Mark Zacharias Inscribed thus:

"Arfa Daily" wrote in message
...
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


Tantalums have kind of a bad reputation for shorting, and that's been
my experience. Not really seen them leaky or open.

Thats been my experience too. I've seen them glow red hot and burst
like a volcano, spraying tiny red hot fragments all over. The
fragments burn for a second or so before dying out. Admittedly nearly
all that have done this have been bead types.

The scratchy sound of the pot is typical of DC leakage in such a
circuit. Can make a volume pot or similar sound "dirty" or make
woofers go "whoomp" when the control is moved.

But then you know this, yes? I know you have loads of experience with
this stuff.

I got the "whoomp - whoomp" in a Yamaha receiver the other day - a bad
muting transistor put DC on the volume control.

Mark Z.

--
Best Regards:
Baron.