Thanks for the good advice from both you and Dave Platt. I'll try
lowering the volume and see if that has any effect.

I'm going to bet that this will probably help matters a lot.

I took a look inside the radio and it's all discrete components. The
audio even sounds pretty good (to my ears). And I can transmit
packets from my Yaesu VX-2R to the Icoms from within my apartment with
good decoding.

If you can gain short-term access to an oscilloscope, it'd be
beneficial to simply watch the waveform coming out of the audio-out
jack. Tune to a busy packet frequency (144.390 will usually get you
boatloads of APRS packets), slowly turn up the volume control, and
find the point at which the audio waveform stops increasing in
amplitude and begins exhibiting a flattening-off of the tops and
bottoms. At this point, your signal is clipping - you definitely want
the volume control turned down below this point. Further reductions
may be beneficial if you're accidentally overdriving the TNC's audio
input circuitry.

Can I build a simple filter that removes everything except 1200/2200Hz
tones?

It's possible, but it's not necessarily trivial. It can be done
fairly easily with active electronics (i.e. one op amp and a dozen or
so passive parts). Take a look at www.tnc-x.com and pull up the
schematic - the receive-audio filter is implemented by U4 and the
parts around it.

It can also be done with a strictly passive circuit, requiring no
power, but that's a bit more involved, and will probably require some
fairly large audio inductors.

What you'll want, in either case, is a bandpass filter which passes
roughly 1000 - 2500 Hz, and rolls off both the low and high
frequencies on either side of that passband.

A well-designed TNC should already have this sort of bandpass
filtering built into its input circuit, I think - adding an outboard
filter between radio and TNC ought not to be necessary.

--
Dave Platt AE6EO
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