What actually Fails inside of Paper Capacitors

[o***@tubes.com]

I've seen so many warnings about these old wax coated paper capacitors
in these newsgroups, and on the web, that every time I see one of them
now, I begin to shudder.

Immediately, I notify the FBI, telling them that there is a wax coated
paper capacitor in my radio, and I know that this thing is deadly. I
carefully explain to them that I have been exposed to it, and instantly
developed capacitor cancer. I also explain that I have heard of entire
cities being reduced to rubble, killing everyone within 100 miles, when
these caps explode. Then I ask them to come remove the capacitors for my
and everyone elses safety.

Yes, I was just joking in this previous paragraph.......

Seriously, I have not been able to find any answers to what actually
happens internally to these caps, which causes them to fail. I googled
for an answer, specifically used the word PAPER CAPACITOR, but all I get
are results for failing electrolytic caps, which I do now clearly
understand what occurs with them.

So, lets say I have an old radio from the mid 1950s. It contains 9 paper
capacitors (plus a few electrolytic filter caps). I am repeatedly told
that I must replace all of these wax coated paper caps, as well as the
dried up electrolytics.

Ok, I know the electrolyte has dried up in these electrolytics and they
are no longer filtering the rectified DC voltage in the power supply,
which I can quickly identify by the 60 cycle hum coming from the
speaker.

Then, I am told I must replace all these smaller wax covered paper caps,
which may or may not be working at the moment. Great, I can take this
advice and I can change them, but it seems that no one (at least not on
the web), can seem to explain what occurs inside of their waxy bodies
that make them fail.

I know they are quite simple. I roll of paper, and two layers of tin or
aluminum foil, with leads attached to the two layers of foil inside.They
are rolled up, with their leads exiting their bodies. Then they are
given an outer shell of paper with their identity printed on it, and are
then coated with wax. I'm sure that if I had a lot of time to waste, I
could even make my own paper caps, but for the low cost they sell for,
that is not necessary, other than for an experiment.

Knowing how they are built, I can only fathom three possible reasons
that they fail.

1. The leads tend to corrode from dialectric corrosion, where they
contact the inner foil. (I have read that moisture can and does enter
these caps, even with the best wax coating).

2. Voltage arcs burn across the paper layer, arcing between the two
layers of foil, and burn a hole into the paper, which allows the two
layers of foil to contact each other, and cause a short circuit.

3. The foil itself decays from age, and loses conductivity in parts or
all of it's roll.

* This does not take into account physical damage, such as crushing or
poking a hole in a cap body.

What else can go wrong on such a simple device?

*NOTE: I mentioned the wax coated paper caps, but am aware the old
plastic coated ones such as the ones called "Bumble bees" (with colored
stripes) are just as bad.

Honestly, I find it hard to comprehend how moisture can get past that
wax, as long as the wax remains intact, but I'll just take the advice
from the experts on that note.

One final question: Does anyone know what kind of wax was used on them?
Was it bees wax, paraffin wax, or something other?

[Michael Black]

Post by o***@tubes.com
I've seen so many warnings about these old wax coated paper capacitors
in these newsgroups, and on the web, that every time I see one of them
now, I begin to shudder.

Here's a simpler way to look at it.

Paper capacitors are an old type of capacitor. ELementary Electronics
about 1971 showed how to make them yourself, some paper, some foil, some
liquid if I recall properly. I think even then I knew they were hideous,
that piece of foil was relatively large, must have inductance. We see
this, paper capacitors were often marked with "+" or "-" so you knew which
end to connect to ground. They weren't polarized, but they might pick up
unwanted signals if you connected the wrong end to ground, it was the
nature of how they were made.

They were also fairly big for the capacitance offered.

But for a time, they were what was available. When you started posting, I
got curious and apparently bypass value ceramic capacitors took some time
to become feasible, which has to explain all the old radios with paper
capacitors. When I got into the hobby in 1971, nobody would suggest paper
capacitors for bypass capacitors (and probably not anything else). "Keep
leads short", but that didn't work so well when the paper capacitor was
the equivalent of a long lead.

So it doesn't matter why paper capacitors fail, there's lots of reason to
replace them anyway because they are an antiquated form of capacitor. The
switch to ceramic bypass capacitors happened because you could get a more
reliable package, but it was also smaller, and lower inductance.

Michael

[o***@tubes.com]

Post by Michael Black
Paper capacitors are an old type of capacitor. ELementary Electronics
about 1971 showed how to make them yourself, some paper, some foil, some
liquid if I recall properly. I think even then I knew they were hideous,

I'd like to see that E.E. article. That would be sort of fun to try,
just for the heck of it.

E.E. does have PDF file reprints of all their issues on
http://www.americanradiohistory.com/Elementary_Electronics_Master_Page.htm

But finding the right issue might be tough...

Guess I'll begin by downloading all the issues from 71. That will take
most of the day on dialup....

[Foxs Mercantile]

Post by o***@tubes.com
1. The leads tend to corrode from dialectric corrosion, where they
contact the inner foil. (I have read that moisture can and does enter
these caps, even with the best wax coating).

No and yes

Post by o***@tubes.com
2. Voltage arcs burn across the paper layer, arcing between the two
layers of foil, and burn a hole into the paper, which allows the two
layers of foil to contact each other, and cause a short circuit.

Not until there is a fault.

Post by o***@tubes.com
3. The foil itself decays from age, and loses conductivity in parts or
all of it's roll.

Not so much the foil as the paper dielectric.

First and foremost, the paper was NOT acid free archival paper.
Mainly because everyone assumed the wax would seal the moisture
out. It doesn't because it is hygroscopic.
So what happens over time is moisture gets into the capacitor
paper, mixes with the acid content eats the metal foil and causes
conductive paths in the paper.
At first the leakage goes up, until some point when the capacitor
fails shorted.

[Terry S]

They break down. The metal migrates thru the dielectric layer -- think about a plating process -- and they become leaky as the dielectric layer becomes resistive. The wax is not a perfect seal, moisture enters and accelerates the process. The ends can become deformed because of stress on the leads, squashing the layers and decreasing the effective dielectric strength. Sometimes adjacent layers of the same potential will short decreasing the capacitance. The paper itself sometimes was slightly acidic, causing the leads to corrode and break off. I've pulled dozens out of radios where the end cap, wax and all, simply fell off. Further, the heat in some radios, from the tubes and transformers, tends to melt the wax, causing it to flow slowly. When the cap starts generating its' own heat, the melting process obviously accelerates.

You've beaten this topic to death -- why is it so hard for you to believe what the experience of generations of technicians have known and proven -- that paper caps suck. Be glad to be rid of them.

Terry

[o***@tubes.com]

Post by Terry S
You've beaten this topic to death -- why is it so hard for you to believe what
the experience of generations of technicians have known and proven -- that
paper caps suck. Be glad to be rid of them.
Terry

I BELIEVE
I BELIEVE
I SEE THE LIGHT
PRAISE THE LORD....
HALLEULIA

But believing only tells me to replace them, it did not tell me why.

I am a curious person, and I dont do anything just because someone says
I should. I want to know WHY!!!!

You did explain this pretty well. Thanks.
At least now I better understand the reason....

[analogdial]

1) Cellouse is marvelously hydrgroscopic. It sucks up humidity like a
sponge. Wet cellouse is conductive. There's no reason to overthink
this. Also, don't assume the paper the manufacturers used was stored
or wound in perfectly dry conditions. That's nearly impossible. I have
no doubt caps made under humid conditions failed more quickly than
caps made during dry times.

2) Wax is NOT a perfect seal against moisture. I'm sure plastics are
better but they aren't perfect, either. Ever see a tupperware container
sweating from the inside? That moisture diffused it's way through the
polyethelyne in maybe a few days or even hours.

Paper caps from the old days that HAD to last a long time were sealed in
soldered metal cans in oil. Too expensive for consumer gear! The
engineers who designed this stuff KNEW the paper caps would be failing
in a few years. So what? Electronics changes fast and the buyer would
almost certainly buy something newer before deteriorating caps killed
the device.

Using good caps in consumer gear would be a waste of money. Cheap caps
were considered good enough.

As far as what kind of wax was used, some sources say beeswax, some say
microcrystalline wax. I don't much care.

[g***@gmail.com]

Post by analogdial
1) Cellouse is marvelously hydrgroscopic. It sucks up humidity like a
sponge. Wet cellouse is conductive. There's no reason to overthink
this. Also, don't assume the paper the manufacturers used was stored
or wound in perfectly dry conditions. That's nearly impossible. I have
no doubt caps made under humid conditions failed more quickly than
caps made during dry times.
2) Wax is NOT a perfect seal against moisture. I'm sure plastics are
better but they aren't perfect, either. Ever see a tupperware container
sweating from the inside? That moisture diffused it's way through the
polyethelyne in maybe a few days or even hours.
Paper caps from the old days that HAD to last a long time were sealed in
soldered metal cans in oil. Too expensive for consumer gear! The
engineers who designed this stuff KNEW the paper caps would be failing
in a few years. So what? Electronics changes fast and the buyer would
almost certainly buy something newer before deteriorating caps killed
the device.
Using good caps in consumer gear would be a waste of money. Cheap caps
were considered good enough.
As far as what kind of wax was used, some sources say beeswax, some say
microcrystalline wax. I don't much care.

"Ever see a tupperware container
sweating from the inside? That moisture diffused it's way through the
polyethelyne in maybe a few days or even hours."

http://www.alphap.com/bottle-basics/plastics-comparison-chart.php

Check the "MTVR" rate for polyethylene in the chart. I believe the moisture you saw came from the food stored inside.

[analogdial]

Post by g***@gmail.com
"Ever see a tupperware container
sweating from the inside? That moisture diffused it's way through the
polyethelyne in maybe a few days or even hours."
http://www.alphap.com/bottle-basics/plastics-comparison-chart.php
Check the "MTVR" rate for polyethylene in the chart. I believe the moisture you saw came from the food stored inside.

No, it's something I've noticed on rare occasions in polyethelyne
containers which were kept outdoors and never contained food. It
happened on cool mornings after a hot humid spell. I atribute it to
moisture diffusing itself through the container with the assistance of
high temperatures boosted by sunshine.

I'll admit I could be wrong. I don't know if the air inside was very
humid at the start. This just anecdotal evidence, not a scientific
study. But there's similiar anecdotal evidence from hams who have used
polyethelyne containers to protect wavetraps in antennas and have
noticed the same sort of moisture buildup.

Anyway, the chart at the noted website does show that polyethylene does
allow a small bit of moisture to pass through. It's my intuition that a
thin film of wax will allow a larger amount of moisture through over the
years, enough to eventually destroy a paper capacitor.

My larger point is that I believe the moisture that destroys caps comes
directly through the wax. There's not reason to imagine sneak paths
along the leads or anything else.

[Michael A. Terrell]

Post by analogdial
1) Cellouse is marvelously hydrgroscopic. It sucks up humidity like a
sponge. Wet cellouse is conductive. There's no reason to overthink
this. Also, don't assume the paper the manufacturers used was stored
or wound in perfectly dry conditions. That's nearly impossible. I have
no doubt caps made under humid conditions failed more quickly than
caps made during dry times.
2) Wax is NOT a perfect seal against moisture. I'm sure plastics are
better but they aren't perfect, either. Ever see a tupperware container
sweating from the inside? That moisture diffused it's way through the
polyethelyne in maybe a few days or even hours.
Paper caps from the old days that HAD to last a long time were sealed
in soldered metal cans in oil. Too expensive for consumer gear! The
engineers who designed this stuff KNEW the paper caps would be failing
in a few years. So what? Electronics changes fast and the buyer
would almost certainly buy something newer before deteriorating caps
killed the device.

Those oil filled bathtub caps failed, as well. The paper broke down,
just like waxed caps, plus the rubber plugs would harden, crack and leak
oil.

[analogdial]

Post by analogdial
sponge. Wet cellouse is conductive. There's no reason to overthink
this. Also, don't assume the paper the manufacturers used was stored
or wound in perfectly dry conditions. That's nearly impossible. I have
no doubt caps made under humid conditions failed more quickly than
caps made during dry times.

Yes they do and we are seeing the failures now. But it's been a pretty
good run for a paper dielectric cap. Much better than the wax covered
caps.

[Foxs Mercantile]

Post by analogdial
Yes they do and we are seeing the failures now. But it's been
a pretty good run for a paper dielectric cap.
Much better than the wax covered caps.

Who are you kidding? Paper caps were already failing after ten
years of service.
The Sprague "bumble bees" and Black Beauty capacitors were
already known failures by the '60s.

[analogdial]

Post by Foxs Mercantile

Post by analogdial
Yes they do and we are seeing the failures now. But it's been
a pretty good run for a paper dielectric cap.
Much better than the wax covered caps.

Who are you kidding? Paper caps were already failing after ten
years of service.
The Sprague "bumble bees" and Black Beauty capacitors were
already known failures by the '60s.

Kidding? I'm saying the only paper caps which lasted more than a few
years were the ones soldered into steel cans. NOT BUMBLEBEES!!
Disagree?

[Foxs Mercantile]

Post by analogdial
I'm saying the only paper caps which lasted more than a few
years were the ones soldered into steel cans.

You did not make that clear. Quoting relevant bits helps.

[Clifford Heath]

What fails outside of paper capacitors is that anyone still gives a shit.

Post by o***@tubes.com
I've seen so many warnings about these old wax coated paper capacitors
in these newsgroups, and on the web, that every time I see one of them
now, I begin to shudder.
Immediately, I notify the FBI, telling them that there is a wax coated
paper capacitor in my radio, and I know that this thing is deadly. I
carefully explain to them that I have been exposed to it, and instantly
developed capacitor cancer. I also explain that I have heard of entire
cities being reduced to rubble, killing everyone within 100 miles, when
these caps explode. Then I ask them to come remove the capacitors for my
and everyone elses safety.
Yes, I was just joking in this previous paragraph.......
Seriously, I have not been able to find any answers to what actually
happens internally to these caps, which causes them to fail. I googled
for an answer, specifically used the word PAPER CAPACITOR, but all I get
are results for failing electrolytic caps, which I do now clearly
understand what occurs with them.
So, lets say I have an old radio from the mid 1950s. It contains 9 paper
capacitors (plus a few electrolytic filter caps). I am repeatedly told
that I must replace all of these wax coated paper caps, as well as the
dried up electrolytics.
Ok, I know the electrolyte has dried up in these electrolytics and they
are no longer filtering the rectified DC voltage in the power supply,
which I can quickly identify by the 60 cycle hum coming from the
speaker.
Then, I am told I must replace all these smaller wax covered paper caps,
which may or may not be working at the moment. Great, I can take this
advice and I can change them, but it seems that no one (at least not on
the web), can seem to explain what occurs inside of their waxy bodies
that make them fail.
I know they are quite simple. I roll of paper, and two layers of tin or
aluminum foil, with leads attached to the two layers of foil inside.They
are rolled up, with their leads exiting their bodies. Then they are
given an outer shell of paper with their identity printed on it, and are
then coated with wax. I'm sure that if I had a lot of time to waste, I
could even make my own paper caps, but for the low cost they sell for,
that is not necessary, other than for an experiment.
Knowing how they are built, I can only fathom three possible reasons
that they fail.
1. The leads tend to corrode from dialectric corrosion, where they
contact the inner foil. (I have read that moisture can and does enter
these caps, even with the best wax coating).
2. Voltage arcs burn across the paper layer, arcing between the two
layers of foil, and burn a hole into the paper, which allows the two
layers of foil to contact each other, and cause a short circuit.
3. The foil itself decays from age, and loses conductivity in parts or
all of it's roll.
* This does not take into account physical damage, such as crushing or
poking a hole in a cap body.
What else can go wrong on such a simple device?
*NOTE: I mentioned the wax coated paper caps, but am aware the old
plastic coated ones such as the ones called "Bumble bees" (with colored
stripes) are just as bad.
Honestly, I find it hard to comprehend how moisture can get past that
wax, as long as the wax remains intact, but I'll just take the advice
from the experts on that note.
One final question: Does anyone know what kind of wax was used on them?
Was it bees wax, paraffin wax, or something other?