Cassette Player (by 2 users)

Motor speed needs to be regulated by PWM. Reducing the current (with a resistor) actually lowers the torque - which, with a given load, does result in reduced speed - however, different length tapes (60min vs 90min), as well as different quality, and even current position in the tape, will change the load and hence the speed.

You need to regulate the speed via PWM. It's sometimes done on a small board built into the motor, in which case you'd have a round hole into which you could insert an INSULATED screwdriver (tiny flathead wrapped most of the way in electrical tape works).

It may be done on the main board, in which case you could find a pot conveniently labeled "motor speed", or perhaps "pulse width" or "pwm" or something to that effect. Find it, mark its current position just in case, and turn it to see if it gets the effect you're after.

And there are some tape decks in the world with no speed adjust - they are designed with a specific speed motor and the sizes of the wheels and gears are calculated for that speed.

Also there may be a mechanical fault/malfunction causing the speed problem.
An initial check and thorough cleaning of the capstan and pinch roller should be done. If there is a buildup of tape oxide or a piece of a broken tape wrapped around the capstan, the tape travel will be too fast (do to the effective increase in capstan shaft diameter).

If the pinch roller isn't fully engaging, the tape can be getting pulled onto the takeup spool at a speed faster than the capstan control.

A different source of trouble might be a slip clutch (possibly an
idler/clutch assembly)which is binding, or otherwise not functioning the way it should.

Another type of motor doesn't have it's speed controlled by an internal or external voltage.. instead, the motor has an internal centrifugal speed controller. This type of motor can be found on older tape decks, and might not have been utilized more recently.

Good luck