Sony WM-F80 keeps switching back and forth or stops, what could be the problem?

Hi, I bought a Sony WM-F80 yesterday and the problem is that it switches back and forth and if I turn off the endless mode it stops after 2 seconds. I fixed it all evening, but I couldn't solve it. I replaced the rubber on the Idler wheel, the drive belt, and there was electronic corrosion.

I also have Sony WM-55 and Sony WM-75 Walkmen with the same mechanism, I haven't experienced such a problem yet. I've already taken the others apart and compared them, but I don't know what could be causing it..


Thank you very much for the ideas!

 

[CSATOLÁS=teljes]53099[/CSATOLÁS]