Instructions for Conversion of Motorola VHF MICOR to Repeater Service

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

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

Modification by: James L. Reese, WD5IYT
March 25, 1996

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 developed for radios having receiver RF/IF board series TLD827?A, or TLD527?A or B; audio/squelch models TLN4310A-2 through TLN4310A-5; and exciter models TLD8261A, TLD8262A, TLD8261B, or TLD8262B.  If your radio differs from these part numbers, all or part of this modification may be different from the descriptions.  Use your judgment to alter these instructions as necessary.

You may want to refer to the modification of a high band receiver to make the receiver tune properly in the 2 meter ham band.

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, S-Com or other commercial controller which requires an active high PL sense signal, the signal at pin 8 can be attached to the "PL Sense input" on the controller. It will drive this input directly. On the S-Com controller, the pull up resistor on the PL sense input must be removed for proper operation.

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.

Go to 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. Ones marked with a star are for uhf only.

Figure 3: Schematic of interface board.

Go to Figure 4: Rough layout of interface board.

Go to RX mod:  Complete receiver modification instructions.

Useful Motorola Part Numbers

6881008E40-D Manual for VHF MICOR
K1007A TX Channel Element for VHF MICOR
K1005A RX Channel Element for VHF 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 diagram 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. After you have constructed the connector, make the following connections inside the control head plug: Jump control head pins 3, 8, and 22 to +12 V.  Jump pins 9, 11, and 17 to ground.  Plug the newly constructed 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: There are some modifications required on 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.  Remove CR201 on the audio/squelch 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.  Check for continuity from pin 3 of the PL decoder to pin 11 of the audio squelch board with an ohmmeter.  If there is not continuity, 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 also want to refer to this mod page "MICOR® mobile "audio & squelch 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.  You will expose the antenna relay with its two associated coax cables running to the transmitter and receiver.  Unplug the receive coax from the antenna relay.

Step 5: Procure a BNC chassis mount connector for a receive antenna jack.  Mount this connector on the top side curved surface 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 the receive coax to this connector routing the cable through a hole which you will drill in the front of the radio chassis.  The method will be obvious once you have examined the unit with the front casting removed.

Step 6: Construct the interface board as described below.  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 transmitter and receiver per the Motorola manual.  Follow the procedures exactly.

Adjust the power set for the desired power output.  I recommend no more than 60 Watts for 110 Watt radios, 40 Watts for 60 Watt Radios, and 30 Watts for 45 Watt Radios.

That's it. you are now ready for major repeating action. 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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