Section 2 - Electrolytic Capacitors

Background

Radio circuits require DC current in order to operate. Any radio which is powered by AC must first convert the AC current into DC current. without going into excessive detail about how AC/DC conveters operate, a major component of the circuit are one or more large capacitors which help to smooth the AC into a steady state DC.

The above schematic shows the location of the electrolytic capacitors in a Philco Model 60. If the electroylytics capacitors are not working properly, you will hear a very large amount of hum coming from the speaker (and possibly nothing else!). If the capacitor fails and shorts out, you could permanently damage the power transformers or other circuit components. In the worst case scenario, you could end up with a fire.

Due to the high failure rate and potentially disasterous consequences of a failed electrolytic capacitor, these capacitors shoudl be rebuild prior to plugging in any vintage radio.

Construction

Below is a typical vintage electrolytic capacitor mounted on a chassis.

Vintage electrolytics were made from metal tubes which contained a liquid electrolyte. These almost always dry out over time. The cans are mounted on the chassis using a friction fit holder. In general, one or more screws can be loosened to allow removal of the capacitor, as seen below.

The positive terminal extends from the bottom of the can. The negative terminal is the can itself. Since you can't easily solder to aluminum, the positive terminal has a crimped on solder connector and the negative terminal is often connected using a small piece of metal inserted between the mounting bracket and can.

Restoration

Step 1 - Cut open the can

I prefer to rebuild the capacitors using the "restuffing" method. I start by cutting the capacitor about an inch from the button. I have found the best tool to be a hacksaw. The cans aren't rigid enough for a pipe cutter, and can generally be cut by hand with little effort. This model of capacitor used a liquid electrolyte, which has completely evaporated. Slightly later models had a semi-solid electrolyte. In either case, the first step is cutting the can and removing the old innards.

Step 2 - Create new solder points

The next step involves creating new solder points for the positive and negative terminals. I accomplish this by drilling a small hole in the bottom of the can, and by removing the original positive electrode connection. I then attach short segments of heavy guage copper wire using copious hot glue. This creates a mounting point for the new capacitor.

Ideally, I would solder the negative terminal of the new capacitor to the can. However, the aluminum prevents me from doing this. My solution is to create a second mounting point as shown below.

In most cases, the can is insulated from the chassis with a ring of cardboard, and a negative terminal soldering point is tucked between the cardboard and can. In this case, I simply move the wires from the old negative terminal to the new one. If the can instead was directly attached to the chassis, I simply run a wire from my new negative terminal to a point on the chassis.

Step 3 - Add the replacement capacitors

The modern replacement capacitors are much smaller and fit easily inside the old can.

Step 4 - Re-Seal the Can

I accomplish this with a strip of metallic HVAC tape. Once the capacitor is placed back into the holder on the chassis, the repair is invisible. It also allows easy replacement of the capacitor in the future while requiring little or no change in wiring under the chassis.

The rebuilt capacitor is now ready for mounting in the chassis. The original negative terminal soldering points can be seem protruding from the cardboard ring.