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TNC-PI and Kenwood TK Mobiles RJ12

This page describes creating a cable to connect the speaker jack and microphone jack of a Kenwood TK radio to a TNC-PI or Kantronics KPC-3.

During assembly, I suggest crimping the wires to the RJ12 first, then apply heat-shrink tubing to the RJ12 end of the wire, then solder the DE9, and finally solder the 3.5mm speaker plug.

Parts and tools required:

There are 3 critical signals, plus ground, connected to the TNC-PI. Those signals connect to the TNC on these pins of the DE-9 connector:

I have arbitrarily selected color coding for the signals using colors available on typical CAT5 wire. I chose CAT5 wire because it is easy to crimp into an inexpensive RJ12 connector.

The color codes are shown for the wires which are connected direction from the DE9 to the CAT5. Pin 1, TxData, is fed through a 10K ohm potentiometer and then into the Blue wire.

Click on the images to enlarge
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The potentiometer allows for a much less stiff adjustment on the TNC-PI. If pin 1 was connected to the mike/txaudio wire directly, the TXLEVEL pot R7 is so sensitive that it is almost impossible to find the proper level to saturate the radio passband, without overdriving. With the addition of the potentiometer in the DE9 connector, it is really easy to set R7.


RJ12 Connector

Please note that the RJ12 connector may be used even on Kenwood TK radios which have an 8-pin microphone connector. The remaining unserviced 2 pins are not used in our application or are redundant with the rear-panel 3.5mm connector.
Also note that the standard Ethernet color codes were not followed and we're crimping the BROWN and BROWN/WHITE wires into the connector but not connecting them at the DE-9 end of the wire.

Strip one end of the CAT5 wire such that there is about 17" of GREEN and GREEN/WHITE twisted pair free, and about 3/4" of the remaining colors.
Crimp six wires into the RJ12 connector as shown, folding the GREEN and GREEN/WHITE pair away from the connector. We’re connecting to 6 pins on the radio. They are described as this by Kenwood:
PSB  — BROWN - don’t care
ME   — BLUE-WHITE — External Microphone Ground
HOOK — BROWN/WHITE - don’t care

Note! This wiring is not anybody's standard. Please crimp with these colors. The two wires on the ends, white/brown and brown, are not important except that they make it easy to place the other four wires in the connector.
The colors are:

Put heat shrink over the cable over where the green wire folds back and all the way up to the connector. I found that a 2" piece of heat shrink was adequate.

DE9 Connector

Strip the DE9 end of the cable back about 3/4 of an inch.

Cut the BROWN wire back to strip point. Cut the BROWN/WHITE wire back to about 1/4" from the strip point. We're not soldering those and they contain signals we do NOT want shorted.


Twist the ground wires, GREEN/WHITE and BLUE/WHITE and ORANGE/WHITE, together. Trim, and solder the bundle of 3 wires to pin 6.

Solder the Green and Orange wires to the DE9 connector pins 5 and 3 as shown at the top of this page.

Solder the blue wire to the center/wiper pin of the POT, and then two outer pins of the POT go to 6-ground and 1-TxData. The blue wire feeds the transmit audio to the TK radio.

When you are ready for test, turn potentimeter R7 on the TNC-PI fully clockwise. Get the TNC-PI to transmit over and over by sending connect requests to a non-existant station, with FRACK set low and RETRIES set high (see TNC adjust and test). Now, while listening on another radio, to the braps being transmitted, adjust the potentiometer in your DE9 shell such that as you turn the transmit level UP (sound gets louder) you turn it up to where you can't hear it get louder with more turning. Set it back to the the breaking point where on one side you can tell it gets quieter, and other other side it plateaus.

Earphone plug

Solder 17" long green wire to center conductor of earphone plug. Solder white&green wire to ground of earphone plug. Note that reversing these two wires on the earphone jack is the number one cause for failure of this cable!

© Tadd Torborg, 2014↝2017 -- all rights reserved