Replacing The Old Capacitors in Rare Radio
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. It tends 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.) If you use radios a lot, so you the peice of mind that all new capacitors provides. This is also the root of 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.
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 Antique Electronic Supply or others. (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.
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 heirloom.
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 parasitic 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 careful, 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. 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 careful, 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!