"Chuck Harris" wrote in
message ...
Richard Knoppow wrote:
here?
Nope, very easy. Just don't use disk ceramics.
Something in a polyester,
or polypropylene would do nicely.


I am curious why you recommend against using disc
ceramics.
BB's are found in a lot of high-quality equipment.
They were supposed to be high-performance deluxe caps
when they were sold but very quickly got a well deserved
bad reputation.

Disk ceramics, particularly Z5U, or X7R, are a bad idea in
any
signal path. The feature that gives them high capacitance
for
their size also makes them highly piezoelectric: meaning
they change
physical dimension with applied voltage. This large
change in
dimension makes the capacitance non linear with voltage,
and also
makes the capacitor's power dissipation change with
applied signal.
That means distortion.

NPO's behave ok, but are of such low values that they
would never
be able to substitute for a capacitor such as a BB.

I have used Z5U ceramic capacitors in high signal areas
and actually
heard them "sing" along with the signal. Not a good
thing!

Their capacitance is also extremely sensitive to
temperature, which
makes them useless in most timing applications. [I did use
one once
as a temperature transducer in a cheapy RF telemetry
device...]

They work adequately in logic circuits as power supply
bypass.

BB's are actually a pretty good capacitor, but they have a
couple
of failure mechanisms that render them useless over time.
They
are an oil filled paper capacitor with a black epoxy case.
If you
look at the banded end of the capacitor, you will notice
the lead
has a bulge where it leaves the case. That is actually a
piece of
brass tubing that was used in filling the case with oil.
After the
case was filled, the lead was stuck into the hole, swaged
to keep
it from falling out, and then soldered shut. The problem
comes when
the BB's banded lead is soldered into the circuit. If no
heatsinking
is used, the solder seal melts, and the oil spoils the
seal. From
that point forward, the oil will seep out of the
capacitor, and moist
air will seep in... it's a slow process that takes many
thermal cycles,
but we are talking about old stuff here. The other
failure mechanism
is the plastic case shrinks over time and breaks the seal
around the
leads, and sometimes even splits the case into pieces.

The BB's are all bad by now, and should be replaced on
sight, but when
they were new, they were a nice high performance
capacitor.

-Chuck

OK about the ceramics. I think there must have been two
series of Black Beauty caps. Not all are oil filled. The
ones I removed from an SP-600-JX for instance, are paper
impregnated with polyester plastic. They are dry and don't
have filler tubes. I suspect the main failure mode for these
is due to the incapsulation. Many seem to have cracked and
even those which have not cracked often have low capacitance
and high dissipation factor. The capacitor winding is
usually physically distorted, flattened for instance. I
suspect that the case shrinks distoring it and also may
allow moisture to enter. These caps were supposed to be much
better than the wax coated paper caps of the time.
Sprague also made a similar capacitor, that is a dry
polyester impregnated paper cap, but in epoxy-dipped casings
called Orange Drops. AFAIK, these have proven quite reliable
and long lived, giving some support to my idea that it was
the molded cases of the BBs that failed.
BBs are found in a lot of high-quality equipment,
General Radio, Hewlett-Packard, etc, used a lot of them.


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Richard Knoppow
Los Angeles, CA, USA