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How To Series - Capacitors

PAPER CAPACITORS

Until about 1960, these were the most common with a range of 1.0 mfd or more down to .001 mfd and consist of two strips of foil separated by layers of thin paper, impregnated with wax or oil. Instead of foil, some had an extremely thin layer of metal deposited on the paper and were known as metallised capacitors.
This construction saved space and rendered the capacitor self-healing in the event of a breakdown, with the metal deposit vapourising around the trouble spot. One synthetic impregnating material used in paper capacitors, PCB, has excellent dielectric characteristics but has been outlawed because it is practically indestructible and can enter the food chain. Be suspicious of metal cased capacitors that leak a clear viscous impregnant. A good paper capacitor will have a resistance measurement of several hundred megohms, but as they were extremely difficult to seal against eventual moisture absorption, many are likely to now measure much less, especially those with cardboard cases. Metal and plastic cased components were used with varying degrees of success. Generally the metal cased type will be found to have survived reasonably well, but there have been some criticisms of plastic cases. In spite of their apparently excellent encapsulation, there are many reports of their failure. Others to watch for are those with thin grey coloured moulded cases, which can develop hairline cracks, which will let in moisture. These were popular at one time as replacements, and if you find any with cracks, don't even bother to test them.

Paper capacitors used for protracted periods on high voltage develop a "memory" and when disconnected can generate appreciable voltages upsetting attempts to measure their insulation resistance. This produces a different reading when the meter leads are reversed. They were used mainly for R.F. bypassing in HT lines, screen and R.F. and I.F. cathode bypassing, and for providing the time constant and bypassing of automatic gain control lines. The amount of leakage that is acceptable depends on the location. A common application was for bypassing cathode bias resistors of only a few hundred ohms. Clearly in this instance, a capacitor with leakage resistance of a megohm is not much of a problem, but the same capacitor in an automatic gain control line could degrade the A.G.C. performance quite seriously.

Another critical location for paper capacitors is the coupling capacitor between audio stages, commonly used in domestic receivers. Leakage here will upset the grid bias of the driven valve and has often been the cause of overloading and short life of the output valve(s). Later models often used ceramic audio coupling capacitors to avoid this problem.
Early in the 1960's many manufacturers abandoned paper capacitors and changed over to buff coloured plastic coated Philips polyesters. These have proved to be excellent in service and rarely need replacing.

MICA CAPACITORS

Along with paper and glass, mica is one of the "historic" 19th Century dielectrics and was the best available in terms of low leakage, low losses (power factor), stability and long life. They were made with both foil and metallised types, the latter being known as "silvered mica". They were frequently used for tuned circuits and as small R.F. coupling and bypassing capacitors.
By the nature of mica, they have to be built up in a stack, making them easily recognized in having a rectangular form, usually having Bakelite cases and are never tubular. They are sometimes very thin and wax coated. Mica capacitors are a bit like the little girl in the nursery jingle - when they are good they are very very good, but when they are bad they are a disaster! A faulty paper capacitor can be a bit leaky but a mica capacitor is more likely to develop a dead short. The reason for this is interesting. Under an electrical stress, metal from the electrodes can actually migrate through apparently solid mica, forming microscopic "trees" or dendrites, often only of molecular thickness. When two dendrites meet, they link up and a short circuit results. Another unfortunate habit is developing an intermittent loss of some capacitance, probably due to disconnected foils. This can be very confusing if the capacitor is part of a tuned circuit.Although they are sometimes still available, many suppliers no longer stock mica capacitors. Polystyrene or suitable ceramic types of adequate voltage rating can replace them.

ELECTROLYTIC CAPACITORS For the past 75 years, electrolytic capacitors, which evolved from electrolytic rectifiers and combine large capacitance with small size, have been an indispensable component in receivers. Before their advent, power supply filtering capacitors were very bulky and expensive and audio cathode bypassing was frequently inadequate. Their dielectric is a thin coating of aluminium oxide on the positive electrode, created by electrolytic action, and they must of course, be connected with the correct polarity to maintain the dielectric. The earliest electrolytic capacitors were liquid filled, but so far as I am aware, were not used in post War models.
The familiar so-called dry electrolytics still need a liquid, but as in "dry cell" batteries this is in the form of a paste. Manufacturers have had remarkable success in sealing these capacitors against evaporation, especially with chassis mounted types. The writer has several receivers still with the original 40 + year old filter capacitors working well. In fact it is quite common to find electrolytics that have outlasted their paper contemporaries. Signs to watch for are increased hum levels or bulging fibre ends. To check out filters is easy, simply bridge the suspect capacitor with a known good one and note if there is a change in the hum level.


TANTALUM CAPACITORS

Tantalum capacitors can be regarded as electrolytics with superior characteristics. They are smaller and have lower leakage than electrolytics, but are available in low voltage ratings only. To the best of my knowledge they were not used in valved receivers and there is no real need for restorers to use them.





THE POLY FAMILY

Replace paper types with plastic dielectrics, and provided they are worked within their voltage rating, the chances are that they will outlast you! There are several types of plastic dielectric capacitors all with names starting with "Poly". As with paper capacitors, both foil and metallized construction is used but they are smaller than their paper equivalents. Make sure that, in valve circuits, replacement capacitors have an adequate voltage rating.
Modem equipment generally operates with low voltages, and capacitor ratings reflect this, with working voltages of 5Ov or 1OOv, but capacitors used for valve circuits, especially if they had any association with the receiver high tension, had ratings upwards of 250 volts. Fortunately 400vw and 630vw polyesters are still in the catalogues and should be used, especially for HT and screen bypasses. Capacitors with axial leads are to be preferred, as, although physically smaller than equivalent paper varieties, they will fit readily in the original wiring and layout. However, radial lead polyesters, intended for printed circuit boards are more commonly available and their small size makes them an easy fit. Their appearance may not be as appropriate as axial lead types.
 

CERAMIC CAPACITORS

There is a greater variety ceramic types than any other species of capacitor. One supplier's site found on the Internet lists over 100 different types! Not only do these have different characteristics, but also many have specialist uses. The two major categories used by later model receivers are general purpose and precision. General Purpose ceramics are not intended for critical applications but are suitable for use as bypass, filter and non-critical coupling elements in high frequency circuits where changes in capacitance caused by temperature variations are not critical.

General Purpose types are sometimes called Hi K and although they make excellent coupling and bypass capacitors, they are quite unsuitable for tuned circuits. Capacitance tolerance is wide, frequently as much as 20% and they can be in the several forms, including tubes plates and discs.
Close tolerance Ceramics will often have markings such as NPO-1% and are larger than the equivalent general types and are used mainly in tuned circuits. They are normally made in values below a few hundred mmf, but their markings are often m1nimal and it can be difficult to differentiate them from general-purpose types. Unless you are absolutely certain about a ceramic capacitor's specification, use polystyrene replacements in critical locations.

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