Instructions for Duplexing a Motorola Micor VHF
by Jim Reese

Instructions for Conversion of
Motorola VHF Micor to Repeater Service

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 TLD8270A; audio/squelch models TLN4310A-2 thru 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 judgement to alter these instructions as necessary.

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 radios. Use a Female connector on the radio end. The standard pinout 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.

    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 3: Schematic of interface board.
    Figure 4: Rough layout of interface board.

    Useful Motorola Part Numbers

    68-81008E40-DManual for VHF MICOR
    K1007ATX Channel Element for VHF MICOR
    K1005A RX Channel Element for VHF MICOR
    KLN-6210APL Encode Reed
    TLN-8381APL Decode Reed
    66-84690C01Contact Removal Tool for MICOR plugs
    TKN-6458ALarge Fuse Holder for Primary Power (40A)
    01-84307A89Empty Control Head Plug less cable, complete with pins

    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 figure 1 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 15pF 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.

    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 using the schematic and board layout in the packet. 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.

    Tuning Instructions

    Tune the transmitter and receiver per the Motorola manual. Follow the procedures exactly.

    Adjust the power set for the desired power output. I reccommend 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.

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    Copyright 1997