Motorola Micor By Jim Reese, jreese@NeoSoft.com
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
jreese@sugar.neosoft.com and I'll try to get you a copy of the
diagrams.
Jim Reese
WD5IYT
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 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.