I know this is an old post but I'm having the same trouble - the string came loose on mine and I can't get it back on.  In the service manual it shows that in one direction the string should be 1/2 turns round the drum coming round the top, and the other end 3 turns round the drum coming round the bottom.  It's really hard to do that without removing the drum, and then it all comes loose again when you push the drum back into place!  The little spring that sits inside the drum, to which both ends of the string are attached, seems to have come loose in mine and there doesn't seem to be anything for the spring to attach too.  I expect there was some little lug or catch within the drum which the spring connected to, and it was perhaps worn or snapped off, so the spring doesn't remain in place.  That's probably the reason the string came loose in fact. I have another non-functioning 901 which is in quote poor cosmetic condition, but the tuning seems to move smoothly.  I was thinking of taking the whole string and drum out of that and putting into the working 901, but I know that will still be really fiddly and hard to do, so I've been putting it off!