This modification of a MICOR UHF to duplex mobile is from Jim Reese,

Why would you want a duplex mobile, you ask?  Once you've experienced it, 
you'll never go back to jap-trac's.

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-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
66-84690C01          Contact Removal Tool for MICOR plugs
TKN-6458A           Large 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 the
ceramic 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

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

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.
     Jim Reese, WD5IYT            | "Real Texans never refer to trouble        |  as deep doo-doo"  --Molly Ivins

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