Restoration methodsAntique capacitors typesPaper/WaxThese are usually 1/4" to 3/4" wide wax covered cardboard cylinders with leads coming out of wax plugs at each end. Occasionally, they can also be found as rectangular metal "bathtub" cans with values stamped on them, but no polarity markings. Paper caps are the most abundantly used type, found in all areas of the radio circuit for general coupling, decoupling, and filtering. They are non-polarity sensitive, with values typically ranging from .001uF to .47uF. Most of these capacitors are clearly labeled with both value and voltage rating, with the actual measured value being quite close. They often have a black band around one end, indicating which lead is connected to the "outside foil" of the capacitor. This end should be connected to the more quiet or less sensitive side of the circuit, so the outer foil acts as a shield to reduce cross-talk with the other components. This is less of a concern if new capacitors are installed, as they have smaller size, better construction and will not be as likely to couple.![]() Paper caps tend to be fragile in nature, and are even more so when they are old or partially melted from the heat under the chassis. Damage to these caps is usually indicated by melting, bubbling or darkening of the outer wax covering, loss of wax from inside or one end, or as loose leads. The caps that are connected to the high voltage parts of the circuit are the most likely to fail. These caps will more often become an open rather than a short when they fail. They can be tricky though - sometimes they can look and test OK, but have electrical leakage when at working voltage that makes them act more like a resistor. This is a big problem if the cap is used as an audio input coupling cap for the input grid of the audio amplifier tube. Leakage here can cause DC bias current through the tube to increase enough to burn out either the audio tube, output transformer, or even the power transformer. In general, replacing paper caps with more robust modern types will greatly improve the long-term reliability of your set. The nature of the paper/foil construction leads to a large physical size for high working voltages or values. This large foil plate area and lead length contributes to the parasitic inductance and series resistance of these caps, which is relatively large compared to modern ceramics or "orange drops". These parasitic qualities limit the effectiveness of these caps at very high frequencies. For this reason, replacing these caps with similar values of ceramic or orange drop types should reduce the overall amount of stray coupling, improve the high frequency response in coupling/decoupling, and improve the overall performance of the set in general. A nice added bonus! Electrolytics![]() Electrolytics are used because they offer the greatest capacity per volume, but are the most prone to failure. These caps are polarity sensitive, and can quickly short or blow up if the voltage or lead polarity is reversed. They will also fail quickly if the working voltage rating of the cap is exceeded. These caps are notorious for leaking out their fluid contents, or drying out and opening over time. They are most often responsible for loudly buzzing or humming sets, and damaged rectifiers and power transformers. Some indications of a damaged electrolytic include bulging of sides, white or tan deposits around the terminals or end seals, leaking of liquid contents, or having completely exploded, leaving sticky wads of foil and paper everywhere! Unfortunately, badly shorted electrolytics more often than not look completely normal. Electrolytics more than a decade old should be carefully tested for both value and leakage current. When in doubt about the age or reliability, change it out. It will give you piece of mind, and your children will thank you for the extra years of trouble-free performance. MicaThese caps can be flat plastic stamp sized square blocks, or small black "pez" sized pills, or even small plastic cylinders. They are usually found in the high frequency tuning and detecting stages of the circuit. Values typically range from the 10's to 100's of MMF (means picofarad or pF). The value will either be stamped on the case as a single number representing it's value in pF, or encoded as a series of colored dots. There are some excellent on-line sources of information for deciphering the color codes of mica capacitors, such as the one at TIPS, HINTS & KINKS. Color codes can also be found in a variety of books such as the ARRL's Radio Amateur's Handbook. The color dot system is similar to the resistor color codes, and there are often several extra dots indicating type, tolerance, and characteristic of the capacitor. These caps perform as well as modern ceramics in high frequency applications, are very stable and durable, and rarely need replaced unless damaged. It is best to avoid replacing these caps unless absolutely necessary, as they affect the alignment of the tuned circuits in the receiver. A set will almost certainly need an alignment after some of these are replaced. Usual signs of damage are cracking of the plastic package, or melted spots from clumsy soldering jobs.Ceramic DiskThese capacitors began to appear in sets in the 50's, and are a more modern style. They are usually small tan, green or red discs ranging in size from shirt button up to a quarter. The values range from a few pF up to .1uF, with voltage ratings from 16V to over 1000V. The value may be clearly labeled, but is usually encoded using the numeric system described below. The voltage rating is usually omitted in favor of a thermal coefficient identifier. Versions of this cap with a 1000V rating are easily available, much smaller than paper caps, incredibly durable, have very little parasitic inductance, and work very well at high frequencies. They are an excellent replacement cap for most paper and mica types, but keep in mind that the working voltage must be verified before using them. Small working voltages are most common in many stores and junk drawers, so it is best to buy those that are specifically rated high enough. When in doubt, assume the rating is too low.Trimmer![]() Air Variable Tuning Condenser![]() Add comment:
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