Every cassette deck runs at a slightly different speed. The tolerance of a machine made in 1990 when brand new was 0.5% to 2%. After 25+ years, it is pretty sure that your 25 year old machine needs new belts and a speed calibration. An exception could be made for machines with direct drive capstan motors. Very few machines had direct-drive capstans because they were very expensive.
How to adjust the speed of cassette decks
Many cassette decks have a hole in the back of the capstan motor. This hole is usually covered by a rubber shroud, and you can insert a small jeweler’s screwdriver to connect with a potentiometer which allows adjustment of the speed. This adjustment is often very sensitive and it’s difficult to get a very precise and stable speed – a very small adjustment can result in a huge speed variance. Better decks might have an adjustment potentiometer on the PC board which allows a much more precise setting of the speed.
We offer a test tape containing a 440Hz A tone and a 3150 Hz tone to assist in setting the speed of your cassette deck.
https://www.duplication.ca/shop/audio-cassette-speed-calibration-test-tape.html
Seems all very logical, but what if you had a Direct Drive Motor deck and want to adjust its speed? Do you easily turn a pot somewhere? Does it require a different resistor in series or do we have to change the diameter of the flywheel to accomplish this task?
Thank you for posting this very helpful tip! Can I assume turning left (counter clockwise) slows the motor and vis versa?
Thank you very much! I thought mine was broken but with your help, I was able to fix it!