I recently got two older Icom IC-2N HTs (2 meter) for use in decoding
APRS packet radio. Nice radios, but I get less than 10% of packets
decoded with either radio.

Mike-

I agree with Dave's assessment of your problem. My first thought was to turn
down the audio level and see if that improved things. Otherwise, check to be
sure your +5 KHz switch is off (or on if appropriate).

Back around 1990, I had an IC-3AT and an IC-2AT connected in an experimental
dual TheNet node, connecting between a 220 backbone network and local access on
Two Meters. I never had a problem like yours.

I'm not familiar with the IC-2N. Could it be a variant of the IC-2A? If not,
then it is possible that model had a differently wired earphone jack, with a DC
voltage on the shield terminal rather than ground. In that case, there could
be a short circuit when connected to a TNC that had its ground also connected
to the co-ax shield on an external antenna. I know this could be the case with
some Radio Shack handheld radios, but have never encountered it with Icom.

73, Fred, K4DII