubber cover. About 5 inches of the interface cable black cover is removed, the 
shield around the brown wire is carefully unwound back, twisted into a wire and the tip soldered to secure 
it from unwrapping. A 4-inch length of proper size shrink tubing is used to cover the shielding wire 
(braid), shrunk into place. 

Squelch Gate Card Wiring: 
The card should be removed from the repeater for conversion using basic tools and a soldering iron. The 
following part numbers may not exactly match values in your specific Control and Audio Service Manual. 
The following module (card) edge numbers and functional descriptions should easily be good enough for 
you to note the obvious manual part number differences. 

Sub Tone "PL" CTCSS Logic: 
Sub tone "PL" detection arrives on the Squelch Gate Module edge pin 4 or 14 to bias transistor Q9 on 
during received PL Module decoded ctcss (sub tone). It's a simple process to first remove diode CR9 and 
use the now empty Q9-side hole to solder the Motorola brand cable white wire. Jumper JU14 should be 
placed on the "PL" pin. The white wire is now active low, approximately +13 volts dc on the collector of 
Q9 will drop to near zero when a valid signal is received with the proper sub tone (CTCSS). 
Squelch Gate Module Jumper JU14 is placed on the PL pin (jumper) selection.

You may remove resistor R36 (10K) if your external repeater controller input circuit provides it's own 
logic pull-up supply. In most cases it will not hurt to leave R36 in, even if your controller has an internal 
logic (pull-up) supply voltage. Some early ACC (and other) brand controllers did not have logic pull-up 
resistors as received from the manufacture (the source of much hair pulling for ACC Controller Owners). 
You must leave R36 in or add a pull-up resistor at/in/on the external controller for some brand controllers.

Audio connections: 
Actually very simple… R74 (680 ohms as labeled in my Control and Audio Manual) is removed 
providing a convenient place to solder in both the required external controller interface cable audio wires. 
The shielded brown (Motorola wire color code standard) wire serves as the receiver audio source and is 
soldered into the empty R74 hole on the Q22 side. 
The available yellow wire is soldered into the empty R74 hold on the C17 side. The front panel Repeater 
Level Adjustment (R66) pot is preset to near full on (CW) rotation. Turn R66 full clockwise; then reverse 
the pot adjustment backwards just a small bit to get off the end of the adjustment limit. At this time, I use 
a small amount of Caig Labs ProGold G5 Conditioning Treatment (expensive but well worth the cost) on 
both the audio level and squelch key pots. 
Receiver COS/COR and Transmit "PTT" Logic: 
Again a simple interface connection, Squelch Gate Module Jumper JU12 is removed and the interface 
cable black wire soldered into hole on the Q17 side of the former JU12 jumper, to serve as the received 
signal indication (COS/COR Logic). The interface cable green wire which serves as the transmit function 
is wired into the hole on the module (card) edge pin18 side of the former JU12 jumper. Note the interface 
cable black wire is not used for ground. The previous version of this modification inserted the COS/COR 
and PTT logic at the Q10 collector. Both versions of this mod similarly complete the same goals, the 
current Q17 version is much more sanitary (clean) and logical. Attention must be paid to setting the 
proper External Controller COS/COR and PTT active state logic. The controller logic state might be set in 
the external controller software or with a typical hardware dipswitch internal to the controller. 

The external wire shielding (ground) or braid (unwound from the brown wire) is now wound with the end 
section "tip" soldered into a now available wire with an added shrink tube cover is soldered to the module 
(card) ground foil. On the part side of the module, I simply route the ground wire to and solder the end 
underneath the groundside lead of resistor R6. This newly made up wound and covered shield wire serves 
as logic, power and audio grounds. 

Limited Controller power: 
The controller supply voltage can be taken right from the module providing you limit the external 
controller current draw to a safe relative value. My interface cable +13 Volts DC red wire has an inline 
3ag type fuse holder added with a 1 amp "safety fuse". The card side of the fuse holder is soldered on top 
of jumper JU9. If JU9 is removed, solder the wire to the now available JU9 hole, which does not lead to 
module edge pin 20. (Jumper JU9 is an option to make the card A+ (+DC voltage) available to module 
edge pin 20.You want to use the other available JU9 hole if the jumper is not in place). JU9 in place has 
about +13 volts dc with limited current. I would not try to source more than 500mA (1/2amp) from the 
module using this connection. Check the expected maximum current draw of your controller to be safe. 

Some Jumper Information: 
JU13 should be set for 1-second delay when first starting out. After you feel the conversion works well 
for you, place the jumper on zero (my choice) or any desired tail-time value available on the module. 
Jumper JU15 is placed on the Carrier Squelch pin (jumper) selection. 
Jumper JU14 is placed on the PL pin (jumper) selection.
The front panel Repeater Level Adjustment (R66) pot is preset to near full on (CW) rotation. The initial 
preset value of R2 Repeater Squelch Key Adjustment is at mid position. 

The above steps should complete the required Squelch Gate Module to external controller interface 
connections. Please recheck your work and replace the module into the MSR after making the proper 
connections to your external controller. Please remember the external controller has active high/low logic 
options/jumpers, which may need to be configured. The described modifications make the external 
logic lines for COS and TX-PTT active low. You are now done with the hardware modifications. 

Some External Controllers have LED status indicators, which are very handy for testing the new interface 
circuit. Proceed to the following Level Adjustments section if your repeater appears to operate normally.

Basic Level Adjustments: 
Receiver to External Controller audio, disable the Repeater Transmitter (simply remove the transmit 
channel element) and place a proper (on the receive frequency) 10uV saturated signal with a modulated 
1KHz frequency test-tone, 3KHz deviation into the receiver antenna port. You should be able to set the 
external controller receiver audio input level using a scope or AC voltmeter, inside or at the controller 
input (or indicated controller audio input test point). Replace the transmit channel element if removed. 

Now comes "the magic of setting the Motorola Channel Element IDC" (deviation control).

First you need an adequate source of test tone (1KHz frequency) audio, which can be from your 
external controller or through the repeater receiver – controller audio path. By default, the transmit 
channel element top mounted IDC control should probably be at least ¾ full on (clockwise) rotation. The 
external controller transmit audio output pot should be min ¾ on. Hopefully you have a Service Monitor 
with a scope display for the following steps. Remove the transmit sub-tone (PL) reed (vibrasponder) from 
the PL Module (card). 

The tight MSR 2000 Receiver IF crystal filters severely limit discriminator audio at higher 
deviations. As you adjust a service monitor receiver injected (1KHz test tone modulated) signal up and 
down in tone deviation, you'll see the waveform (scope display at the receiver discriminator output) 
audio start to compress or distort at ~4KHz deviation. As you rise above 4KHz test signal input, the IF 
filters in the receiver will start to seriously limit the available receiver discriminator audio. The receiver 
IF filters comprise an effective limiter function, which make up the corresponding transmitter deviation 

External Controller to the Transmitter audio, enable the transmitter and enable the repeater carrier 
squelch transmit function in the external controller. I set up a 1KHz tone into the transmitter and 
watch on the service monitor (transmit display) scope to see where the injected transmit test tone 
sine wave starts to distort more than a little bit. The initial waveform distortion test is most often done 
using the repeater controller (through controller programming) or receiver through-path as the tone 
source. Note: many external controllers have an audio level control for internally sourced (generated) 
test tones. Rotate the transmit channel element IDC for at least 6KHz to 7KHz heavily distorted (as 
displayed on a service monitor scope) transmit audio deviation. Reduce the tone source level until you see 
the transmitter audio deviation waveform return to an approximate sine wave (clean-up near normal). 

I make a judgment call on how far above that first transmit test tone distortion point is the actual hard 
limiting with an ugly displayed audio waveform. Most often, the hard limiting happens less than 2 or 
3KHz deviation above the first signs of real audio sine wave distortion. With the injected test tone value 
set just at the edge of distortion I set the transmit channel element IDC control to first show a distorted 
waveform at about (plus & minus) 5.5KHz to 6KHz transmit deviation, with no sub tone enabled. 

The described TX channel element IDC adjustment is not the expected or dependent audio limiter 
for maximum transmit deviation. Normal system operation depends on the Receiver IF filter bandwidth 
to limit the transmit audio. Once you preset the Transmit IDC pot to start limiting hard at 5.5KHz to 
6KHz deviation, you simply set the external controller transmit audio level control using a receiver port 
injected 10uV saturated signal at 3KHz deviation, 1KHz test tone frequency. Since the receiver to 
controller value was set in previous steps, match the transmit deviation to the 3KHz receiver input signal. 
I often add a small bit of extra transmit deviation to cover any base-band noise on the transmit signal, 
which is often about 100 to 200Hz added deviation onto the transmit signal. Force your repeater 
controller to transmit test/CW tones or voice audio and set those values as desired. Average uncompressed 
voice audio is less than 35% duty cycle. Set external controller tones for 1KHz to 3.5KHz deviation, 
depending on frequency. Set controller synthesized voice audio to be loud and display on your service 
monitor scope with maximum voice peaks near the full system deviation (say 5.5 to 6KHz) deviation. 

Using injected receiver antenna port test signals, you should see the receiver input to transmitter output 
deviation ratios start to quickly fall apart (un-match) above the 3KHz benchmark. The scope-displayed 
transmitter to receiver input ratio should start heavily compressing above values of 4 to 4.5Khz receiver 
test signal deviation. The receiver to transmitter deviation ratio centers on the 3KHz initial value set point. 
Although we wish they would be, the receiver-to-transmitter deviation ratios are never linear values.
If your repeater has the PL Module (card), replace the transmit sub tone reed (vibrasponder) and check the 
transmit sub-tone (ctcss) deviation without the 1KHz test-tone audio. The level should be under no higher 
than 700 to 800KHz deviation; lower values down to even 400KHz deviation are just fine. Note the total 
system deviation is now the additions of the normal transmit audio with the sub tone deviation. There 
is no variable adjustment for the ctcss (sub-tone) level except the channel element IDC control, which 
should probably be left alone. If your sub tone deviation level is objectionably high, PL Tone Generator 
circuit fixed resistors might be changed to reduce the level below 800 to 900KHz deviation (which is 
pretty high, but not a major problem). My target sub tone value is (plus and minus) 500Hz deviation. 

With the transmit sub tone again enabled and a 3KHz deviated 1KHz frequency test tone, the output 
deviation should be near 3.6Khz deviation. As you increase the receiver (input) signal tone deviation, the 
transmit deviation will start to compress (depart from the original linear input VS output relationship) 
when you raise the test signal deviation above approximately 4KHz deviation. Below 3KHz deviation, the 
input signal should track similar to the output deviation with the additional sub tone deviation level added.

Notes worth knowing: The squelch adjustment on the R1-Audio module controls when the receiver R1 
Audio and Squelch Gate Module audio mutes (gates) and is separate from the Repeater Squelch Key 
adjustment. The typical settings place the R1-Audio adjustment to open the Squelch Gate Module receiver 
audio gate circuit at a received signal level slightly higher than the Repeater Squelch Key Adjustment. 
This example removes squelch closing crash noise, even during carrier squelch operation. Very nice 
indeed. Note: There is a critical alignment procedure described on/for the R1 Audio & Squelch 
Module page, under the page manual Notes: #5 for controls R7 and R25, which must be completed 
before your final squelch key and R1 Audio Squelch control adjustments (described below).

Another example with relative number values: Set the Module Squelch Key Adjustment with a .5uV 
(micro volt) signal into the receiver antenna port, to enable (key) the transmitter. Then set the R1-Audio 
Module Squelch adjustment to open the audio gate at .7uV receiver RF-signal input. Normal receiver 
signal strength will be constantly changing and the .2uV "dead band" difference will rarely be present for 
any significant (or even noticeable) length of time. The above described squelch circuit action is a 
composite operation of the Micor fast slow audio squelch board circuit. You'll find the repeater operation 
received signal characteristics very pleasing to repeater system users ears.

Normalize (clean up) your repeater and securely mount the external controller to prevent damage. If 
you're using a repeater out of its normal operating range, consider turning down the RF Power Amplifier 
output to 60% to prevent problems. Normal frequency in-range equipment operates at the manufactures 
rated values. Understand there are both intermittent and continuous duty cycle RF Power Amplifier 
models, which determine maximum transmit lengths. 
Parts (Motorola Parts (800) 422-4210) Motorola Part Number
Quality Shielded mic Wire used for interface wiring 3083560A02 $1.10/ft 
(MSR 2000 Manuals) 
VHF Base and Repeater Station Manual 68P81061E50-C
Control and Audio Instruction Manual 68P81061E40-C
UHF Base and Repeater Station Supplement 68P81061E55-C

I hope this modification works well for you. Please feel free to contact me if you find errors or have 
specific questions. Enjoy… 

Skipp May P.O. Box 192 Elmira, CA. 95620 (707) 678-4187 nospam4me@juno.com 
This text is made available on the http://www.radiowrench.com/sonic web page.