Issue on Yamaha's TC-720 Dual Capstan Cassette Deck - no correct Tension of Tape across the Head

There is to observe an unwanted looping of the tape instead a correct tension across the head.
First idea was to replace the belt by a new one.
After done this the unwanted looping wasn't gone - check out the image.
The left capstan rotates a bit faster than the right one - so as if the capstans had been swapped.

What is therefore the reason ?
Is it a typical effect on this model ?

post #1098 under
https://www.diyaudio.com/community/threads/what-did-you-last-repair.313739/page-55describes in general, what mechanism is it exactly, that maintains the exact proper tape tension across the heads in such cassette drives with dual capstan

This thread don't provide the wanted information:
http://www.hifi-forum.de/viewthread-185-4462.html
Maybe one of the member knows more - thank you very much for an advice.
 

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