Everett M. Greene wrote:
Now, if there are any EEs present, perhaps they can explain
why aircraft radios tend to fail in this manner whereas
nothing much of anything else in the electronic world fails
in a manner to produce heat and smoke. Are avionics units
designed so close to the components' limits to cause this
failure mode? I can understand a transmitter going up in
smoke (while transmitting), but a receiver?

I'm an EE. In my day job I design guitar amplifiers. Let me tell you,
I've made a LOT of unintentional smoke at my workbench. When I was
in school a fellow student explained to me that the smoke contained
inside those little electronic components is what makes them work.
It's magic smoke. If you let out the magic smoke, the electrons stop
doing whatever it is they're designed to do.

Seriously though, maybe the propensity of aircraft radios to fail in
this manner has to do with the fact that on average they're pretty old,
and constructed mostly of discrete components. Modern electronics are
more highly integrated into power- and space- miserly "chips", or ICs
(integrated circuits). ICs also allow for more circuitry in a smaller
space, so thermal protection, over-current protection, and other
similar support circuitry can be incorporated into a design without the
weight and space penalties that made them impractical for aircraft
radios "back in the day". It's a theory, anyway.

-R