A brief look at the causes
An Article by Erwin Timmerman
In a cassette deck the tape is transported by means of a thin shaft, called the capstan, which turns very fast. If you look into your cassette deck, you can see the capstan to the right of the playback head. It'll look like a small steel pin. After you press play, a larger rubber wheel moves in and holds the tape tight to the capstan. This large rubber wheel is called a pinch roller. The tape actually gets pinched in the space between the capstan and pinch roller during recording and playback.
Neither of these two parts are perfectly round, nor mounted at exactly a 90 degree angle. Also, the motor that drivs these two pieces never turns at a perfectly constant speed. All of these slight variations in speed and size causes small fluctuations of tape speed with every rotation, one with a high frequency, created by the capstan (flutter), and one with a lower frequency, created by the pinch roller (wow).
When you're very perceptive to tonal heights and tuning, this wow and flutter can be really annoying. Flutter can be heard as some sort of strange fast vibration (trembling) of the sound, much faster than a normal vibrato. It is easiest to hear this in high pitched, sustained sounds (church organ, steel string guitar). Wow normally isn't as obvious as flutter. It's a very low speed pitch modulation that is generally not audible, or shouldn't be. If it does become audible however, it usually is so overbearing that the tapedeck becomes unusable. It sounds like a slow, deep vibrato. A bit like playing an old 45 single without the corresponding adapter.
What if I'm fluttering?
If your deck is suffering from wow and flutter problems, it's time for a service call. Generally, there is nothing you'll be able to do at home to cure the problem. The only thing you should try before sending it to the shop is to thoroughly clean the capstan with head cleaner, and clean the pinch roller with rubber cleaner. DO NOT use head cleaner on the pinch roller, this only amplifies the problem by causing the tape to stick and slip uncontrollably.
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