hi jason
did you see the "field repair notes" on sk grimes site?
i have never done it, and found it after i sent mine to be repaired ...
http://www.skgrimes.com/545/index.htm
when mine was in the shop getting worked on, i ended up getting a 500 back.
it cost me $15 on FEEbay, and i have been using it for 4 years now without a problem. the 500 back is thinner, and has a different ( better ) roller mechanism ... but doesn't have the "stop" when you remove the paper, so you kind of have to know when to stop pulling the paper out or you will remove it completely
good luck getting yours fixed!
john