Motorola Micor 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 
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 

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 
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 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.