Instructions for Duplexing a Motorola Micor UHF
by Jim Reese

Why would you want a duplex mobile, you ask? This describes how to take a standard UHF micor mobile radio and make it work as a full-duplex mobile radio. Since Texas is high-in, low-out on UHF, this also includes which cap changes are required to make the radio receive below 445 Mhz. There are diagrams referenced in this text. If you are unable to figure out what to do without the diagrams, mail me at and I'll try to get you a copy of the diagrams.

Jim Reese

Conversion of Motorola MICOR UHF Radio
For Use as a Duplex Mobile

This conversion is not for the faint-hearted. Make sure you are very familiar with the operation of the Micor radio before attempting this conversion. As with all modifications of this type, insure that the radio is operating correctly BEFORE any modifications are made. It is much easier to fix it before you hack it up. Do not attempt this conversion without a service manual. You need the PC layouts and tune up instructions from the service manual in order to perform the modification.

Useful Motorola Part Numbers

68-81015E70-HManual for UHF MICOR
KXN-10245ppm Channel Element for UHF MICOR
KXN-10292ppm Channel Element for UHF MICOR
KLN-6210APL Encode Reed
TLN-8381APL Decode Reed
66-84690C01Contact Removal Tool for MICOR plugs
TKN-6458ALarge Fuse Holder for Primary Power (40A)

Crystal Ordering Information

When ordering crystals, specify the KXN-1024A channel element number. Always order the crystal on the RECEIVE frequency. If your radio will be a high transmit unit, the crystal will be LOW side injection. For low transmit radios, order the crystals on HIGH side injection. This will flip the transmit offset to the other direction.

Conversion Instructions

Step 1:
Make sure that the radio is operating properly by tuning it up with one of your crystals before any modifications are made. If you are using the radio as a high transmit unit, you must make the capacitor changes in step 2 for the radio to work properly.

Step 2:
If your radio will be receiving below 445 MHz, change C117 to 27pF, C119 to 39pF, and C125 to 12pF on the receiver board. This is not necessary if the radio will be receiving above 445 MHz.

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. First, remove JU-905 on the control board. 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. Add capacitors at the following points on the audio squelch board. Add a 100pF cap between P903-5 and P903-6. On IC-201, add 15pF caps between pins 3 and 4, and between pins 3 and 5. Add 100pF caps between pins 6, 7, 11, and 13 of IC-201 and ground. On IC-202, add 15pF caps between 5 and 9, 5 and 13, and a 33pF cap between 5 and 15. This makes the board less susceptible to RF. Keep the leads on these caps as short as possible.

Step 4:
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. 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 plier or hemostat. 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:
Mount a BNC chassis mount connector on the top side of the front casting on the side opposite from where the lock is located. This will be the receive antenna connection. 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.

Step 6:
This is the toughest part of the conversion, the circulator modification. 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 or green rectangular unit with a wire coming from the relay is the reject load for the circulator. This is a ceramic 75W 50 Ohm resistor. The relay switches the output port of the circulator between the receiver and the reject load. Be EXTREMELY CAREFUL when soldering on the reject load, as the top terminal can break off of theceramic very easily. I suggest cutting the wire from the relay, removing the relay, and then removing the wire from the load resistor. Once the relay is removed, wire the dummy load back to the output port of the circulator which is on the common side of the relay. Refer to the manual 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.

Tuning Instructions

Before tuning, disable the receiver AFC by soldering a wire from the "AFC OFF" trace on the receiver board to ground. The procedure for disabling the AFC is described in the receiver tuning instructions section of the service manual. Tune the radio per the Motorola manual. Once you have achieved this, you need to tune the circulator. The following procedure should be followed: Remove the power control board, and power the radio with a supply having a current meter. 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. Key the transmitter and tune the three circulator capacitors for maximum power output. 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. 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. Turn the drive limit pot 1/4 turn clockwise, or until power just starts to fall off. That's it. 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 cooling space around the PA heat sink fins when the radio is installed. The Micor PA is not easy to fix, and when it blows, it blows big.

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Copyright 1997