Mark Hansen wrote:
We have a plane in our club (1978-ish C172N) that has a problem
with the pilot's push to talk switch. It is mounted on the yoke
handle (as if from the factory, not like an after market).

When you press it gently, it works fine. If you press it too hard,
the radio continues to send the carrier, but voice is no longer sent.
...

Mark,

I have never seen a yoke-mounted PTT switch which breaks the MIC line.
Usually,
the yoke mounted switch is only a single pole normally-open switch,
which is wired to connect the PTT line to airframe ground while the
switch is depressed.

If pushing harder effects the Mic audio, this may be happening
indirectly depending on what type of intercom/audio panel is in the
aircraft. Frequently, there is logic inside the intercom which switches
the headset mic from the intercom function to the selected transmitter
when the PTT switch is depressed. It is possibe that pushing hard
causes a high-resistance in the yoke-mounted PTT switch, such that the
resistance is low enough to key the transmitter, but not low enough to
cause the mic to be switched from the intercom bus to the transmitter.

The solution is to replace the yoke-mounted PTT switch.