Hi,
Here’s the [https://www.manualslib.com/manual/567039/Sony-Wm-D6c.html|service manual]
On p.16 it describes the tape speed adjustment procedure which requires a test tape (Sony WS-48B) and a digital frequency counter.
Searching online indicates that the test tape is no longer available but there is a lot of information about making your own test tape. Just search for ''Sony WS-48B''
You will also need other test equipment to make sure that the device is set up correctly - see p.17, as trying to adjust by ear usually doesn’t work
Hopefully a start.