Instructions for Conversion of Motorola UHF MICOR® to a Repeater

This page was personally approved by James L. Reese for internet publishing here at

 NOTE: This now includes all figures and tables scanned from Jim Reese's information packet.

Original modification by: James L. Reese, WD5IYT
March 25, 1996

Updated February 15, 1999 by: Phil Salas, AD5X
Phil's updates include better printing figures and text additions to make the conversion easier. Included also are tuning procedures and details on the construction of a tuning meter.

This modification is for the 450-470 MHz MICOR® radio.  This frequency band radio has a 16.7 MHz offset oscillator permitting a 5 MHz transmit offset. In order to use a 406-420 MHz or 470-512 MHz MICOR® radio, you will need to change the crystal in the offset oscillator to 16.7 MHz for proper amateur repeater offset.

Read through this entire procedure before beginning.

This modification is not for the faint-hearted.  Be sure you are very comfortable with the operation of the MICOR® radio before you attempt this modification.  A thorough understanding of the way the MICOR® radio and control system works is absolutely essential for the success of this conversion.

Modification Notes:

This modification was intended to be used with radios serving the Armadillo Intertie System. Whenever "Molex Pin x" is mentioned, it refers to the Armadillo Standard connector. This is a small, 9 pin Molex connector which serves as the interface for all of the Armadillo System radios. Use a Female connector on the radio end. When using this modification for individual use, the pin out can be disregarded. The standard pin out of the connector is:

      1 - Ground
      2 - Receive Audio Output
      3 - n/c
      4 - Transmit Audio Input
      5 - COS Output
      6 - PTT Input
      7 - n/c
      8 - PL Decoder Output (Sense)
      9 - PL Select Input

The COS output and PTT input are open collector to ground signals.  Pin 8 goes to +9.6 V when PL is decoded.  Pin 9 is ground for PL, open for Carrier.

These modifications allow "PL and Squelch" operation.  This means that when in PL, the normal squelch circuit still operates.  This avoids long noise bursts upon unkeying.

If using an Advanced Computer Controls controller, the signal at pin 8 is run to the "PL Sense input" on the controller.  It will drive this input directly.

Order the crystal after determining the crystal frequency as follows by reading this information:
The MICOR UHF Mobile.

Disable the receiver AFC by jumping the printed circuit pad under the "AFC-off" hole on the RF/IF cover  to ground. Why? Two reasons. First, the UHF mobile MICOR® uses one channel element (receive), and the transmit frequency is also determined by it, For normal low-side injection, a user that is off frequency will be pulled on frequency by the electronic warping of the channel element, thus pushing the transmitter off frequency by that same amount.  And second, if the receiver is operating with the local oscillator injection on the high side, the phase of the receiver discriminator is 180 degrees out, so a user that is off frequency is pushed even further off frequency.
For more information on understanding the concept of the MICOR® receiver go to The Motorola® MICOR® "Sensitron" High Band Receiver  Even though this information was written for the high band MICOR receiver, a lot of the information is pertinent.

List of Tables and Illustrations:

There are several illustrations which are detailed below.  Refer to them as needed when modifying or constructing pieces of this project.

Table 1:  This is a complete hookup chart for the control head plug and interface board.
Figure 1: Schematic of control head plug, with simple diagram.

Figure 2: Board Layout of audio squelch board detailing added capacitors.

Figure 3A: Detail of circulator unit showing location of major components before modification.

Figure 3B: Detail of circulator unit showing location of major components after modification.

Figure 4: Schematic of circulator unit before modification. Click on it for a larger image.

Figure 5: Schematic of circulator unit after modification. Click on it for a larger image.

Figure 6: Schematic of interface board. Click on it for a larger image.

Fgure 7: Rough layout of interface board.


Useful Motorola Part Numbers:

6881015E70-H Manual for UHF MICOR®
KXN-1024 5ppm Channel Element for UHF MICOR®
KXN-1029 2ppm Channel Element for UHF MICOR®
KLN-6210A PL Encode Reed
TLN-8381A PL Decode Reed
0184307A89 Empty Control Head Plug less cable, complete with pins
6684690C01 Contact Removal Tool for MICOR® plugs
TKN-6458A Large Fuse Holder for Primary Power (40A)

Step-By-Step Instructions for Modification:

Step 1:  Make sure that the radio operates properly BEFORE any modifications are done.  If there are problems with the radio, they will be easier to fix before the mods are done.

Step 2:  If you are satisfied with the operation of the radio, construct the control head plug per the figure below and table 1.  Use miniature pots and switches inside the control head plug. It is rather tight inside, so be careful to check that the connector will re-assemble before you drill it up.

Most control heads do not have the full complement of pins installed.  Therefore, formerly used pins 12, 13, and 14 are employed.  Pin 12 apparently is never used but is wired to a small relay near where your interface board (built later) will go.  Cut this wire loose from the relay and add a 6" wire extension to it.  Pin 13 is the F3 select line and goes to the junction of CR903/CR907 on the control board.  Pin 14 is the F2 select line and goes to the junction of CR902/CR906 on the control board.  Remove these four diodes and attach short wires (6") to these junction points to connect to the interface board you will build later. Finally, permanently enable the F1 channel element by adding  JU909 on the control board (wire from junction of CR901 and CR905 to ground).  Also, ensure that JU1 is in place on the control board, as this enables the offset oscillator.

After you have constructed the connector, make the following connections inside the radio:
Jump control head pins 3, 8, and 22 to "A+" on the control board.  Jump pins 9, 11, and 17 to ground on the control board.  Next locate the two feedthrough caps which power the Transmitter Power Amplifier.  They are just to the left of the control head plug on the bottom side of the radio.  One will have a red wire, and the other a black wire.  Jump from the feedthrough with the red wire to the "A+" trace on the control board.  Jump from the feedthrough with the black wire to the ground trace on the control board. There is a reverse polarity protection diode on the control board which can be used to make these connections.  Next, plug your control head plug into the radio and verify proper operation with the new control arrangement.  If there are problems here, troubleshoot them NOW.  Do not wait until later, as you may be chasing more than one problem.

Step 3:  You must make some modifications to the control board and the receiver audio/squelch board in order to make the radio full duplex and to make preparations for the interface board.  First, remove JU-905 on the control board.  Then, jump the F1 channel element to ground as described in the Motorola manual.  Next, jump pins 1 and 4 of the audio squelch board.  On the later version audio squelch board, there is a place for a jumper (JU-202), on earlier units, just make the jump with wire wrap wire.  Next, remove a jumper on the audio squelch board which goes from pin 3 of the PL decoder to IC-202 pin 8. This is near the pins which the PL board plugs into.  Next, ring from pin 3 of the PL decoder to pin 11 of the audio squelch board with an ohmmeter.  There should be continuity here.  If not, add a jumper.  The later boards have a trace from pin 3 of PL to pin 11 of audio squelch, on earlier boards, pin 11 is unused, and you should jump from PL decoder 3 to audio squelch 11 with some wire wrap wire.  Add 15 pF capacitors between the following pins on the two chips on the audio squelch board.  Figure 2 details the location of these caps.  On IC-201, add a cap between pin 3 and 4, and between pin 3 and 5.  On IC-202, add caps between 5 and 9, 5 and 13, and two caps between 5 and 15. This makes the board less susceptible to RF.  You may want to refer to this mod page "MICOR® mobile "audio & squelch board." However, DO NOT cut the circuit trace leading from IC 202 pin 11 as called out in this mod! Finally, it is a good idea to raise the input impedance of audio input to the MICOR® exciter.  This is done by removing R301 (R439 on some models), a 270 ohm resistor on the UHF exciter board.  Or just cut the trace between R300 and R303 on the exciter board.

Step 4:  Next, carefully remove the front casting from the chassis.  This is done by removing the four screws top and bottom as well as two screws on the control head plug.  This is kind of tricky, so be careful to remember how you got it apart so you can re-assemble it later.  Especially pay attention to the way the front connector is held in place with the two plastic pieces that the screws go through. Next, examine the Power Amplifier section of the radio and notice the miniature connector which connects the output of the PA to the circulator.  Unplug this connector from the circulator using a needle nose pliers or hemostat.  Next, turn over the radio and remove the power control board.  This will expose the top plate of the circulator.  Remove the circulator by carefully removing the sensing wires which connect to the power control board and the two screws which hold the circulator in.  You will have to unplug the receive antenna coax from the preselector unit in order to remove the circulator.  Set the circulator aside for later modification.

Step 5:  Procure either an SO-239 or a BNC chassis mount connector for a receive antenna jack. If a SO-239 is used, make certain to use a hood on the back of the connector to totally shield the socket. Mount this connector on the top side of the front casting on the side opposite from where the lock is located.  Be very careful to locate this connector so that it does not hinder the operation of the latch mechanism.  Attach a small coax to this connector and route it to the receive antenna jack on the preselector unit.  Drill a hole in the front of the radio chassis to pass the coax.  This will be obvious once you have examined the unit with the front casting removed. The jack on the preselector unit takes an RCA plug with a short pin.  Examine the plug on the original cable and cut off the pin on your new RCA plug to match.

Step 5:  If your radio will be receiving below 445 MHz, change C117 to 27 pF,  C119  to 39 pF, and C125 to 12 pF on the receiver board.  This is not necessary if the radio will be receiving above 445 MHz.

Step 6:  This is the toughest part of the conversion, the circulator modification. Figures 3, 4, and 5 detail the conversions.  Remove the cover from the circulator unit.  You will notice that there is a circulator, an output filter, the antenna switch, and the circulator reject load.  There are three trimmer caps, only one of which has an access hole in the top plate.  Measure and drill the top cover so that you have access to all three trimmers from the outside.  This is necessary because the cover affects the tuning of the circulator.  After drilling the cover, set it aside.  You must now remove the antenna relay.  This is a small relay on the right side of the circulator.  The small dark red rectangular unit with a wire coming from the relay is the reject load for the circulator.  The relay switches the output port of the circulator between the receiver and the reject load.  You must remove the relay and wire the dummy load back to the output port of the circulator which is on the common side of the relay.  Refer to the diagram for the circuit. The easiest way to accomplish this is with a small piece of teflon coax (RG-188).  Run from the circulator port to the reject load.  You can solder to the circulator case for the shield on the load end of the coax. Replace the cover on the circulator and reinstall it in the radio.

Step 7:  Construct the interface board using the interface schematic and board layout links above.  Install this board on the three unused mounting tabs near the rear center of the radio.  These tabs are above the control board.  Wire the board up as described in table 1.  At this point, the radio will be operating full duplex, and you should be able to put the radio in PL by flipping the switch on the control head plug to the PL position.  Verify that you have +9.6 volts at the 9 pin molex connector PL SENSE pin when the correct PL code is being sent.  Also verify correct receive audio gating and COS action.

Tune Up Instructions:

Tune the radio per the Motorola manual (ADJUSTMENTS Chapter 6).  The receiver adjustments are in Section 5, the Offset Oscillator adjustments are in Section 6, and the Exciter adjustments are in Section 8 of this chapter.

For the receiver alignment, you can use a talkie at its lowest power setting with a 20 dB attenuator for the initial front-end adjustments.  You can purchase 20 dB type-F attenuators from Marlin P. Jones (P.N. 5049-TV) for $2 each (800-652-6733) as well as F-to-BNC adapters.  However, as the receiver tune-up proceeds, you will be unable to get a low enough level signal even with additional attenuators due to radiation from the talkie, cables, attenuators, etc.  The 3rd harmonic of a low power 2-meter rig with 20 dB attenuators, or a 140-150 MHz signal generator with 20 dB attenuators works well.  The 3rd harmonic of an MFJ-259 running from an external 12V supply that has been left to stabilize for at least 10 minutes makes a good signal generator for this (again, with as many 20 dB attenuators as necessary). Put all the attenuators on the signal generator.

In Exciter adjustment procedures steps 5 and 7, change CW to CCW and CCW to  CW for repeaters where the transmitter is offset 5 MHz low in frequency.

Now, key the transmitter and adjust the trimmer capacitor in the channel element for the correct transmit frequency.  Make sure the offset oscillator adjustment has been done first, or your receiver may be off frequency!
Once you have tuned up the radio, you need to tune the circulator.  The following procedure should be followed:

Step 1:  Remove the power control board, and power the radio with a supply having a current meter.

Step 2:  Attach a jumper or clip lead from feedthrough C527 on the Controlled Stage in the PA compartment and feedthrough C536 on the driver stage in the PA compartment.  This will force the radio to maximum power output.

Step 3:  Key the transmitter and tune the three circulator capacitors for maximum power output.

Step 4:  Reinstall the power control board, and preset the drive limit pot fully counter-clockwise.  Set the power set pot to the desired power output level.  I recommend no more than 50W out for a 75W radio, and no more than 60W out for a 100W radio.  The 25 and 45W radios are rated continuous duty at 25W output.

Step 5:  Key the transmitter and tune the center circulator capacitor (the only one accessible from the top of the power control board) for minimum current draw.  You should be able to make several Amps difference without affecting the power output.

Step 6:  Turn the drive limit pot 1/4 turn clockwise.

That's it.  you are now ready for major repeating action.  Remember to always set the receive frequency first when setting frequency, as this affects the transmitter also.  Set the transmitter with the offset trimmer coil on the exciter board.  Make sure that you have adequate forced air cooling on the PA at all times during operation.  The MICOR® PA is not easy to fix, and when it blows, it blows big.

This page prepared by: Kevin K. Custer W3KKC
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