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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. This cable uses a 6pin microphone connector and is compatible with the
TK705, TK805, TK808d,
TK705d, TK805d (note: See article on an optional improvement to the TK705d/TK805d here)
TK730, TK830 (except that the earphone/speaker plug is replaced with wiring to the Kenwood molex connector)
TK740, TK840, TK840(N)
TK760, TK762, TK760G, TK762G, TK763G, TK860, TK862, TK860G, TK862G, TK863G (and high power H varients)
TK780, TK880.
Even though many of these radios have 8 pin mike jacks, the 6 pin varient is perfectly suitable.

This cable, including the potentiometer in the D-shell connector, is available pre-made for $25. Click here to order and send a Paypal payment to TNC Radio Cables:

Click here to buy a cable for $25. This will enter a PayPal dialog which will ask you to authorize $25 from your PayPal account, or to select to pay without a PayPal account.

pays to TNC Radio Cables
in Woodburn, OR
radio-and-tnc-tk-radio-to-tnc-cable-i6_02269cr_251 radio-and-tnc-tk-radio-to-tnc-cable-i6_02272sc

Build your own

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:

I have had very good luck with DE9 connectors and audio plugs from Parts Express.
I recommend:
090-550  parts-express 9 Pin Male Solder Type D-Sub at 45 cents each
090-695  parts-express 9 Pin D-Sub Hood at 58 cents each
092-158  Neutrik Rean NYS226L 3.5mm Mono Plug Nickel 0.25" Entry at 73 cents each.

Try to find real copper CAT5 or CAT6 wire. You'll need about 14 inches per cable.

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 directly 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
2017_02_tk_radio_wiring_i6_11270cr 2017_02_tk_radio_wiring_i6_11271cr 2017_02_tk_radio_wiring_i6_11268cr

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 though only the middle 4 are important. Based on the image of the RJ12 just below, this is the order from left to right.
HOOK — BROWN/WHITE - don’t care
ME   — BLUE-WHITE — External Microphone Ground
PSB  — BROWN - don’t care

Note! These wiring colors are 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!

Where applicable, make sure you put the plastic insulator, spring, and screw-on cover over the wire, in the proper order and orientation, before soldering!


Adjusting the built-in Potentiometer

The potentiometer in the DE9 connector should be adjusted to about the middle of its range.
The ideal setting would make it so the TNC's TX-level would be at about 3/4 of TNC pot's range at the point of best transmit audio level. Without the pot built-in to the DE9, the TNC would hit the best audio threshold at about 1/4 turn. Also, the threshold is very sharp. By building the potentiometer into the cable, we are reducing the sharpness of that threshold and thus making it easier to find.

The transmit audio level setting is achieved by generating [nearly] continuous packets through the transmitter while changing the potentiometers. Lacking a deviation meter, I have found a reasonable level by listening to the transmissions with a local receiver. Adjust the audio to maximum, and then slowly turn the potentiometer until the transmit audio starts becoming quieter. That's probably the best level to leave it at.

For more information, consult TNC and Radio Adjust and Test

© Tadd Torborg, 2014↝2019 -- all rights reserved