Little brown silver mica capacitors

M

mike diack

Guest
For the third time in as many months I have come across one of these
little brown buggers that has gone wierd. Test it on a regular meter
for leakage - OK, test it on a Cap meter - OK, Put it on a megger
(these things are 500v rated) - short circuit !!!. The consequences of
the failures were unspectacular in the first two cases, but the
downstream effects in the last one (being used as a compensation cap
in an output triple of a 5000 watt Crown amplifier) were awesome
(and, for the owner, expensive). These caps were not being used
outside their ratings (about 200v in the case of the Crown). Small
value non ceramic HV caps are a lot less common these days. Has anyone
else noticed failures in these components ?.
M
 
If your megger is in the voltage range of the caps and they fail testing,
then I would put in new replacements. This would be the simplest and least
risky approach.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"mike diack" <moby@kcbbs.gen.middleearth> wrote in message
news:4059ee75.891279@News.xtra.co.nz...
For the third time in as many months I have come across one of these
little brown buggers that has gone wierd. Test it on a regular meter
for leakage - OK, test it on a Cap meter - OK, Put it on a megger
(these things are 500v rated) - short circuit !!!. The consequences of
the failures were unspectacular in the first two cases, but the
downstream effects in the last one (being used as a compensation cap
in an output triple of a 5000 watt Crown amplifier) were awesome
(and, for the owner, expensive). These caps were not being used
outside their ratings (about 200v in the case of the Crown). Small
value non ceramic HV caps are a lot less common these days. Has anyone
else noticed failures in these components ?.
M
 
On Thu, 18 Mar 2004 16:26:28 -0500, "Jerry G." <jerryg50@hotmail.com>
wrote:

If your megger is in the voltage range of the caps and they fail testing,
then I would put in new replacements.
Click to expand...
It is and I did, what I'm looking for is whether this is an isolated
incident or if there is a general unreliability with these components.
It makes a difference if someone's expensive piece of kit is shredded
by a dodgy part.
cheers
Mike
 
"mike diack" <moby@kcbbs.gen.middleearth> wrote in message
news:4059ee75.891279@News.xtra.co.nz...
For the third time in as many months I have come across one of these
little brown buggers that has gone wierd. Test it on a regular meter
for leakage - OK, test it on a Cap meter - OK, Put it on a megger
(these things are 500v rated) - short circuit !!!. The consequences of
the failures were unspectacular in the first two cases, but the
downstream effects in the last one (being used as a compensation cap
in an output triple of a 5000 watt Crown amplifier) were awesome
(and, for the owner, expensive). These caps were not being used
outside their ratings (about 200v in the case of the Crown). Small
value non ceramic HV caps are a lot less common these days. Has anyone
else noticed failures in these components ?.
Click to expand...
Silver mica capacitors are among the most reliable of electronic components.
Are you sure? Some megohmmeters exceed 500 volts.
 
On Thu, 18 Mar 2004 19:00:40 GMT moby@kcbbs.gen.middleearth (mike
diack) wrote:

For the third time in as many months I have come across one of these
little brown buggers that has gone wierd. Test it on a regular meter
for leakage - OK, test it on a Cap meter - OK, Put it on a megger
(these things are 500v rated) - short circuit !!!. The consequences of
the failures were unspectacular in the first two cases, but the
downstream effects in the last one (being used as a compensation cap
in an output triple of a 5000 watt Crown amplifier) were awesome
(and, for the owner, expensive). These caps were not being used
outside their ratings (about 200v in the case of the Crown). Small
value non ceramic HV caps are a lot less common these days. Has anyone
else noticed failures in these components ?.
Click to expand...
I've always found them to be extremely reliable.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
"Charles Schuler" bravely wrote to "All" (18 Mar 04 17:34:26)
--- on the heady topic of "Re: Little brown silver mica capacitors"

CS> Silver mica capacitors are among the most reliable of electronic
CS> components. Are you sure? Some megohmmeters exceed 500 volts.

Some silver mica caps were prone to exhibit metal migration problems.
They could also be contaminated by moisture if the lead seals broke.

.... 'Keep the smoke inside.' -- 1st Rule of Electronics.
 
On Friday, 19 Mar 2004 11:01:20 -500 "Asimov"
<Asimov@-removethis-bbs.juxtaposition.dynip.com> wrote:

Some silver mica caps were prone to exhibit metal migration problems.
They could also be contaminated by moisture if the lead seals broke.
Click to expand...
The ones which were famous for the silver migration I think were
completely open to the atmosphere. I've seen exactly one bad one which
had been sealed, but that was a very old one of a style that was
sealed into a plastic can (from the 50s.)

I've never seen a bad one of the style he described, the brown dipped
versions.

Have you?

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
Jim Adney wrote:
The ones which were famous for the silver migration I think were
completely open to the atmosphere. I've seen exactly one bad one which
had been sealed, but that was a very old one of a style that was
sealed into a plastic can (from the 50s.)

I've never seen a bad one of the style he described, the brown dipped
versions.

Have you?
Click to expand...
OP mentioned this is a tone compensation cap in an output triple of a
5000-watt Crown amplifier. I don't know what a "an output triple of a
5000-watt Crown amplifier" is but it sure seems likely that a 500 vdc
rating would be inadequate in the presence of 5000 watts of audio (or RF).
I suspect they are being damaged as opposed to "going bad"

-BillM
 
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