Back      

Repairing Philco wet electrolytic filter capacitors

Granddad's radio needed a cup of water to make it work.
 
In the 1930s and '40s some radios had filter capacitors in the power supply whose electrolyte was mostly distilled water. Known as "wet electrolytic" capacitors, many people probably didn't realize that water was a component of the radio set.
 
Here is a 1939 Philco "wet" capacitor. Freed from the chassis after 80 years, most of the water in it is gone.
Can we fill it back up? No* but what follows is a suggested way to repair it.
 
* Theoretically, you could drill a small hole in the end-cap and use a syringe to refill the can. I wasn't going to attempt it, or use the radio
   with a compromised filter capacitor.

 
 
ADVISORY NOTICE: Please be aware there may be some pressure inside the capacitor. The patent stamped on the end cap (which is actually the patent for the end cap, not the capacitor) describes how it is shaped a certain way so the pressure doesn't blow it off. Most of this pressure is generated by heat radiating from the vacuum tubes while the radio is on.

Patent No. 1,969,630 can be seen here.
 
If the capacitor has a crimp-on connector, just cut it off. You can try to remove it, but I couldn't get mine off the wire without chewing it up. I didn't put much effort into it, I admit.

So to start, draw a line around the outside of the capacitor about 3/4 inch from the bottom end. Gently work around it with a hack saw till you cut through the can. Don't cut right through it as if it was a piece of pipe. Have paper towels under the work.
 
Once you cut through in one spot, any liquid still inside will come out. Have extra paper towels standing by in case you have a full one. Ironically, a full one probably works perfectly but there is no way to determine how full it is without cutting it open. That's OK, the odds of finding a full one are miniscule.

Notice on the left photo there is a pencil mark perpendicular to the cut. This is to help align the parts when they are put back together.

The electrolyte is  a combination of distilled water, boric acid and ammonium borate. You can get it on you, but don't drink it. If you have an urge to drink it, seek immediate psychiatric care.

The electrolyte is very pure. The slightest trace of contaminants would have affected the properties of the capacitor. It has a distinctive odor, which for some reason I keep smelling.

 
This is what was inside. The cylindrical screen is plastic. It insulates the center electrode from the aluminum can. As a matter of fact, nothing touches the can but the water, yet the can is the negative side of the capacitor. (Not all Philco capacitors look like this inside.)
 
 
Grab the center electrode with pliers and pull it out of the rubber stopper while twisting it back and forth.
 
Sand the inside of the bottom piece down to bare metal, drill a hole, and attach a solder lug.
Make sure it is tight; 200 - 300 volts will be going through it. If you don't have a solder lug, use a piece of wire.
 
Take your replacement capacitor and solder a piece of heavy wire to the POSITIVE side. This is 14 gauge from some house wire.
 
Insert the heavy wire into the rubber stopper till the new capacitor is seated. Mark the wire a bit above where it exits, remove it, then apply heat-shrink tubing up to the mark. Reinsert the capacitor and solder the negative end to the solder lug. You now have a working replacement Philco capacitor.

 
Next we need to put the two halves of the aluminum can together. You will need a thin cardboard tube. In this case, the diameter of the tube was too large, so it was cut down the back and overlapped. A cutout is needed to clear the solder lug.
 
Next, glue the cardboard tube inside the bottom part of the housing. I used J-B KwickWeld. Use whatever you want that works.
 
When the glue has set on the bottom half, glue the top half on.
Black epoxy made a black seam. Next time, I'll use clear.
If this needs to opened again, it can be cut with a utility knife along the seam.

That thing looks a medical device in a sci-fi movie.

If you are able, test the capacitor before it goes back into service. Bend the anode so the wires will be soldered in the exact same place as before.
Mark the can with the new value and the date. Don't use a felt tip marker because the ink will fade over the years.
 

Note 1: This particular capacitor was rated at 12 mfd. They don't make 12 mfd capacitors anymore, so I used a 22. I could have used a
            10 mfd and a 4 mfd in parallel to make a 14.  Also, two 22 mfd in series would have made an 11 mfd. I was too lazy.

Note 2: Obviously you need to observe the voltage. The original was rated at 300 volts. I used a 450 volt replacement.

 

 
Cutting the capacitor in half is OK if you are going to repair or refurbish a radio. But what if you want to RESTORE the radio? Steve Geary wrote in with this method. Hold the capacitor in a lathe (or the chuck of a drill if you don't have access to a lathe.) While it is spinning, cut it at the crease to the end cap by holding an X-ACTO knife against it.

He uses aluminum solder and a butane torch to add a small pool of solder to the inside of the can. The negative side of the replacement capacitor can then be soldered with regular solder to the aluminum solder. When the top is glued back on, it's not apparent that it was ever opened.

Steve knows a little about radios. He built a "barn" just to house them all.
 

 
Where do you get capacitors now that Radio Shack is closed? You can usually find them along the side of the road after it rains.
 
A Philco capacitor water bottle!
Somebody make this so I can get one. Since it's my idea, I expect a free one.
 

 
  BACK