Controls
The keypad is easy to disassemble and clean. Attention because the most modern models (Mod. 501A) incorporate 2 fibre washers in the central area. I believe that these washers act as a spacer to facilitate the operation of the central button (cursors).
Note the presence of 2 fibre washers just below the central rubber (the cursors).
Copper tracks can be cleaned with alcohol. Rubbers must NOT be cleaned with liquids containing alcohol or solvents.
A trick to remove dirt and dust from the rubber parts is to use good quality adhesive tape that does not leave a sticky residue. If the contacts are damaged, you can use conductive paint.