Originally posted by manacho2005 Are you using a teleconverter? If not it may be gunked up in the back. Cleaning may cost more than a replacement, but I'm sure if you've had it for 28 years, there's some sentiment....
I think there was some crud. It hasn't been dropped recently, and I've not got a teleconverter. I've only ever really used it on a K1000.
I just put it face down on my desk and tried to undo one of the screws on the rear plate. I had to push quite hard, but it wouldn't budge.
Weirdly, pushing down on the back of the lens seems to have unblocked whatever it was! For now, at least. Thanks for the suggestions, but for the moment it seems to be back on form. :-)