88 Where can I get look-alike wax paper caps?
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
88 Where can I get look-alike wax paper caps?
I have an early 1930's radio and want to replace the wax paper
capacitors, but want to keep the chassis looking original. Where can I
get look-alike wax paper caps?
The manufacturers who made these discontinued them years ago. You
will probably find repairs from the 1946-70 era in old radios using
paper capacitors molded in plastic, and even these are difficult to find
nowadays. While recent axial lead caps with mylar and other plastic
dielectrics work well, as long as the voltage rating is adequate.
Generally, 400 volt caps will work, but 600 or 630 volt caps are safe
in any set with an 80 or 5Z3 rectifier. However, they don't look
anything like the old axial lead capacitors.
It is possible to melt the wax out of old capacitors, salvage the
cardboard sleeves, and install new axial lead caps inside. While
axial-lead caps are somewhat hard to find, and tend to be expensive,
they are still made, and are usually small enough to fit in the old
cardboard sleeve.
Dan Schoo, who does this type of restoration regularly, kindly wrote up
a procedure for salvaging the old sleeves and putting new caps inside,
and made it available for inclusion in the faq. Here it is:
Rebuilding Wax Filled Paper Capacitors
by Daniel Schoo
From: schoo@fnal.gov (Dans Cockatoo Ranch)
A paper capacitor is a type of capacitor that was used
extensively in radios from the thirties through the fifties. They
are made of wax impregnated kraft paper and two thin metal foils
cut into long narrow strips. The foils were placed one on each
side of the kraft paper and rolled up along the long dimension
into a rod shaped assembly. The foils were skewed such that they
extended a little past the paper at the ends of the rod, one on
each end. This provided an electrical connection point to each
foil over it's entire length. The voltage rating of the capacitor
was controlled by the thickness of the paper. Thicker paper could
hold off a higher voltage. The capacity was controlled by the
surface area of the foils. Longer wider foil wraps would have
higher capacity. This is why higher voltage and/or higher
capacitance values would require a larger size for the capacitor.
The lead wires were attached to the foils extending out from the
ends of the rod. The entire assembly was then slipped into a
cardboard sleeve and the sleeve was filled with wax. Later types
were molded into plastic shells and had paper labels attached or
were printed with colored bands or text to indicate the values.
The black band around one end of the sleeve and the words
'outside foil' indicate the lead that is attached to the foil
strip wound on the outside of the kraft paper. This is important
in some applications and tells the assembler which lead to use
during construction. Paper capacitors were used for higher
voltages at medium to small capacities. The voltage ranges are
usually from 100 to 600 volts and from .0001 to 1 microfarad in
capacity. They typically fail by becoming leaky and allowing DC
current to pass.
The purpose of rebuilding an old type paper capacitor is for
appearance only. When restoring an old radio to operable
condition, some owners desire to keep the appearance of the
components under the chassis as close to original as possible.
When certain components fail such as capacitors, it is not
possible or even desirable to replace them with original types.
To keep the original appearance, the old component is taken apart
and a new one is hidden inside the old shell.
After you have determined that a capacitor is bad, or if you just
want to replace one because you have a basic dislike for them,
remove it from the radio. Begin by melting out the wax potting.
Wear eye protection and use a heat gun, blow drier or small torch
with a hot air attachment like a Master Ultratorch. Do not use a
torch or other open flame on the capacitor as this will apply too
much heat in a small area and probably cause it to burn. Not much
heat is required to melt the wax but it has to be steady and even
to heat up the entire body of the capacitor. Hold the capacitor
sleeve with a long nose pliers and heat it slowly until all the
wax has dripped out. Discard the old wax. Some paper capacitors
have cardboard end disks. For these, the ends of the sleeve are
rolled inward to retain the disks. Unroll the end crimps and
smooth them out. Remove the end disks with a small screwdriver.
After the end disks are out continue to heat the capacitor until
the rest of the wax is out. After most of the wax has run out,
hold the capacitor with an insulated pad, grab a lead with a
pliers and pull the insides out. If the wires come off, push the
insides out with a small screwdriver. If the insides are stuck in
the cardboard sleeve, it may be necessary to drill them out. Pull
out the wire leads and drill a small pilot hole down through the
center of the capacitor. Drill another larger hole about half the
diameter of the sleeve. You should be able to dig out the rest of
the insides with a small screwdriver. Be careful not to puncture
the sleeve.
Once the sleeve is cleaned out you can install the new modern
capacitor. The most difficult part of this is to find a suitable
capacitor that will fit into the paper shell. You can substitute
a new capacitor with an equal or higher voltage rating than the
old one but try to get as close as possible to the original
capacity. Fortunately many of the modern capacitors are much more
compact than the old paper ones. Modern capacitors use plastic
films like polycarbonate, polypropylene, polystyrene, and
polyester which is also known as Mylar. The most common is Mylar
and is suitable for many replacement applications. These do not
degrade with time like paper does and should give years of good
service. The popular Sprague "Orange Drop" is a Mylar capacitor.
These are not suitable for use in this application because they
are designed for printed circuit mounting and the leads are
radial. This means that they extend out the side of the capacitor
at a right angle. You must use an axial lead capacitor with the
leads extending out in line with the capacitor body.
Slide the new capacitor inside the old sleeve and center it. If
there is a lot of space around the new capacitor such that it is
loose you can wrap a few turns of plastic tape around it to build
it up. Slit the tape down to about a quarter of an inch wide and
wrap it in a band around the middle of the capacitor until it
fits snugly in the sleeve. After the new capacitor is centered in
the sleeve you can fill the ends with wax. You can get beeswax at
any well stocked hardware store. It comes in small tan blocks
about three inches square and one inch thick. Cut off a small
chunk and place it in a small metal can. Prepare the can by
bending a pour spout into the top edge and make sure it is clean
and dry. Wear proper eye and skin protection when heating the wax
just in case it spatters. Heat up the wax slowly with a heat gun,
a hair dryer or small torch. Remove the heat when all of the wax
has melted and be careful not to overheat it. If it begins to
smoke remove the heat immediately. Support the capacitor in a
vice or tape it to the edge of a table top. When the wax has
melted thoroughly, pour it slowly into the end of the sleeve just
up to the edge. If the capacitor had end caps leave enough room
to reinstall them. When the wax has cooled sufficiently, flip
over the capacitor and fill the other end. Allow the capacitor to
cool completely before installing it in the radio.
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