Just put a random belt into a Sanyo JJ-P4.  3.7 inch inside circumference.  Powered it up, sounds like it's running, but the gears aren't turning... I'm guessing the belt is creating too much tension for it to turn?

I measured out the belt path with some string, I was wondering how close should the belt's circumference be to this.  In my case, I measured out ~7 inches... so a 3.7 inch circumference belt just might be completely off.  No more grabbing random belts and putting them in.

I attached a picture of the guts if anybody's interested.

I realize this is an old thread but I have the Sanyo JJ-F4 which appears to be nearly identical, but has the radio as well. It also needs a belt and I need to figure out a size myself, and I also am not sure exactly the belt path. I've never taken a player apart before.