Old capacitors pose several types of problems for restorers of vintage radio sets (TSF):

  • their identification - nature and characteristic values
  • their replacement with current components
  • their possible reforming (chemical capacitors)

Principles

The capacitor is a dipole composed of 2 metal plates separated by an electrical insulator. This insulator blocks  direct current and allows alternating currents to pass. This behavior makes it possible to transmit a variable signal between 2 stages  of a tube device (fig.2). This property and the ability of a capacitor to store electrical charges allow  the filtering of the 50Hz (or 100Hz) component of the rectified high voltage power supply of TSFs (fig.1).
Capacitors do not withstand the test of time well. It is therefore common to replace them to make an old radio work correctly. Two steps in this operation:

  • Read the capacitance value; marking codes have evolved over time; then find the nearest standardized value.
  • Determine the component technology (chemical, mica, paper) and find its current equivalent.

view of a filtering capacitor fig.1 filtering capacitor
view of a coupling capacitor fig.2 Coupling and decoupling

Capacitance Units Expressed in Centimeters

1 centimeter is worth 1.113 picofarad capacitor principle and calculation form
structure of a
capacitor

Today, the unit of capacitance for a "paper" and mica capacitor is the Farad (F). This unit comes from the SI system ( International System). The farad, a poorly adapted unit, is expressed using its sub-multiples. The table opposite describes the most common sub-multiples in TSF:

               
Unit Value F Use
The picofarad (pF) 1F / 1 000 000 000 000 tuning, decoupling and HF (*) coupling
The nanofarad (nF) 1F    / 1 000 000 000 decoupling and HF coupling
The microfarad (µF) 1F    / 1 000 000 LF (**) decoupling and HT (***) power supply filtering
* HF: high frequencies ** LF: low frequencies *** HT: high voltages

In the SI system, the unit of length is the meter and the electrical permittivity of vacuum ε is 8.854187 x 10-12 si. Until the 1950s, capacitor values could be marked in centimeters (cm). This unit from the CGS system (Centimeter, Gram, Second) is close to the current picofarad (pF). In the CGS system, the unit of length is the centimeter and the electrical permittivity of vacuum ε is 0.0795775. By applying the capacitor calculation formula opposite, a capacitor with a capacitance of 1 cm (10-2m) therefore has an S/e ratio of 12.566371. This same capacitor in the SI system will have a capacitance of: 8.854187 x 12.566371 x 0.01 which is 1.113 pF In TSF, apart from precise HF tuning capacitors with a 2% tolerance, the manufacturing tolerances of industrial capacitors used in radio sets of that era were around 20%. One can, without too much risk, equate cm with pF.

Capacitances: Old Markings

 
Old Marking Unit Symbol Value
mF microfarad µF 10-6 F
mmF or µµF picofarad pF 10-12F
1/1000th nanofarad 1/1000 10-9F
T (Tausend pF) nanofarad T(*) 10-9F
Other difficulties may arise during the identification of capacitors. Today, the units are sub-multiples of the Farad (F). Opposite is a correspondence table with the old markings.
(*) Germany

Some Old Models

capacitor image
Here a capacitor marked .02 mF which is 0.02 µF current or 20 nF or 20000 pF
capacitor image 2
Here a 10,000 µµF capacitor which is 10,000 pF or 10 nF
capacitor image 3
On radios from the 1920s, this type of capacitor is often found placed in parallel on headphones. they are expressed in mF (µF today). The component in the figure on the left is worth 3/1000 of µF therefore 3nF. The thousandth therefore corresponds to the current nF.


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