Casio MT-400V filter repair

Hello everyone! I’ve just picked up a Casio MT-400V with a faulty filter section - the ADS and cutoff sliders do not seem to work, although the resonance slider does. I can adjust the cutoff using the internal trimmer pot so I believe the filter circuit is working. The non-functioning sliders look physically good - I’ve cleaned them and tested continuity on them as well as the wiring coming from the slider daughter board and the ribbon cables connecting the main boards. Apart from a couple of archived posts from owners with similar issues I can’t find anything much on line - certainly no service manual or schematics so I was wondering if anyone here has any experience with this issue? Any pointers would be much appreciated!