BB capacitors were molded as well as the technology of the day
would allow... That is not why they cracked.
The problem is the plastic they used, like most plastics, shrinks
slowly over time, and the wire leads, and especially the solder
sealed oil filler tube, that penetrate the plastic, don't.
This can cause a crack to form at the weakest point, which is
usually at the mold line formed by slight misalignment of the
mold halves.
When you injection mold plastic, the plastic's internal bonds
stretch out to fill the mold. When the plastic cools, it is
frozen into a state where those bonds are stretched.
Over time, under the continuous stress of these stretched bonds,
the plastic will cold flow into a more relaxed state... becoming
slightly smaller.
The problem is if the plastic is constrained by something inside of
it that cannot shrink.... such as the wire leads, a gear's hub,
or an oil filler tube, something has to give.
If the plastic shrinks around a metal tube, the thickest parts of
the plastic will break the thinest parts forming a crack, and a leak.
I was told by Deane Kidd, that in the early days, Tektronix created
a situation, ripe for failure with BB capacitors, by putting them
tightly fitted into ceramic or bakelite terminal strips. When the
capacitor's leads were soldered, the heat traveling down the very
short leads would melt the capacitor's solder seal, allowing the oil
to mix with the solder, and make a leak. In the worst cases, the oil
would drool out leaving the capacitor incapable of withstanding its
rated voltage.
Not to mention it made a greasy dirty mess in their otherwise very
nice looking scopes.
-Chuck Harris
On Thu, 4 Nov 2021 14:40:42 -0400 "greenboxmaven via groups.io"
<ka2ivy@...> wrote:
Sprague "Bumble Bee" and "Black Beauty" condensers were molded
improperly, and they will crack, especially the oil filled ones, and
let in moisture. That seems to react with the oil and make it
conductive as well as corrosive. Once this happens, leakage gets very
bad, especially when the condenser is heated by surrounding
components. Many efforts have been made to bake? and otherwise
restore them, but once the problem begins it can not be reversed. The
dielectric charicteristic? is perported to be superb for audio
circuits, and AudioPhools will freak out if these condensers are
replaced, that is until the power transformer, rectifer, output
tubes, and filter choke in their amplifiers are all incinerated. Not
only did Hammarlund use them in receivers, Collins and National did
as well. Tektronix used them in their equipment, and too many
consumer electronics companies to recall all of them.
?? Bruce Gentry, KA2IVY
On 11/4/21 13:45, Richard Knoppow wrote:
Who made those? Some early advertising for Sprague Black Beauty
caps said they were plasticized paper. The main problem with these
is some sort of production problem they had. I have dissected a
number of "bumble bee" caps, these are actually Sprague Black
Beauty caps with striped value markigns. I found the windings were
distorted. I suspect this happened during the encapsulation but
don't know. Some of these caps, the higher voltage ones, were oil
impregnated. The oil was introduced through a tube which formed one
of the leads. It is common for all the oil to have leaked out due
to a lack of sealing at the solder joint between the tube and the
lead. That may have been due to excessive heat when the caps were
installed. The most notorious of the BB caps are those in the
Hammarlund SP-600, however, they were very widely used including in
a lot of -hp- gear. However, in the receiver it is common to find
cracked or split caps or even caps where part of the casing has
come off. I have not seen that in any other make of equipment.
Perhaps just a bad lot or perhaps some difference in installation.
Nonetheless, ALL paper caps become degraded with age. The usual
symptom is an increase in series resistance. Leakage, which is
another term for parallel resistance, is sometimes a problem but
not as often. I don't think any of these caps, even the real oil
filled transmitting caps, was intended to have a lifetime of over
about ten years. The life problem was known and described in some
engineering texts. It was dependent partly on voltage; the closer
the voltage to the rated voltage the shorter the life of the cap.
Likely it was some sort of electrolytic degradation of the paper.
In any case, paper caps should be replaced with modern plastic film
caps. These for the most part will last forever and generally will
improve performance perhaps beyond the original. There is a good
amount of information on the web about the relative merits of
various kinds of plastic dielectrics. Polypropylene seems to be
about the best especially for RF use.
On 11/4/2021 9:33 AM, Dave Wise wrote:
Even the later plasticized-paper ¡°Prokar¡± and ¡°DiFilm¡± versions
can cause trouble in some applications ¨C they may not be leaky,
but their Dielectric Absorption and Dissipation Factor are
¡°through the roof¡±.
Dave Wise
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of
*greenboxmaven via groups.io
*Sent:* Thursday, November 04, 2021 6:25 AM
*To:* [email protected]
*Subject:* Re: [HP-Agilent-Keysight-equipment] HP200CD oscillator
ALL Sprague "Black Cat", "Black Beauty" and? color striped "Bumble
Bee" condensers should be considered bad on sight. Unfortunately,
they are common in older equipment of all makes and models. The
630V. polyester condensers are a fine replacement. The only
condensers that are polarized are electrolytics, poyester
condensers are not polarized.
??? Bruce Gentry, KA2IVY
On 11/4/21 8:51, Richard Merifield wrote:
??? My unit has Black Cat 0.5MFD 400v axial capacitors for C8
??? and C9. Would 630v polyester unpolarised caps be ok
??? replacements?
