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Micor Duplexer Mods
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Conversion of MICOR UHF to duplex mobile
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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 [email protected] 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.
--
Jim Reese, WD5IYT | "Real Texans never refer to trouble
[email protected] | as deep doo-doo" --Molly Ivins
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