Capacitors

After tubes, one of the most mysterious components found in Amps and other equipment has got to be a capacitor. These components fail - sometimes with spectacular effects, other times with a subtle change in the sound (or picture) quality of the device where they are installed.

What is a capacitor?

A capacitor consists of two conducting surfaces separated by an insulator which is called a dielectric. The modern component is descended from a Leyden jar which was a glass jar lined inside and out with metal foil.

These devices have two properties that are useful in a circuit. The first is that a capacitor can store an electrical charge, and the second is that an AC signal can travel through a capacitor while a DC current can only flow for the time it takes the capacitor to reach a full charge. This means that there are capacitors in the power supply of any unit that runs on AC power to store power from the peaks of the power cycle to power the unit during the valleys of the power cycle. A weakness in the components used here can result in a sixty cycle hum (fifty cycles in Europe) in the overall signal.

Another place you will find capacitors is in transferring the signal between stages in an amplifier. In both tube and transistor equipment each stage requires DC power, and a capacitor can block that voltage from reaching the next stage. These are the coupling capacitors that are generally between the plate of one tube and the grid of the next.

Other caps shunt the signal from travelling between stages via the power supply lines so that the signal from one stage does not get a chance to disrupt the operation of the rest of the equipment. In a radio, you will also find capacitors that remove the high-frequency carrier signals left over from the transmission, from the audio signals recovered by the detector.

The audio signal does pass through some of the capacitors in any audio Amp. be it a hi-fi or a Guitar amp. In many solid state amps, the coupling capacitors that the signal must pass through are large electrolytic units. Since tube equipment tends to have stages with higher internal impedance, it is normally designed to use much smaller values of capacitance between stages.

Capacitors can also be used as part of an oscillator, such as is found in the horizontal and vertical defection circuits of TV sets.

Why do some folks say you should just replace all capacitors when servicing an old radio?

Older capacitors - particularly in units that have spent time in storage, can cause all sorts of problems, some of which can cause damage to related components. While wholesale replacement is often not required, it can side step some potential hazards.

The properties of capacitors depend primarily on the dielectric. There are two basic types of capacitor construction.

Large value capacitors such as are found in power supplies and between stages in transistorized equipment have two plates made of aluminium foil, with an conductive paste between the plates. (the electrolyte) The dielectric is actually a thin layer of aluminium oxide on one of the foil surfaces. This layer is maintained and is reinforced when the capacitor is charged in normal use, and can deteriorate over time if the capacitor has not seen regular use. The coating will be destroyed if the capacitor is connected in reverse. If gaps have developed in the coating, a large current will flow through the capacitor, and you run the risk of the case bursting, accompanied by much bad smelling smoke. The current flow can also overload other components sometimes causing a melt down in the plates of the rectifier tube. Most manufacturers expect that the capacitor will be put into service within a year or two at the most. Any unused electrolytic or any used one that has not been used in the last year or two should be slowly charged to working voltage over the course of several hours while checking for any heat build-up or leakage- electrical or physical. This slow "forming" process will allow the film to redevelop evenly on the aluminium foil

If the paste between the plates dries out, the capacitor will lose capacity, and may not be able to fulfil its role, or may also overheat. Electrolytics in equipment that has not been used for some time are best checked with a capacitor tester, or alternatively charged slowly by means of an external power supply, or by operating the unit from a variable transformer and gradually increasing the supply voltage to normal, while keeping a close eye for leakage and any abnormal colour in the plates of the tubes..

Whenever one is handling electrolytics, it is very important to observe the polarity indicated, Anytime the polarity is reversed, the capacitor will appear as a dead short, and the insulating film will be damaged. Acrid smoke will fill the workshop within seconds. It is possible to buy special non-polar electrolytics, but they are made in limited values and are more expensive. They are used for some applications like the cross over network in loudspeakers.

Smaller value capacitors - sometimes called fixed capacitors because they don't depend on a conductive paste for operation can also have age related problems. Until the 1970's one of the most common materials for the dielectric in a fixed capacitor was paper. The paper would be oiled or waxed, and rolled up in a cylinder with the foil plates in between. Problems start when moisture gets into the paper, and allows a small amount of current to leak between the plates. This is the equivalent of attaching a resistor in parallel with the capacitor. Depending on where the leaky capacitor is in the circuit, leakage may or may not have a noticeable effect on circuit operation.

Unfortunately the most popular way of sealing capacitors in those days was to dip them in wax, which can crack, or even fall off in the intervining years. Time has revealed that even the moulded plastic cases that some makers used cannot fully protect paper capacitors from becoming leaky

Any leakage in a capacitor in the grid circuit of a tube, especially when the resistors nearby are often many kilohms, may upset the bias on the tube, which in turn may increase the plate current to an excessive value. (alternatively it may just create distortion - or reduce the gain of the stage to point where the unit stops working). Leakage in a bypass capacitor on the other hand will sometimes only show up when it gets so bad that the capacitor starts to get warm.

Capacitor leakage in the sync and deflection circuits in TV sets can make it impossible to prevent the picture from rolling..

The foil in paper capacitors was normally attached to the leads by a pressure contact inside the case, and this may have oxidized or deteriorated in other ways over the years. The result may be noise or intermittent operation, which can be difficult to trace.

The advice to just change them all springs from the fact that to accurately test for leakage, you have to unsolder one end of the capacitor from the circuit. A complete test may involve using a heat gun to see if the capacitor opens when it reaches operating temperature, as well as other tests, which are often best done with an old tube type capacitor tester which itself is becoming an antique. (Although some modern digital capacitor testers are around, most don't use the hundreds of volts that the capacitor must endure in tube circuitry.)

Modern "paper" capacitors are actually made with a plastic dielectric - often polyester or polypropylene and so they are much more resistant to moisture. Some folks will even use polystyrene capacitors but they are expensive and tend to be rather sensitive to heat. At audio frequencies there is little to pick from the available material choices. The small difference in price between polyester and polypropylene is generally not a concern.

Modern caps generally have an epoxy coating and improved connections between the leads and the foil. Switching paper capacitors to new plastic capacitors can chase away many potentially hard to track problems.

Small disc shaped capacitors are also found and they use a ceramic plate as the dielectric and so they often are more resistant to deterioration than paper. Other materials are also used such as Mica, and again these can have a long life if they have not been physically damaged. Ceramic and mica are often used by designers in the Radio frequency stages of receivers, as the mica units will normally hold their values over a wide temperature range, and some ceramic capacitors are designed to vary in value by a predictable amount as the temperature changes, this property can be designed in to offset drifts in values of other components. For this reason you should be careful if you want to interchange a disc capacitor with a paper or plastic unit. Disc Capacitors which don't change value with temperature are usually stamped NPO. Since they are made of a ceramic disc, ceramic capacitors tend to have less accurate tolerances for values. -20% +100% is not an unusual specification.

Where the change in value with temperature is not a factor, and where a ceramic disk has a suitable value tolerance, it can often replace a paper unit. Likewise a paper unit can often be substituted for a disc unit where their is enough space for the larger component. In that case one should make an effort to ensure that the replacement is designated as "non-inductive" particularly when working in the RF and IF stages of a radio.

One variant that you may find in some old radios is the ceramic network. This is a small ceramic rectangle with several leads, that uses both ceramic capacitors and deposited resistors on the same ceramic base to replace many components. The network could significantly reduce size and manufacturing cost, particularly with Printed Circuit construction. (one component to install rather than many - fewer leads to solder). If one of these become defective, the chance of finding an exact replacement is rather small, and you may have to research the equivalent circuit and reconstruct the functionality with separate parts.

So I should be wary of NOS capacitors then?

I think so. (IMNSHO) A paper or electrolytic capacitor that has been stored 30 years is just as likely to have problems as the one that is already installed. If appearance is an issue, as in antique radios with exposed components, some folks will go to the trouble of hiding a modern capacitor in the case of the older original. (thankfully, Plastic capacitors tend to be a fraction of the size of the paper units they replace - likewise modern electrolytic tend to be smaller than the originals)

In some cases, the value you need is not readily available from current production, and then of course, one must explore old stock components.

I would definitely want to do a serious leakage test at full working voltage on any capacitor that I was planning on installing in tube based equipment.

My radio uses a .05MFD 400V cap, and the dealer does not list that value

the electronics industry decided in the 60's that it would be a good idea to start specifying so-called standard values for capacitors. In this system there is an equal difference between values. you should find that your dealer will have a .047, and possibly a .056 with the same voltage. .047 is the closest match.

As far as voltage ratings go, you may have more trouble finding the voltage you need. Transistor circuits seldom run on more than 24 volts and so many lines of capacitors top out with 50 volt units. 400 and 630 volt units are still made by folks like SBE and Xicon. When replacing a paper capacitor you can always use one with a higher voltage rating if it will physically fit, but cannot use a lower rating, so many folks will only buy the 630 volt units as spares.

Electrolytics are a little more fussy. If they are not charged near their rated voltage from time to time, they will never get a chance to form fully and may start to leak. A general rule of thumb is that the voltage rating should not be lower than the working voltage and also not more than twice the working voltage unless other wise specified by the capacitor maker on their data sheet. A Google search will often turn up the makers data sheet for current production capacitors on the web.

A buddy warned me not to buy a big metal capacitor at a junk yard, why?

Large rectangular Metal cases were sometimes used for oil filled paper capacitors. These are the sort of units used in power distribution florescent lighting and motor starting applications. For a good part of the 20th century, the oil was mixed with an additive called askerol, or PCB. This chemical was also used to fill large transformers. Although it has excellent electrical properties, it is now considered to be an environmental hazard because it is associated with cancer. It is best to avoid any large metal caps unless they are marked "PCB free". If you have an old oil filled cap that is leaking, you should consult the environmental authorities in your jurisdiction to see if it requires special disposal.

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