Anyone read anywhere what the failure mechanism is for solid dielectric caps
of the tantalum variety ? In my experience, no matter what the value,
working voltage, or format (bead, bullet or box), they always seem to fail
leaky. Not open or short (well, very occasionally short). Just leaky.

Example. Today, I had a Mesa Boogie combo cross my bench. Very odd problem
in that when the 80Hz slider in the graphic was advanced in the 'boost'
direction, the audio suddenly went very distorted, and then disappeared.
There was also a slight 'scratchiness' to this pot, which did not feel like
a bad or dirty track.

When I got the graphic pots board out, it was actually quite a simple
affair, with each of the 6 bands having just a pot, one resistor, one choke,
and one cap. On the 80Hz channel, this cap was a 3u3 tantalum bullet, and it
was 2k leaky. Why ? The device is under no voltage stress at all in this
position, being subject to low signal levels only. I wonder if it's some
kind of internal 'growth' like the dreaded tin whiskers, which causes it ?

A new cap (used a 3u3 tant bead that I had in stock) restored normal
operation of the equaliser, and all scratchiness in that band disappeared.

Arfa