When you run old walkman, it is very probable that it will not work. In 90% of cases, problem is caused by aged belt. In most cases this is the only problem that these players have.
Correct belt is critical for good walkman functionality.

There is no universal belt existing. Some players use one belt, some use two or more belts. Some use thin belts, 0.5 mm, while other use thicker belts, up to 1.0 mm. Belt diameter varies from 18 to 80 mm.
It is necessary to find correct belt for your model. Try to search your model on this site. If you cannot find it, then you need to measure belt dimensions yourself, and find suitable belt in “by-dimension” category.

How do I measure belt dimensions?

If old belt still exist in solid state, then you can easily measure half length using ruler. Then you can easily calculate internal diameter. Please note that old belt is extended and original belt should be little smaller. Usually it is 90% of what you measure.
If old belt does not exist, or it is melted, then you can measure belt circumference by stretching a piece of string along the belt path.

Tip: It is not necessary to use exactly the same belt as original. New belt can be thinner, it can be made of material with different properties, quality of the belt will also be worse than original.

Example: you measured half-length of 110 mm. Circumference is 220 mm. Internal diameter is 220/Pi = 70 mm. 90% of 70 mm is 63 mm. It means that you can buy belt of internal diameter 64 mm (closest to 63 mm).

How do I know that the belt is aged?
Belt is aged when it is
– too hard, i.e you must apply strong force to expand it, and you feel that it is not flexible enough
– it has cracks on surface
– it is too soft. Try to expand it and then release it. It must return to original size quickly. If it is “lazy”, then the belt is aged
– it already changed consistence to liquid

How do I replace belt?

There is no universal rule for that. Each player is organized differently. It is usually necessary to remove the back cover. Then belt can be directly accessible, or it can be hidden under PCB.
PCB can be attached by screws, or you need to de-solder some points, in order to flip it. Sometimes it is necessary to dismantle mechanics, or part of mechanics, to access the belt. Some players have more belts (drive belts, counter belt) and it can also be hidden inside mechanics. Generally, belts in later models are easier to replace.

Tip: You need to clean all pulleys using cotton wool and IPA, before you install new belt. Also, do not forget to adjust correct speed at the end.

Be careful to route belt correctly.

Cassette belts production nowadays is financed by enthusiasts who do not have millions of budget for research.  Back in days, big producers like Sony, had budget for each model they produced. Now, we’re happy that at least some spare parts exist. Do not expect new belts to have same dimensions and quality as original. Also, price will be higher – they’re produced in hundreds, not in millions