TM721A.16

The mods to do 9600 baud packet  using a K9NG or G3RUH modem are  fairly simple
to do with the Kenwood TM-721/621 radios.  Look at your schematic.

     Receive  data  is  taken  from  the dicriminator output. Look for the 3.3K
resistor comming off pin 9 of the  last IF stage. Connect a 1K series  resistor
from the modem RX input to the junction of the .001 and 3.3K.

     PTT  is taken  off pin  2 on  the microphone  connector. TX  data  is  fed
through a 1K series resistor to the top of VR3 (the deviation conltrol).

     This method has been tested and works  quite well on 440 MHz and 220  MHz.
The mods haven't fully been tested on the 2 meter portion of the radio.

     The TM-731 radio is different enough  to cause problems. What needs to  be
done is to add another RX Mute  transistor. (Look at the schematic and see  how
the TX deviation  is shorted to  ground during receive.)  Use a plain NPN and a
pair of 10K resistors (copy the one  used on VR3 in the schematic). Take the TX
data and feed it to the collector  through a series 1K resistor, and then  feed
that through another 1K resistor to the  wiper of the deviation pot. This was a
result  of Kenwood  changing the  deviation pot  value from  10K to  100K.  (It
wasn't broken, but they fixed it anyway.)


     As  with  any  modifications,  take   reasonable  care,  and  I  won't  be
responsible if you  trash your radio.  BE CAREFUL; it  is full of surface mount
devices.  

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This MOD file was written by Jeff, WA6FWI @ WB6YMH.#SOCA.CA.USA.NA
Schematics and layouts are available by request through the author.

Good Luck!


 73 de Chris KB2LPW @ KB2LPW.#NYC.NY.USA.NA
 SYSOP of PDSNET ARC and the Helping BBS's