The main tuning capacitors in old radios are often full of crud, grease, and dust. Similarly, ham radio operators working the hamfest circuit looking for linear amplifier and antenna tuner parts often find just what they need, but the thing is all gooped-up. Several things can be done about it. First, try using dry compressed air. It will remove dust, but not grease. Aerosol cans of compressed air can be bought from a lot of sources, including automobile parts stores and photography stores.
Another method, if you have the hardware, is to ultrasonically clean the capacitor. The ultrasonic cleaner, however, is expensive; unless you have one, do not rush out to lay down the bucks.
Still another way is to use a product, such as Birchwood Casey Gun Scrubber. This product is used to clean firearms and is available in most gun shops. Firearms goop-up because gun grease, oil, unburned powder, and burned powder residue combine to create a crusty mess that is every bit as hard to remove as capacitor gunk. A related product is the degunking compound used by auto mechanics.
At one time, carbon tetrachloride was used for this purpose . . . and you will see it listed in old radio books. However, carbon tet is now well-recognized as a health hazard. DO NOT USE CARBON TETRACHLORIDE for cleaning, despite the advice to the contrary found in old radio books.
