I used this: https://www.cromwell.co.uk/shop/lubricants-and-chemicals/cleaners-and-degreasers/cfc-free-electro-contact-cleaners/f/18505
It's safe on plastics, there is a version of the same stuff with a better nozzle
Sorry for the late response, this is a common problem with these machines given their age. The 788 can be opened up from the rear. Pull off fader/pot knobs. lay face down on a padded bench and remove screws from around the edge. Two more screws need to come out from the centre, you can work this...