Paper/Wax
These are usually 1/4" to 3/4" wide wax covered cardboard cylinders with leads coming out of wax plugs at each end. Occasionaly, 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
These are most commonly either a large aluminum cylinder mounted to the top of the chassis, or a large cardboard cylinder under the chassis with several colored leads. Both of these types usually contain several independent sections, joined to a common ground at the cylinder case or lead. Each section may have a different capacity value and working voltage. These caps are almost always used for power supply filtering. Common values range from one to several hundred uF. The value, polarity, and rated working voltage should be clearly labeled on the side of the cylinder.
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.
Mica
These 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 Disk
These 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 paracitic 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
These small valued adjustable capacitors are often square white ceramic blocks with two overlapping metal plates separated by a thin shim of mica. They are adjusted by turning the screw head that compresses the plates together. They are also used in the high frequency receiving and antenna circuits. They can be found mounted to the chassis, or built into variable tuning condensors, or the top of IF transformers. Values can range from 2 to 100pF, and are used for fine adjusting or "aligning" the resonant frequency or impedance of a tuned circuit. Trimmers rarely fail, except through corrosion or cracking of the ceramic body. It is best to avoid adjusting these trimmer caps until you have the proper test equipment to verify the effects of what you are doing. Without a clear schematic or alignment chart, it is difficult to determine what function each one has.
Air Variable Tuning Condenser
This main tuning cap is composed of a set of large rounded aluminum fins that mesh together when tuning the radio.
As each fin is separated by a small air gap, they are referred to as "air dielectric". They are usually variable from a few pf up to 365-500 pF, with two or more independent sections and padder trimmers in parallel with the main fins. The trimmers set the minimum value of the main condensors, and are set so that the sections "track" together. The sections adjust the resonant frequencies of either the RF amplifier stages, or the RF preselector and oscillator circuit to receive the desired radio station. These caps only fail if they become bent, corroded or worn out. Clean them with a small stiff paintbrush and alcohol, or an electronics spray cleaner. Replace the rubber grommets that insulate the condensor from the chassis if they are crumbling. Clean and tighten the mounting screws, and clean the connections of any corrosion. Poor connections here will lead to no
signal or flaky reception. Put a drop or two of light sewing maching oil on the shaft bearings at the front of the condensor, and a little at the rear of the shaft too to keep it moving freely.
Testing Capacitor Values and Leakage
Warning: Many capacitors, especially large value electrolytics in the power supply can remain charged to a dangerous voltage for minutes after the power is disconnected. You could get a nasty shock if not careful! It is a good idea to make sure these large caps are discharged by touching a 100 Ohm resistor with alligator clips on the ends across each cap before doing any work on the chassis.
To properly test the capacitors, all of the tubes should be removed from the chassis, and at least one lead should be disconnected from the circuit to prevent the other components from affecting the reading. Otherwise, some of the caps can look bad or give weird readings. If you plan to work on radios often, a good LCR (Inductance(L) Capacitance(C) Resistance(R) ) meter is valuable for eliminating much of the guesswork in testing capacitors. It is a small investment that will be well worth it. Some Digital Multi-Meters (DMM's) do come equipped with a capacitance meter, but some might not be able to read small caps in the pF range, or filter caps with large values.
If you have only an ordinary multimeter, the only test you can perform on a cap is to determine if it is shorted. (This can be of limited use, as many capacitors go bad by opening, or leak when voltage is applied) Normally, a good cap should measure as an open (infinite resistance) to a DC ohmmeter. The one exception is with large valued electrolytics. If you use an analog ohmmeter (with meter needle rather than LCD display), you will observe a momentary bounce in the needle then the leads are first applied. This is due to the momentary short that the cap looks like as it charges to the small voltage presented by the probe tips. If you then reverse the test leads, the needle should bounce the other way. With just a regular Ohm-meter you can still do an adequate repair job. Measure each cap to confirm that it looks like an open with one lead disconnected. (if not, 86 it!) Then, replace any paper caps showing the signs of damage discussed earlier, and replace all of the electrolytics to be safe. If the set still has problems, you will need to use good troubleshooting skills to compensate for uncertainty about the caps. Often the easiest way is to replace all of the paper caps with 1000V ceramic or orange drops, as this will eliminate any doubt about them.
With an LCR meter, each cap in the chassis should be tested. The values measured should be between +/- 20% of the printed value, and the leakage current should be low (It is best to compare leakage of several new caps to get a feel for what leakage is considered low, as it tends to be relative.) Paper caps will usually measure a little bit over what is stated.
If an electrolytic measures open, shorted, or is much lower than it's printed value, or the leakage is high they should be replaced. (Electrolytics will naturally have a higher leakage than other types, so again, compare to some new ones of similar value for reference.)
I think it is also a good idea to test each new cap before it goes into the chassis to check it's value. But don't worry, if you don't have an LCR meter, you still have a good chance at success, since all of the obviously bad caps are going to be replaced anyway, and the new ones are probably fine.
Reading Ceramic Capacitor Values:
The IEC numeric system used on ceramic capacitors is very similar to that used for resistors, with two numeric figures followed by a multiplier (the number of zeros), and a letter indicating the tolerance. The base unit for this system is in pF, so you may want to convert that number into uF for larger values. The voltage rating may or may not be printed on the cap.
A quick primer:
The fundamental unit of capacitance is the Farad, a HUGE value.
This is broken into microfarads (uF)
and picofarads(pF) (and sometimes nanofarads) for ease of use.
1 microfarad (1 uF) = one millionth of a farad! (1,000,000 microfarads = 1 farad)
1 nanofarad (1 nF) = one billionth of a farad! (1,000 nanofarads = 1 microfarad)
1 picofarad (1 pF) = one trillionth of a farad! (1,000,000 picofarads = 1 microfarad)
A ceramic capacitor with a number like "103J" translates to a 10 followed by 3 zeros or 10,000pF (.01uF) with J = 5% tolerance. The number "101M" translates to 100pF, with a tolerance of 20%.
A quick look-up guide: 
101 = 100pF = .0001uF
102 = 1000pF = .001uF
103 = 10,000pF = .01uF
104 = 100,000pF = .1uF
332 = 3300pF = .0033uF
223 = 22,000pF = .022uF
Tolerance Code: (+/- %)
M=20%
K=10%
J=5%
C= +/-0.25pF
Sometimes other types of capacitors will use this same coding. You might also see a coded number like "2R2" in which the "R" just represents the decimal for 2.2uF.
Combining Capacitors For New Values
Occasionally, you might need to make a value of cap that you can't easily find
in the catalogs. Or, the correct value is available, but the loose tolerances
prevent you from finding one that is close to the value that you want. In either
case, you can combine two caps, for a new value that is given by these rules.
Caution: Be sure that each capacitor is rated to handle the entire voltage that the equivalent cap will be exposed to, and mind the indicated polarity if you are connecting electrolytics together! The equivalent cap should perform very well, but remember to solder the leads, and keep them very short and tidy.
Replacing The Old Capacitors in Your Set
When you begin to restore a set from the 50's or older, it is hotly debated whether or not you should replace all of the paper capacitors or not. Purists and collectors feel that they should only be replaced if absolutely necessary, and if so, the new ones should be hidden inside of the old cap casing for appearances. I tend to believe that since the work is under the chassis it is not visible, it should have little impact on the collectible value. (unless it is a rare and valuable set, in which case you shouldn't be using it anyway.) I use my radios alot, and I would like my children to as well, so I like the peice of mind that all new capacitors provides. This is also the root of my bias that aging electrolytic capacitors should be replaced whether they test bad or not. These older style caps are prone to failure, and it's always just a matter of time before they do. I would hope that the sets I'm working on now will give years of faithful service when I'm done. Replacing them is the only way that I can be confident they will.
Replacing Paper Capacitors
The paper caps with values below 0.1uF should be replaced with modern 1000V ceramic capacitors, that can be easily purchased through
MCM Electronics , Antique Electronic Supply
or others such as Allied Electronics .
(Radio Shack doesn't have them.) The modern value conventions are a little
different than they used to be. The .02, .03, and .05uF caps of yester-year are now sold as .022 .033 .047uF caps. You can usually expect that the modern caps will measure a little less than the stated value, so try to replace caps with the next (only slightly) higher value. For example, replace a .005uF with a .0056uF rather than the .0047uF. But before you expend a lot of effort trying to find the exact values you want, remember that most capacitors vary 10% to 20% away from the stated values when manufactured anyway, and this fact was taken into account when the radio circuit was designed. A paper cap can usually be replaced with a value within about 20% of the printed value without noticably affecting circuit performance. In critical areas, trimmers are used to adjust for these variations, but variation in paper cap values should not affect the alignment of the set. Very rarely is an EXACT value ever needed, so don't sweat it! If you should need to make a value that is closer, use the series and parallel capacitor rules found above.
Replacing Mica or Ceramic Capacitors
These caps are very durable, and rarely need replacement unless way off value or damaged. These caps are used in the high frequency RF and IF resonant circuits that could fail to operate at the correct frequency if they are replaced with a different value. By not replacing them you can reduce the amount of possible re-aligning needed to get the set working well. If you must replace one, carefully select one that closely matches the original or schematic value to keep the frequencies in the ballpark. Expect to have to do some tweeking when you are done, so get a schematic and an alignment sheet. You'll need it.
Replacing Electrolytic Caps
When replacing electrolytic filter caps, you have many options available to you. Some new or NOS chassis-mounted aluminum cylinder caps are still be available through catalogs and over the web, but can be very expensive or hard to find due to the many parameters that must me matched. The correct number of sections, their values, ratings, and size must all be fairly closely matched. Usually it is much easier to just install separate individual axial lead caps of the right value. These types of electrolytics are much smaller than they used to be, are widely available and cheap. I usually install axial caps under the chassis. There's usually lots of room once the many paper caps have been replaced with smaller ceramics. It is best to avoid using the original can terminals for connecting the new caps. Disconnect the wires from the old can and leave it completely disconnected, but still mounted to the chassis for appearances. - The electrolytic sections still in the old can could short in the future, rendering your new repair job useless. There should be several places available to connect the new caps to, using tube socket legs or terminal strips that were electrically connected to the old tie points.
If you prefer a more discrete repair, the guts can be removed from the old cylinder, and the new caps hidden inside the can. Be careful, as this is hard to do, and you might damage the cylinder while doing it. Be sure to liberally tape the new caps up to avoid shorts against the inside of the metal sleeve. Some hobbiests go so far as to pot the new caps in wax, but this is unnecessary if well insulated. Remember to use new caps with a voltage rating at equal or greater to the original value, or at least 150% of the working DC voltages expected. When in doubt as to the voltage, just use quality 350V or 450V units available from Antique Electronic Supply or MCM Electronics .
If you don't have a close value on hand, a power supply filter can usually be replaced with one as much as 50% over the original value without too much trouble. Keep in mind that using values much higher could result in increased transformer and rectifier surge current when powering the set up.
Wiring and Soldering Tips
Replacing the capacitors in your radio could turn out to be a complete disaster if the job is not done cleanly and carefully. I would recommend that on your first attempt, that you practice on a low quality cheap set from the thrift store before you try it on the family hierloom.
Until you get some good practice in, you should also invest in a schematic for the set. This will really come in handy if you should get something crossed or forget where a wire goes. It is a good idea to replace cracked or burned wires, other bad components and capacitors at the same time, as you only want to unsolder the terminals once if possible. Be sure to work on one terminal at a time, making sure that each wire or component lead is re-installed on the terminal before you move on. Plan out each replacement first, to be sure that when done the new leads and wires will avoid shorting with other connections. You should also keep a pen and notebook close to make quick drawings in case you might forget, or have to quit before you're finished. The new component should be clear of moving parts and away from hot power resistors, and be secure against vibrations. I will often slip a short length of heat-shrink tubing or cloth insulation over the leads of the new cap before installing it to reduce the shock and short hazard. New wires and leads should be as short and neat as possible, sitting as close to the position of the original ones as possible. This will keep the inter-component paracitic capacitance and inductance as close to original as possible. If you should later find the set motor-boats or squeals, you might need to nudge some of the new wires or caps around with a plastic stick to see if a new part got too close to something sensitive like the oscillator coil.
The solder used in these old radios will usually be dirty and contaminated with dust and grime, making clean unsoldering a little difficult. I recommend using a good mechanical "solder sucker" available at Radio Shack rather than desoldering braid. Be very carefull, however, with loose solder splatters or drips that could rattle around and short things out. Be patient, and uncover and lift wire loops slowly rather than pulling them off to avoid splattering hot solder or damaging the terminals. Use a good quality modern rosin core solder, and a soldering iron of at least 25 watts for the job (Please not a soldering gun!). Make sure the tip is the right size to make good contact with the terminal and wires without touching and melting things around it. Hold the iron to the terminal, heat it up, then start to apply the solder to the terminal, letting it melt the solder rather than the iron tip. Let the solder flow liberally around the joint, but avoid runs or drips down the terminal. Be sure to keep the wires steady as the joint cools, and carefully inspect the connection for shorts or "dry" areas. The solder must bond well to both metals being joined, and the joint look clean and shiny with nice flowing contours. If the joint looks muddy, crystalized, blobby or jagged, try using a higher wattage iron or cleaner, newer solder.
Completely removing the wires and leads from the terminal can sometimes be difficult. Some tube socket pins and terminal strips on an old radio chassis will have many wires and component leads crowding each tie point. In other places, the terminal might be buried under lots of other stuff or hidden in the corner. In these cases, you can often splice into an old cap lead. Fortunately the older paper capacitors are usually much bigger than the new replacements, and have long leads. You can clip the old cap leads at the body of the cap, leaving at least 1/2 an inch of wire lead hanging from the terminal, to which you can connect the new cap. Bend a small loop in the end of this lead with needle nose pliers, and connect the new cap to this point. Doing this will make the job much faster, cleaner, and reduce the chance of damaging something as you dig down to the terminal.
Be sure to work on recapping your chassis for only a few hours at a time. I know it's fun to do, but believe this night-owl when I tell you it's really easy to make mistakes when you get rummy. Worse, you might find yourself compromising on the quality of the job if you are anxious to get it working. If you don't have time to do it right this time, you'll never have the time or interest to do it again. Be patient, work carefully and methodically, and you'll be very proud of how the repair looks and works. When you're done with the recapping, treat the set as if there might be a short in it when you turn it on for the first time, because there might be. Be ready to quickly disconnect the power at the first sign of trouble. If there is, you'll be glad if you have the schematic, because you'll have to do some troubleshooting. Yet another reason to be careful to avoid problems beforehand! But if you were carefull, you shouldn't have any troubles, and will be amazed at just how much better the old set works. Plus, you can also feel good that the new caps will probably never give you any trouble for a long long time!
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