Tape Transport Problems
Clean capstan and pinch roller. (See How to Maintain a Cassette Deck.)
Check pinch roller for erosion and cracks.
Remove visible tape oxide flakes with anhydrous alcohol and a cotton swab.
Listen for normal motor sound.
Inspect belts (usually thick black rubber bands) connected around motors and reel spindles inside deck for wear. Replace broken, brittle or stretched belts.
Replace belts if you hear excessive wow and flutter or an obvious variation in speed.
Replace motors if they won't move a good belt.
Check all connections--tape deck to receiver or amplifier, amplifier to speakers and so on.
Make sure your deck is biased for the right type of tape (I, II or IV) and that the noise reduction method used in playback is the same as the one used during recording.
Demagnetize erase and record heads if deck is not completely erasing tapes. (See How to Maintain a Cassette Deck.)
Replace heads if you hear or measure a loss of high frequencies.
Replace heads if they are visually worn or grooved after cleaning.
Replace heads if you hear both sides of tape at once. (Check the tape first.)
Have the azimuth and bias adjusted if prerecorded tapes or tapes made on another deck sound wrong.
Have the head gap realigned if tapes sound muffled or garbled.
Have the meters recalibrated if distortion exceeds what's indicated during recording.