There is a right way to clean a cassette deck, and there is a wrong way to do it. Knowing the difference can save you a good deal of frustration and expense.
The cassette player runs the magnetic tape through guides and across the surface of the heads, with the heads being responsible for the pickup and playback of the sound. Without getting technical, this part is fairly easy to understand to anyone who has watched the tape moving around inside the deck. The problem is that oxides from the tape eventually coat the surfaces of the guides, rollers and heads, resulting in a drop in signal quality. A very dirty cassette deck may even break or 'eat' the cassette tapes.
The problem doesn't stop there, either. Since the tape is magnetic (the reason cassette tapes should never be stored near an energy source like a speaker magnet or power supply), a small charge can build up on the heads that also destroys quality playback and in extreme cases can demagnetize the tape. It can be intensely frustrating to start to listen to one of your special tapes, only to find that there is nothing left there to listen to.
The aggravation can become a great deal worse though, if the cassette deck isn't cleaned properly. Improper cleaning can result in damage to or destruction of the heads, rollers, and guides. This makes the deck virtually useless.
It is important to know the wrong way, and why it is wrong, to avoid mistakes. We can then look at the correct way to clean the cassette player.
Many stores sell cassette head cleaners, which consist of a cassette tape case that contains a strip of abrasive material. This is inserted into the player and run like a standard cassette, often with a few drops of solvent, usually alcohol, added to the strip. The reason this is bad centers on the word 'abrasive'. Cassette heads are fairly delicate electronic components and this kind of cleaner can wear away the surface of the head, scratch it, and eventually will result in costly repairs or replacement. Depending on the quality of the heads, this can also happen quite rapidly.
Think of what would happen if you cleaned a pair of eyeglasses or sunglasses with sandpaper and you will have a good idea why this isn't a good idea.
The good news is that it isn't very difficult to clean the cassette deck the proper way.
First, purchase some q-tips. The ones you can buy in a pharmacy will work in a pinch, but it is best to get the kind that are sold at electronics stores specifically for cleaning heads. The handles are longer and sturdier, and the cotton tips are wound tighter so there is less cotton residue left behind.
You will also want to get some alcohol. Isopropyl (rubbing) alcohol will do, but again it is best to get the alcohol from electronic supply as it is specially designed to remove the oxides without leaving behind a film that will dry on the heads.
Open the cassette carriage so you have access to the heads. In some systems, this requires carefully removing the carriage cover.
Dip the q-tip in the alcohol, and then gently rub this on the guides, rollers, and heads. Rotate the q-tip as you swab the surfaces, and when the q-tip gets dirty, throw it away and use another. The surfaces are clean when you can no longer pick up any of the dark oxide residue.
Allow the deck to air dry at least a half hour before attempting to play any cassettes, and before replacing the carriage cover if necessary. Using a clean dry q-tip is also helpful, but still allow the alcohol to evaporate.
Alternately, the deck can be taken to a qualified repairman who will generally have the equipment and alcohol on hand, plus the knowledge to use them.
Cleaning a cassette deck periodically is a necessity, unless you have money for replacements. However, it isn't hard to do, it isn't very expensive, and as long as you understand what not to do, the results will usually be pretty good.