3/13/92 From : WA6FWI @ WB6YMH.#SOCA.CA.USA.NA To : INFO @ ALLUS Date : 920308/2019 Msgid : BF 8269@WB6YMH, 44910@WA0PTV $8269_WB6YMH Subject : 9600 baud mods TM-721 and TM-731 Path : KA3SFC!NM3G!WB6YMH From: WA6FWI @ WB6YMH.#SOCA.CA 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. 73 es GE from Jeff, wa6fwi@wb6ymh.#soca.ca (schematics and layouts are available on request.)