The wire contacts of the stop tabs will oxidise (just like the basses). Carefully scour the contact-wires with a sandpaper or a little grindstone. Scouring can can be done directly in the upper part of the organ (after disposal of the tab panels ). It is also possible to overturn the upper frame to the upside. Therefore you have to remove the two foremost screws of the left and right side from the frame. As soon as the frame stands up the contact-wires can be scoured through a special made slot (as shown here). N.b: instructions on this site are based on the standard version; be aware that there are slight differences with the theatre version!