Basically find another unit on Ebay, take it apart, and take notes for re-assembly. There's quite a chop-shop Roomba industry on Ebay. IRobot designed a clutch assembly for the brush drive that basically never slips, so the spur gears get trashed. The software design is good, the bells-and-whistle factor is high, but the implementation, QA, and durability is poor. Sorry. We have several and I'm tired of fixing them.
Lee
April 2007