I have recently changed the belt on my aiwa hsp-x101 (havn't seen many of these on ebay) , the new belt is slightly longer in length because the unit now plays a little slower than i remember. I have checked this against my wm-dd9,wm-d6c and wm-dc2, all sound noticably faster. How can i accuratly set the speed with out using any expensive technical equipment, i will need to repeat this procedure as have over 130 walkmans collected over 25+ yrs.  

I had an opposite experience when I used belts slightly too thick on a JX707 - it caused the speed to be faster. The best way to calibrate it? This is what I do, record something from your PC preferably quite a long track onto tape using your WM-D6C which is quartz speed controlled. Then playback from your PX101 in one ear and play back from your PC in the other ear the same track and same channel and try and match the speed. Selecting a long track will give you a better chance to speed match and also help let you know if the PX101 is running faster or slower over a greater length of time.

The PX101 has AMS (automatic music search) so it should be fairly easy to get back to the beginning of the track.

If the length of the belt is not perfect, has not changed only the rate but also the wow and flutter. It is not recommended set the speed, but it is much better to insert a belt with the right fit.