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wrote:
I work on fire alarm systems in outlying areas that transmit a signal
to the fire department via 72 Mhz AM.

The crystal-set and old-TV-tuner ideas that have already been mentioned;
of those two I like the crystal-set idea the best. (A super quick and
dirty version of the crystal set would be to hook a piece of wire to one
end of a diode and feed the other end of the diode to the receiver half
of a phone line tone tracer.)

An extension of the old-TV-tuner idea is to get one of the portable
radios that had TV-band audio (try the thrift store) and see if you can
hear it. Yes, TV audio was FM, so this might not work.

There is probably a scanner that can do it, but a lot of scanners have a
hole between 54 and 108 Mhz, to avoid VHF analog TV channels 2-6 and the
FM broadcast band. Also, some scanners don't allow you to pick the
modulation independent of the frequency; they might be "hard wired" to
AM for air band (108-137 MHz) and FM for everything else. I have one
that is hard-wired like that, and another one which lets you choose.

If you happen to have a junk Cessna or Piper up on blocks in your yard,
the marker beacon receiver in it is 75 MHz AM. The audio (Morse code
between 400 and 3000 Hz) is decoded and sent through some filters to
light up one of three lamps. Usually there is a way to access the audio
before the filters; you'd just have to tune it down 3 MHz somehow.

Matt Roberds