Modifications for the Kenwood TM-621

TM-621 to 9600 baud


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.

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


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