This modification of a MITREX 440 to a 9600 full-duplex repeater
is from William Lyman, lyman@a3bee2.radnet.com:

     I have successfully completed a assembling a 9600 fullduplex
data repeater. I used a Mitrex T44 with an MFJ 1270 TNC and Tapr
9600 baud modem with bit regen.
     This repeater in now in use at Bethany CT, Portland Ct, and 
 Bridgeport Ct and they are all working GREAT.
     My first feeling was it wasn't going to work. I though that
internal desens would kill the receive.
     Here are some hits and suggestions for making this work Please
remember these mods for a Motorola Mitrex T44 UHF model.

     1) Be sure to get the elements temp comped! I didn't at first
and as it got used it drifted badly as the unit got hot.
     2) I maked 2 of the mods that MoAmps published for increasing
high frequency response at the limiter/Detector. If you are on a
fringe than this will really help.
     2a) Remove C240 a .0022 cap. This can be achieved by
physically grabbing it with pliers and breaking it away from the
PC board. Or you can remove the shield cover and unsolder it. I did
the later and that what I suggest.
     2b) Remove C451 a 220 cap between U403a pins 2 and 3.
     2c) Replace R231, 75.k resister with 2.2k resister
     3) Receive audio is (Det) is available on pin 11 of the
control cable connector.
     4) Lock receive element on. Please note this is for fullduplex
only this will cause unit receiver circuitry to be on all the time.
If you did this mod in simplex the modem would false by hearing you
only transmitted signal and would take longer time to lock.
     4a) Locate Q1 and add a jumper across the e and c junction
     5) The following are for transmitter mods
     5a) locate pin 15 of the interconnect board. This is not pin
15 where the control cable connects but pin 15 on the connector
that connects the interconnect PCB to the main PCB. Solder wire
from this connection to pin 1 on the backside of where the control
cable connects. With unit facing you (lock towards you) this will
be the top right most pin. You will see either a white jumper or
a coil (small) that goes from pin 1 to the rest of the circuitry.
Remove one side of this jumper.
     5b) Remove one side of each of the following caps.
C507,C508,C509. This will remove the splatter filter allowing you
to use Q503 and q504 to drive the element. You could drive the
element directly but I found that there was distortion when driving
the element hard. This also matches the Impedance on the Element.
     6) Xmit audio is now available on pin 1 on the control cable
connector.
     7) Rec/Xmit ant connections
     7a) On my unit the xmit connection to the ANT relay was hard
wired on to the final AMP. I didn't have any small coax so I left
the ANT relay in. My transmitter connection is at the existing ANT
connector. For the receive I unplugged (RCA JACK). Trace back from
ANT switch. I taped the jack and left in place. I drilled a hole
in the top cover over where the RCA jack plugged in. I then made
a cable with an RCA jack thus making this the Receiver connection.
I only drilled the hole in the cover because covers are easy to
come by if I make a mistake.
     8) Cable connections
     8a) The following connection to the motorola female connector
should be made.
pin 17 to pin 6 this is negative
pin 4 to pin 12 to + fused at 2 amps (Rec/Xmit enable)
pin 19 + fused at 20 Amp
pin 13 PTT
pin 1 xmit audio
pin 11 rec audio
     9) Summary
     9a) it is very important that the receive cable be of the best
quality. Remember it is VERY close to the Xmiter. When you adjust
the Xmiter follow the manual very close. You can't have any spurs.
     9b) I did experience some desense from the xmiter circuitry
in the range of .03 uV. I find that to be almost non existent.
     
     There might be other ways or mods. These have worked for me
in 3 installations. The mods are made so if you decide to resale
the radio. Just replace cover, install splatter caps, Install
jumper on pin 1, And remove jumper from pin 15 interconnect to pin
1 on cable connector. And resale.

     I am running the unit at 50 watts. It get warm, well very
warm. I would suggest that you place some blower fans blowing on
the heat sink. You could get a T74 (110 watt) unit and run it all
day at 50 watts, But they are very hard to come by.

          73's and good luck


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