Interfacing an HTX-202 to a Kantronics KPC3+

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If you're like me you found your budget stretched by purchasing a TNC for packet radio at the last moment for an ARES club activity. I liked the Kantronics KPC-3+ Packet Communicator - a 1200 baud AX.25 TNC which will not drain my station's battery very quickly, and will help me learn a lot more about amateur packet radio. I purchased one, connected it to my computer with a DB25-to-DB9 serial cable, and configured it.

What 2 meter radio sitting around can be pressed into service as the radio side of the packet radio station? Hmmm, how about that Tandy/Radio Shack HTX-202 handheld transceiver! It is built solid, and also connects nicely into the 13.8V battery power. I'm glad to put this bulky (by modern standards), blocky, bulletproof old standby back into service.

Now to hook them up. I searched and searched the Internet but no diagram came up which matched my situation exactly, and no source described it clearly. Sources conflicted as some appeared to be armchair suggestions, and those who had done it seemed to think it was a no-brainer. I stared again and again at the available KPC3+ MX schematic and pinout documents and HTX-202 user manual and schematic for advice.

What I came up with is to the right. You also have to open the Kantronics KPC3+ case by removing the two screws on the sides, and move Jumper 9 (J9) on the circuit board from NOR (closer to the edge of the board) to HT (closer to the center of the board).

Note: I have ignored shielding the cable for now. I don't know *what* kind of noise it's going to pick up in my shack and retransmit.

Originally I did not install the 2.2k ohm resistor because I saw on the KPC3+ schematic that setting J9 already will try to clamp the MIC line down toward ground, but measuring the voltage indicated it really didn't budge from about 0.65V, and as a result the HTX-202 did not go into transmit. Shorting MIC to ground will kill any mic audio, but it did cause the HTX-202 to go into transmit, so I thought I'd try a 2.2k ohm resistor to ground, and installed in the circuit, that seems to work. My thought? The builtin resistor in the KPC3+ is too wimpy for the HTX-202's burly design.

Adjusting the HTX-202 is pretty easy. With the MIC/PTT plug installed and SPKR plug removed, turn on the HTX and rotate the volume to 9-o'clock, a quite volume to human ears. Open the squelch all the way (turn down, to the left) and listen for packet traffic. Increase the squelch until it turns off audio between receiving each packet burst. Reinstall the speaker plug, and watch the KPC3+'s receive LED on the front panel. Adjust volume just loud enough that it lights up for each burst of radio data received.

Good luck! You can reach me at kn1x (at) if you have any questions. Feel free to browse my general ham radio page or my personal home page.