Slide projector plus 'magazines' (Brincliffe Edge S7)

Prinz 1800 concord remote control projector plus 10 magazines for holding the slides, specific to this model. All working beautifully apart from, it seems, the focus which only moves slightly. Is dusty & an old model so this might be fixable with just a good clean, or with a bit of electronic know how. Received recently from a kind freegler. Decided to pass it on cos unfortunately my (hundreds of) slides are already stored in an incompatible type of magazine.