Restuffing Dubilier Cub Capacitors

My antique radio restoration logs

There is a particular type of tubular paper-wax capacitor used in antique radios that was made by Cornell (perhaps later to become Cornell-Dubilier?) and branded "Cub".  They are quite unique in construction and cannot be re-stuffed with new capacitors in the usual manner.  There are two types that I have encountered.  The internal construction is the same for both types and is as follows:

• The waxed paper and foil sandwich is wound onto a 1/4" diameter wooden dowel, with alternate layers of foil extended to each end of the dowel.
• Metal end caps are crimped onto the ends of the dowel over the foils.  The caps have the lead wires soldered to the inside of the cap.  This establishes a connection between the lead wires and the foils.

The finished capacitor is then constructed in one of three ways:

• The capacitor is dipped in some sort of wax to seal out moisture and a paper label is attached covering the wax. 
• The capacitor is enclosed in a cardboard tube with the ends crimped tight around the metal end caps.  The exposed metal end caps are then covered by smaller cardboard tube covers.
• The metal end caps are totally enclosed inside the crimped ends of a cardboard tube and sealed with a small amount of wax.  The cardboard tube covers for the metal end caps are essentially inside the outer cardboard tube in this case.

First Type (wax enclosed)

• Select an appropriate axial tubular film capacitor to replace the original.  I generally use 630 volt capacitors regardless of the original capacitor's voltage rating.  As is usual in antique radio repair, +/-20% is close enough unless the capacitor is used in critical circuits (such as tone controls).
• In order to rebuild the capacitor, the paper label first has to be removed intact.  In some cases I was able to apply heat from a heat gun and the glue released.  In some other cases, the glue would not let go.  In this case I chipped away as much wax as possible, applied heat from a heat gun, and was able to slide the label off of one end of the capacitor intact.  
• Remove the two metal end caps from the wooden dowel by twisting and pulling with pliers.
• Remove any wood that remains inside the end caps (I use an Exacto knife with a #11 blade).
• Measure the lead length that projects from the end caps, as these  leads will be replaced with new wire which needs to be at least as long as the original leads (for ease of reinstallation in the radio).  If the leads of the replacement film capacitor are not long enough, then a new lead must be spliced onto the capacitor, close to its body (so that the splice will not be visible - hidden by the end cap).
• Remove the old lead wires from the end caps by heating with a soldering iron and shaking out the molten solder.  Once the leads are removed, a small hole will remain in the end of the cap (make sure this hole is clear of solder).  
• If the paper label is still glued together, find a paper or plastic tube or an old paper wax capacitor that will just fit inside the existing label (any form of tubing of the correct diameter will work).  If using an old paper-wax capacitor then first clean out the old contents.  The empty capacitor case or tube is then restuffed using a new 630 volt axial film capacitor wrapped in strips of paper towel to hold it in place inside the tube.  
• Seal the ends of the tube with melted rosin (discarded RCA Radiola Superheterodyne catacomb potting compound) or hot melt glue (which can be tinted with tan flat hobby paint).  I support the capacitor with the open ends of the tube upward, melt the rosin in a metal pet food container over an alcohol burner (empty hobby paint jar with a hole in the lid and a cotton rope "wick"), then simply pour the liquid rosin into the ends of the tube.
• The original end caps are then soldered to the projecting lead wires by holding them in a vise, filling them with solder, and then inserting the capacitor lead wire (and positioning the capacitor before the solder hardens!)  The two metal end caps should be installed so that the spacing between them is the same as the original capacitor.
• I normally do not apply wax (rosin in my case) to the metal end caps as in the originals, but sometimes do. Even without the wax, the result is still very close to to the originals in appearance.

The capacitor on the bottom is an original Cub capacitor.  The capacitor on top is a reproduction using a new film capacitor, while retaining the original label and end caps.

Second Type (Cardboard Tube)

• Select an appropriate tubular film capacitor to replace the original.  I generally use 630 volt capacitors regardless of the original capacitor's voltage rating.  As is usual in antique radio repair, +/-20% is close enough unless the capacitor is used in critical circuits (such as tone controls).
• Measure the original projecting lead length on both ends of the original capacitor.
• Melt the wax around the end cap covers (between the end cap cover and the outside case) using a soldering iron.
• Pull the insides out one end of the capacitor (this will usually remove the crimp from the end of the case).
• Remove the cardboard end cap covers from both ends (these cover the metal cap underneath)
• Remove the metal end caps with the original leads (crimped to wooden dowel, foils wrapped around dowel) by twisting and pulling with pliers.
• Remove any wood that remains inside the end caps (I use an Exacto knife with a #11 blade).  I sometimes have to drill out the wood first if a lot is present.
• Unsolder and remove the original leads from inside the metal end caps
• Solder a piece of #20 buss wire to each end of the replacement film capacitor, close to its case (the length of the buss wire should be equal to the original leads in length, or longer)
• Solder one end cap to the #20 buss wire - inside the cap (careful not to damage the capacitor body!)
• Slide the outer cardboard end cap cover over the metal end cap on one end
• Tin the inside of the other metal end cap (the hole must be clear and slide over buss wire)
• Insert the other end cap on other lead (melt solder, insert wire, make sure it can still MOVE, for now)
• Insert the replacement capacitor inside the original outer case (uncrimp the cardboard on the other end as needed for clearance)
• Install the other cardboard cover on the metal end cap
• Recrimp the outer case cardboard so it is tight around cardboard inner end cap cover.  Push it back in place using a screwdriver or similar tool - make it look like the other end if possible.
• Position the metal end cap and cardboard cover inside the outer cover, appropriate distance as original
• Solder the metal end cap to buss wire lead (apply heat on the outside)
• Seal the ends with melted rosin (around the cardboard end cap covers where it contacts the outer case).

Here is a completed capacitor.  It looks similar to the first (wax coated) type, but this type has the markings printed on the cardboard tube cover rather than having a paper label.  Also, the metal end caps are insulated vs. the first type where only a wax coating is applied.

Third Type (Cardboard Tube, No End Caps)

• Select an appropriate tubular film capacitor to replace the original.  I generally use 630 volt capacitors regardless of the original capacitor's voltage rating.  As is usual in antique radio repair, +/-20% is close enough unless the capacitor is used in critical circuits (such as tone controls).
• Measure the original projecting lead length on both ends of the original capacitor.
• Using a new, sharp single edge razor blade, make a cut about 1/16" from one end of the capacitor and about 1/32" deep!  If done correctly, the entire contents of the capacitor can be removed by pulling on the component lead on the cut end.  The crimped end and insulating cardboard sleeve will also come off.  The cut must be deep enough to allow removal of the contents, crimped end, and insulating sleeve as a unit but not deep enough to slice through the insulating sleeve.
• Remove the insulating sleeve and end crimp as a unit and save them.
• Remove the metal end caps with the original leads (crimped to wooden dowel, foils wrapped around dowel) by twisting and pulling with pliers.
• Remove any wood that remains inside the end caps (I use an Exacto knife with a #11 blade).  I sometimes have to drill out the wood first if a lot is present.
• Unsolder and remove the original leads from inside the metal end caps
• Solder a piece of #20 buss wire to each end of the replacement film capacitor, close to its case (the length of the buss wire should be equal to the original leads in length, or longer)
• Solder one end cap to the #20 buss wire - inside the cap (careful not to damage the capacitor body!)
• Insert the replacement capacitor with the attached end cap inside the original outer case until the end of the metal end cap is flush with the end of the capacitor body.
• Slide the other metal end cap over the opposite end of the replacement capacitor's lead (I do NOT solder the metal end cap to the component lead).
• Slide the insulating sleeve and end crimp over the metal end cap
• Insert the insulating sleeve inside the capacitor body until the end crimp is flush to the cardboard tube.  I use melted rosin to seal the parts together, then wipe off any excess.

Here is an example of a completed capacitor.  Note the cut just visible on the right end.  Neither crimped end of the tube is disturbed using this method.