Hello Suzanne,
If it is the very big nylon gear on top they used to be able to unscrew by four screws. two either side and the whole cam could pull out, I am trying to remember on that model if that was the case or not, if it is possible for you you send a picture I can soon tell you, if you don't have the screws either side of the cam it will be a major job and will mean the topshaft out.
Ken
drivewithken@blueyonder.co.uk, April 2008