From: ard at p850ug1.demon.co.uk Sorry to follow up my own message, but I spent last night thinking about it, and I think I now understand it. The things that are important are (a) the belt is elastic and therefore stretches, and (b) the drage from the spools means that the tension in the belt is not the same all the way round. There is more tension in the belt on the takeup side than on the supply side. Supppos that a certain length of belt has gone past the supply spool (and thus wound off that length of tape). As it goes round the takeup spool the belt will be stretched a little more, so it will move the surface of that spool by more than the amount that the surface of the supply spool was moved. It will try to wind more tape onto the takeup spool, thus tensioning the tape. For obvious reasons this effect is small. It helps to have the tape fairly well tensioned before using the cartridge. If you're reassembling a defective one, I'd tension the tape as much as possible by hand. If you could make a cartridge with the belt round the outside, and with the drive roller possitioned far enough forward to let the bead get in contact with the tape (inside the loop formed by the belt), it wouldn't work. There'd be more movement on the supply side, the tape would get ever more slack. -tony