You can download the Atari 1050 Field Service Manual from atarimania.com - it's here: http://www.atarimania.com/documents/atari-1050-field-service-manual.pdf
I printed out a copy for the very sad day that my 1050 stops working. I'm fortunate enough to have a 1050 in museum condition (except the outer carton which is a bit tatty) but I got some DOS 2.5 disks from Canada a few weeks ago and the first two didn't work, the third one did but then the first two were working as well... I don't know if it just needed to warm up or if there's a creeping problem setting in!
It is thirty years old though... anyone got sources for spare belts / rubbers, or equivalents? I've just got a USB SIO2PC and I want to populate as many disks as possible!