ICOM Handheld to TNC Interface To interface a ICOM handheld to a TNC do the following. Directly connect the speaker output from the handheld to the audio input connections on the TNC. Connect the sleeve on the mic jack to the ground connection on the TNC. From the tip of the mic jack on the handheld connect a 1 mf cap to the TX audio input on the TNC. From the tip of the mic jack on the handheld connect a 33 K resistor to the PTT line on the TNC. Please remember that almost all problems in interfacing a TNC to a radio come from one of two sources. Setting the volume control too high on the radio causes the audio to be overdriven to the TNC. This causes retries or in the worst case no reception. Im- properly wired cables are the second most frequent cause of problems.