Comprehensive Modification of a
MICOR Compa-Station Base/Repeater Station

By Kevin K. Custer W3KKC

Please read the entire article from top to bottom if you are serious about converting your MICOR Station or Repeater to the best operating repeater you could have!
(in my opinion, of course)

Concept: The modification presented here is geared toward allowing the connection of an outboard repeater controller to a Motorola MICOR Compa-Station Unified Chassis, and eliminating most of the Station Chassis Modules.

This modification will let you eliminate most of the Control Chassis Modules, as most are useless when an add on repeater controller is used. This modification is NOT recommended if you want to keep the Station totally factory controlled or if you prefer to use the Squelch Gate Module in repeater service. In addition, some factory voting schemes require the use of the original modules, so be sure of your intentions before you do any modifications. My feeling is; when you connect an after market repeater controller, like an SCOM, NHRC or other modern unit, most of the original control modules are no longer needed or desired. Why? Because they tend to keep doing things on their own, and unpredictable results usually happen. In most cases, the only module needed is the Station Control Module which allows easy keying of the exciters powered stages. Modification of the Station Control Module will allow disabling of the PTT to easily check for desense. This information will work on either a Low Band, High Band or UHF Station. The main concept of this article to create a simple transmitter and receiver from the MICOR Unified Station, with a real good logic interface. After connecting a quality repeater controller, your MICOR Station/Repeater it will be the best operating machine money could buy.

Description: In my opinion, the operation of the carrier squelch portion Repeater Squelch Gate Card in a MICOR Repeater Station (RT) is terrible, especially when using an add on repeater controller. This card is used to provide two major functions, first is the keying of the transmitter from signaling from the receiver, second is audio muting. These functions have two different controls to set the threshold of keying, and when audio muting occurs. I have found it extremely difficult to set the controls to work in conjunction, and the result is usually an imbalance where either the repeater keys, doesn't pass audio, sends the courtesy tone and drops, or else the audio passes during the courtesy tone and / or tail and the repeater transmitter drops out early. The modification described here will allow the stations original "MICOR Receiver" squelch control and circuitry to provide both functions thus eliminating the problem of setting a bunch of controls. Most controllers handle all repeater functions anyway, so I simply remove this card. Since you may be modifying a Base Station, the repeater squelch gate card was not supplied anyway, and with this modification, not desired. I feel the original squelch circuit in the receiver deck allows for smoother squelch control and the use of it will retain the famous dual hysteresis squelch function allowing mobiles to be in flutter without the squelch clipping them off. This is especially true in UHF operation.

Some people confuse the operation of the Repeater Squelch Gate Card and the Audio and Squelch Board because of the reference to the dual squelch circuits, however the operation of the squelch circuit of the Audio & Squelch Board is a single circuit with dual hysteresis. The Repeater Squelch Gate Card doesn't have variable hysteresis, just two controls that are difficult to set. For more on the operation of the MICOR Receivers bi-level squelch, go here.

Advantages: The modification presented here will allow the following advantages when an outboard controller is used:

  • Ease of interfacing to an outboard controller by providing stable consistent operating logic.
  • Elimination of most of the station chassis control modules means less items to maintain.
  • Allow the use of the stations squelch control to provide the functions of generating a quality cos, and muting of the receiver audio if necessary.
  • Employing "And Squelch" will allow for crashless squelch tails on strong signals.
  • The stations volume control will manage the audio level to the local speaker allowing you to monitor receiver activity without interaction of the repeat audio level.

  •    This is handy when checking for desense or station maintenance.

    The Station Audio & Squelch board was never intended to be used by itself in muting the audio to the local speaker, as many times a hiss is heard even when the station is squelched, so modification of the Station A&S board will allow more pleasant monitoring of the received signal. See the link stated in this article to understand this modification.


  • This modification is involved and requires knowing a bit about the MICOR Station, or at least having a manual to refer to.
  • After doing this modification, it isn't easy to convert the Station back to using the factory control circuitry.
  • This modification won't allow many of the factory cards to work, so be certain of your intentions.

  •    I don't see this to be a problem in either ham or commercial use, especially if you have a spare controller for when lightning strikes or whatever.

    The Conversion in General: The use of a Station Service Manual is needed if you are not familiar with the MICOR Station unit. The Station should be tested for basic working functions before modification, as troubleshooting may be more difficult after doing some modifying. In some instances, all of the modification procedures presented here may not be necessary.

    Links are provided to individual articles to aid in the modification process, and may be band specific.

    Modifications relating to RF, I-F, and Squelch:

    Receiver Modification: HF Stations:
    RF and I-F Board
    Station Audio & Squelch Board

    Receiver Modification: VHF Stations:
    RF and I-F Board
    Station Audio & Squelch Board
    VHF preamp conversion

    Receiver Modification: UHF Stations:
    RF and I-F Board
    Station Audio & Squelch Board

    Transmitter Modification: HF Stations:
    Exciter and Harmonic Filter

    Transmitter Modification: VHF Stations:
    Exciter Board

    Transmitter Modification: UHF Stations:
    Exciter Board
    Band Pass Filters and the Circulator (Antenna Network)

    Modifications relating to the Chassis:
    Base Station Modification:
    If you are converting a base station configuration go here first and modify the receiver interconnect board for duplex service.

    Station Remote Control Modules: (all bands)
    Remove all station remote control modules except the "Station Control Module."

    Modification of the Station Control Module (TLN 4635 B) is presented in another article and will allow the PTT to be disabled for checking for desense. Modifications to the module are also available for installing a PTT lamp, and insuring that keying will not be inhibited by an outside source. The Station Control Module will provide the function of keying the exciters A+ and A- stages as original, but from an outside source like from an add on repeater controller. This modification is not a necessity, but is very nice for station maintenance.

    Channel Elements:
    Supply the receiver channel element its ground signal to enable it at all time. This is accomplished by connecting pin 30 of the receiver interconnect board pins {J2, bottom row of 30} that come through the backplane, to ground. Pin 30 is R1 oscillator select.

    For the Transmit Exciter, you have a choice:
    1.)  To enable the exciter all the time, connect pin 19 to pin 20 on the top row of 30 {transmitter} interconnect board pins that come through the backplane. These are the 3 sets of 10 pins sticking through the backplane board on the top. Pin 19 is F1 channel element select and pin 20 is ground.
    2.)  For those who don't care to have the transmitter oscillator running all the time, you can use the (switched ground) logic provided by the Station Control Card that was originally intended to drive the antenna change-over relay to key the exciter channel element during PTT. The Station Control Module Modification explains how to do this. Ensure that you place the transmit and receive channel elements into their respective F1 positions.

    Information relating to Audio:
    Receiver Audio: On the backplane board, where the 30 receiver interconnect board pins come through, jump pin 6 to 14 with a wire. This allows the preamp audio from the receivers Audio & Squelch Board to be fed into the local volume control to provide amplified receiver audio to drive a local speaker. This eliminates the need for the Line Driver card. If the audio is too hot and the action of the control seems all on the lower end, resistance can be placed in series with this connection to limit the amount of maximum audio to the local speaker. A value of 100K is what I use and makes the pot centered with a moderate amount of audio for testing.

    Receiver Audio De-Emphasized, Squelch Gated, and possibly PL filtered
    If you have a controller that requires deemphasized, squelch gated receiver audio, connect your controller to pin 14 on the receiver interconnect board pins that come through the backplane. This will supply quality deemphasized audio to the controller. If an optional OEM PL decoder is installed, the audio from pin 14 will have PL filtering if Jumper JU-201 is cut on the Audio & Squelch Board. The Station Audio & Squelch Board can be rewired for better muting of the receiver audio, and for supplying a better active low COS. The muting part of the mod might be necessary when using a controller that needs squelched audio. This mod is certainly nice when listening to the station locally, but is not a necessity. The COS rewiring part of the modification will generate an active low (negative logic) COS signal from the famous MICOR Squelch chip; in my opinion, the absolute best place to get the COS.

    Receiver Discriminator Audio
    If you require discriminator audio for your controller, connect to pin 15 on the receiver interconnect board pins that come through the backplane. PL has not been filtered at this point and the feed is directly from the emitter follower buffer amplifier for best audio quality. This is where we feed modern controllers like the NHRC line that have de-emphasis and squelch gating in the controller. This is also the best place to hook-up an aftermarket PL (CTCSS) decoder like a Com-Spec TS32/64.

    Transmitter Audio (De-emphasized): For stand alone repeaters using 4 pin exciters not requiring flat audio, or HF and VHF units employing a 3 pin exciter, I inject transmit audio directly into the backplane at the 30 horizontal transmitter interconnect pins at pin 24 (exciter input). Audio delivered to this point will need to be adjustable to set the amount of "mike gain". Most controllers already have a "transmit audio" control, so an additional pot is usually unnecessary. If your audio source doesn't have level control, a simple pot can be added to the backplane with the wiper going to pin 24. Something around 10K will be fine.

    I also ensure that I have the jumpers set up on the exciter for the appropriate audio input level that is available. I also cut the bias resistor to remove the voltage from this trace. Most stations have the resistor clipped already, some don't.
    See here for eliminating the bias resistor.

    I cut the trace leading from pin 24 into the backplane web so there will be no additional audio path. I have seen MICOR repeaters where the audio doesn't fully mute when sending touch-tones and so on. This is because there may still exist an original audio path. Cutting the trace on the backplane and injecting audio directly to the exciter is best. Depending on how much audio your controller is capable of providing, you may need to amplify the audio to the exciter in order to get the proper amount of processing. Read on.

    Some folks don't properly understand how to set up audio into an FM transmitter. Most transmitters need driven with an certain amount of audio to provide some clipping (processing.) Clipping is necessary to provide punchy louder audio without over deviating the exciter. Too much clipping exhibits objectionable distortion, while not enough makes soft audio with no guts.

    FM radios usually employ some sort of a mike gain and a deviation pot. The correct way to set up audio into an FM exciter is to apply audio and set the mike gain control so some clipping occurs on the peaks, then adjust the deviation (IDC on the MICOR) for 5 kc. But... there is no mike gain control in the MICOR so now what do I do? Some controllers have enough audio to drive the processing circuitry in the exciter into clipping. By using the 'audio output' or 'transmitter level' pot on the controller, you should be able to adjust it like a mike gain control to get the amount of clipping you want, then set the deviation to 5 kc. Some controllers don't have enough audio output level to provide clipping and a buffer amplifier is needed to provide more audio to the exciter. I use a LM-386 audio amp circuit to provide the needed amplification and provide a "mike gain" control. See a Radio Shack parts substitution manual for a schematic or go here and build this circuit. LM-386's are a buck a piece and work very well for this. Replacement via a socket is nice when lightning strikes. After setting the buffer amplifier for some clipping, adjust the IDC for 5 kHz deviation.

    Transmitter Audio (Pre-emphasized): For repeaters needing to employ flat audio response I simply inject audio from the controller into a brick wall audio processor/filter into the PL injection input on a 4 pin exciter. Stations already have true FM exciters from the factory and employing "Flat Audio Modification" is easy from the standpoint of the station. If you don't understand what this is or what it means, do more research on the subject or do the mod as suggested above in the "De-emphasized" section. For those wanting to bypass the OEM MICOR processing and use something better, go here and look at this product.

    Information relating to Signaling:
    Push To Talk: For connecting the PTT, I go directly to the Station Control Module interconnect pins on the backplane board. These are the vertical pins that connect directly to the module. I connect to pin 5 (local ptt.) I also Modify the Station Control Module to provide a PTT disable switch by rewiring the "Line Disable" switch.

    COS: This information is only pertinent if you do the Audio & Squelch Board modifications.
    For COS, I connect to the 30 horizontal receiver interconnect pins (J2) to pin 13. This pin connects directly to pin 8 on the Audio & Squelch Board. Logic is inverted or negative at this point meaning 0 volts for unsquelched receiving a signal and voltage when not. If your controller requires positive going logic, you can invert the supplied logic with a transistor switch with this circuit.

    This circuit can be run from 12 or 9.6 volts to provide that much voltage when receiving a signal. The circuit will give a ground when not receiving, (positive logic.) Of course, this inverter circuit isn't necessary with most modern repeater controllers, as you can use the active low signaling provided with many of them.

    PL: For PL, I usually use the OEM MICOR decoder deck to produce PL logic, I connect to the 30 horizontal receiver interconnect pins (J2) to pin 10. Logic is positive. I make sure that the PL board doesn't interpret the COS. They should work separately and the "And Squelch" mod will insure they do.

    If you are using a after market PL decoder, a good place to get audio to drive it is pin 15 of the receiver interconnect board. This connection (called receiver discriminator) will allow clean consistent detection of the CTCSS tone so you can set the levels the way you want or need to make the repeater sound right without interpretation of any level setting controls affecting the way the PL tone decodes.

    AND SQUELCH: Go here to understand what this is and how to make it work for you.

    Additional Information:
    Squelch gated audio is usually not required for newer style controllers like the SCOM, Arcom, ICS, or NHRC controllers. Squelch gated audio can be can be had from either Pin 14 of the receiver interconnect board pins or some choose to use the "MICOR Muteboard" for supplying a controller with squelch gated, PL filtered, deemphasized audio, and a quality positive logic COS source. The Muteboard is nice for delivering audio to an ACC or other controller that requires squelch gated audio when you don't want to (or can't) modify the Station Audio and Squelch board because of needing to drive the OEM card deck.

    Kevin K. Custer  W3KKC   <email me with comments or questions.
    Original html Date  12-1-1998
    Revised 12-5-1999, and 01-23-2000 Additions of controller connections and explanations.
    Revised 07-27-2003 Additions to text about being certain of your intentions on eliminating the SGC.
    Revised 12-12-2004 Added HF exciter modification reference.