I think a spring in the gearbox has broken. The gearbox lever is sloppy and doesn't return to it's ordinary position. I can shift gears if i fiddle a lot with the lever so the gears seems all right. Is the spring situated inside the crancases?? I wonder if I have to split the engine in order to repair this?

Yes! I seems possible! I can see here: https://www.tr1.de/cgi-bin/service.cgi?key=xv750se&lng=eng&flg=0&pge=3018
that the shifter can be removed if you remove the left side engine cover as you said. The spring comes out together with the shifter. Many thanks George! I should remove the cover anyway to change the o-ring for the starter engine (it leaks oil there).
In the mean time I can drive with #2 (the red TR1 from 83, http://www.buf-online.kristianstad.se/~lars.sandstrom/mars04.JPG) so all is fine!
Regards,
Lars