
TM-721 & 9600 Bps.

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 discriminator output. Look for the 3.3K
resistor coming off of 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 control). This has
been tested and works quite well on 440 MHz and 220 MHz. The mods haven't
been fully tested on the 2 meter portion of the radio.

The TM-731 radio is just enough different to cause problems. What needs to
be done is the addition of 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 on the
schecmatic. 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 broke, but they fixed it anyways.)
This message is being uploaded to the local BBS with the modified TM-731.
As with any modifications, take resonable care, and I won't be responsible
if you trash your radio. BE CAREFUL it is full of surface mount devices.
TM-721 & 9600 Bps.

The interface to my 721 was guessed at and seems to work quite well.  Audio
directly from pin 9 of FM IF chip, and PTT on pin 2 of mic connector.  TX
input
to the collector of muting transistor of PLL audio input. The local 9600 node
seems to be quite busy and that may be the cause of seemingly poor operation.
A local friend of mine is about to get another 9600 card for his 1278 TNC so
we
will have a clear frequency to do testing between our rigs.

73 & God bless from Chris/AB6FA  Lompoc CA.
