Should I replace paper capacitors during my radio restoration project?

History

Paper capacitors, sometimes known as paper/wax capacitors, were widely used in electronic equipment in from 1930 until the late 1950's due the fact that could be manufactured easily and at a low cost.

A paper capacitor consists of two long pieces of tin foil sandwiched between wax paper tightly rolled up so that the tin foil was exposed at the end of the roll. Wires were soldered to tin foil exposed at the end of the roll. The assembly was stuffed in a cardboard tube and sealed and protected with a coat of wax.

Many paper capacitors have a black band closest to one of the wire leads. This is to indicate the foil closest to the outside of the foil/wax paper roll. Paper capacitors are not polarized and can be put into the circuit in either direction, although, radio manufactures of the day did specify which connection the lead closest the black band should be connected, which was typically closet to ground potential. For some reason the prevailing consensus of the day was that paper capacitors wired this way would last longer if wired in this fashion.

Some paper capacitors, called "molded" capacitors, are housed in plastic case. These are just paper capacitors with just a prettier facade.

Paper capacitors range in capacitance from 0.001 uF and 1.0 uF and can have a working Voltage up to 500 Volts, depending on their construction.

Should they be replaced?

The consensus  of hobbyists that restore vintage radios and Amateur Radio enthusiasts that restore "Boat Anchor" or vintage receiver/transmitter equipment is a resounding "YES".

They site the following reason to replace paper capacitors:

- Old paper capacitors are inherently unreliable.

- Paper capacitor's capacitance change with age.

- As paper capacitors age, their Effective Series Resistance (ESR) increases and their leakage resistance decreases. These are not good trends for these parasitic resistances in a capacitor.

- Corrosion at the lead ends will cause the ESR to increase to a point that it effectively takes the internal capacitor out of the circuit.

- Aged paper capacitors with their yellowing cardboard body and dripping wax, due to heating by vacuum tubes, become unsightly.

- Paper capacitors deteriorate with age due to acid in the paper.  

What is a good paper capacitor replacement?

The general consensus of antique radio and Amateur Radio enthusiasts is to use plastic/polyester film capacitors due to their small size, low cost, and stability.

I use metalized film polypropylene capacitors to replace paper ones in my restore projects as they are more stable at higher frequencies than polyester capacitors but cost about the same.

When replacing paper capacitors:

1. Power off the device you are restoring, just saying!
2. Make sure the working voltage of the replacement capacitor is equal or greater than the original.
3. For most vintage radio restore projects, capacitance values are not critical.  You could probably get by with a replacement .047uF capacitor to replace an original one valued at .05uF. Remember, the original capacitor had a tolerance of 20% meaning that its value could have varied between .04uF and .06uF at the time of manufacture!
4. New plastic/polyester metalized film capacitors do not have a marking to indicate the lead attached to the outside foil. They can be installed in either direction.
5. New plastic/polyester metalized film capacitors are not polarized, you do not have to worry about attaching one lead to the circuit with lower potential voltage.

Below is a picture of my 1947 TRAV-LER Model 2050 AM Radio with its original paper capacitors, circled in red.

Here is a picture of the my 1947 TRAV-LER Model 2050 AM Radio with newly installed metalized polypropylene capacitors, circled in red. I believe replacement of the paper capacitors really improved the appearance under the chassis. I know, who looks under the chassis? I would know these ugly paper capacitors were there and it would bug me!