Old capacitors present several types of problems for vintage radio restorers:
- their identification - nature and characteristic values
- their replacement with current components
- their eventual reforming (electrolytic capacitors)
Replacing old capacitors
The capacitors encountered in old radios can be classified into six categories:
- power supply filtering capacitors
- electrolytic decoupling capacitors
- non-polarized decoupling capacitors
- coupling capacitors
- fixed tuning capacitors
- variable tuning capacitors
- adjustable capacitors
electrolytic capacitors
fig.1 HT Power Supply of a TRF radio 
fig.2 First capacitor 
fig.3 two axial capacitorsand 1 radial
power supply filtering capacitors
Its value often takes the values 8, 16, 32, 50 µF. It withstands rated voltages from 350 to 450V. It is single or double; It is polarized. It is packaged in the form of an aluminum tube. It is screw-in or axial. Before starting the radio, when possible, it is reformed or simply changed with values approximately within 20%.Some capacitors show 2 voltages; for example 450/500V. The first corresponds to the maximum in steady state. The second is supported for a few seconds. Replacement in this case is done with a component supporting 400V rated voltage.
| Old value (µF) | New value (µF) |
|---|---|
| 8 | 10 |
| 16 | 15 |
| 20 | 22 |
| 32 | 33 |
| 50 | 47 |
Several replacement techniques
1 – the suspect or defective axial capacitors are unsoldered and replaced with new ones. The unsoldered capacitors are placed in a plastic bag in which a description of the intervention is slipped, not forgetting the year and the author. For aluminum capacitors that screw onto the chassis, the connections of the old one are unsoldered and a new axial one is placed under the chassis. The disadvantage of this method is that the interior appearance of the radio changes; the advantage is the speed of the intervention2 – the new capacitors are dressed with the sleeves of the old ones (or facsimiles) for the axials. Not always easy to do. For aluminum capacitors that screw onto the chassis, they are disassembled, opened, and their contents emptied. A radial component is placed inside. It is closed without leaving any traces. It is screwed back in and resoldered. Advantage: no change in appearance.
3 - the capacitor is reformed. Electrolytic capacitors consist of 2 metal armatures between which a liquid electrolyte bathes. Upon initial startup, an insulating layer "forms" on one of the plates. This extremely thin layer easily gives the capacitor a capacitance in multiples of 10µF. This type of capacitor is therefore polarized. After a long period of non-operation, the oxide layer deteriorates. To reconstitute it, it must be "reformed". That is, let a direct current pass until the insulating layer is reconstituted. The repolarization current will be limited to less than 10 mA to avoid heating that could damage the component. The depolarized capacitor behaves like a resistor varying from a few Ω at the beginning of the operation to a few MΩ at the end of the operation. If after several hours the current in the capacitor does not decrease significantly, discard it.
Electrolytic decoupling capacitors
fig.4 electrolytic decouplings 
fig.5 paper capacitor 
Sheet of labels to download
Non-electrolytic coupling and decoupling capacitors
Coupling capacitors ensure the passage of the HF or IF radio signal between two stages. Decoupling capacitors are placed in parallel on a resistor so that the dipole thus formed offers a quasi-zero resistance (called impedance) for HF or IF signals. New axial capacitors can be easily dressed with the sleeves of the old ones (or facsimiles). The PDF sheet opposite can be downloaded. After cutting, simply wrap and glue the label with the closest value.
Tuning capacitors
fig.6 750pf micas
fig.7 old VC
fig.8 trimmer
fixed tuning capacitors
Coupled with the coils, they participate in the tuning blocks. Often with a value around 200pF with an accuracy of 2 to 5%, they are of "mica" technology; very solid, they rarely need to be changed. Sometimes placed in the anode circuits, they must withstand up to 400V.
To replace them, modern substitutes can be found, except when the values are of the 177pF type. This is very rare except on measuring devices. Some models are marked in centimeters, which is a CGS unit very close to the pF.
variable tuning capacitors
Simple on reaction radios, double and coupled on superheterodyne type radios, they are solid. They rarely need to be changed. At most, a drop of oil to restore their tuning flexibility. If they need to be changed, this type of component is almost no longer manufactured. They must be obtained from swap meets. In addition, the double model is very often asymmetrical. One cell is dedicated to selecting the transmitting station (50 – 500pF), the other to varying the local oscillator (30 – 350 pF). The 500/350 ratio is given here as an example. It can be different, which complicates the search for a replacement.
Adjustable capacitors or trimmers
They are placed in series and parallel on the variable tuning capacitor described above. They are rarely damaged.
Sources and references
[1] R. Besson, "Technologie des composants électroniques, Tome 1, Edition Radio, Paris, 1977.
