[R-390] Lightning crashes in audio

[Tisha Hayes]

Mica cap failures have been understood for about sixty years when there
were unexplained failures in high-reliability applications (subsea
communications cable amplifiers). There are a variety of failure
mechanisms, including;

High voltage transients (sometime in the life of the mica cap). This blows
a tiny little puncture through the mica laminates and eventually---

Silver-ions migrate through this gap
Carbon tracking from contaminants

Because it is not a constant failure the capacitor gets noisy as this
microscopic pair of filaments (silver, or carbon or anything else) begin to
arc over at voltage.

Capacitors that we are dealing with today have had a literal lifetime of
experiences and any of these may have caused a pinhole in the mica. Also,
since mica is a mined mineral there are different grades of mica that is
used in caps. They would classify mica based upon its optical clarity but
just how many trace contaminants (iron molecules from the geology the mica
was formed in) or other defects just do not get caught in the mass
production of capacitors.

*Ms. Tisha Hayes, AA4HA*