So I finally got myself to restart my long-dormant hobby, playing the piano. Back at folks I would have a real piano that I would have permission to move into my place, but I have not bothered with it - yet. Instead, I have an old (20+ years) but good Roland KR-500 keyboard (complete with stand and pedals), but even that has been unused for a few years. Alas, some keys do not work unless I use more Force than Darth Vader. So I start googling for the nearest Roland service centre's contact info, wondering whether they still have the spare parts and know how to repair my 'board.
By sheer luck, I run into a DIY article (or a forum post, rather) detailing how to (try to) fix keyboard keys that are mute, play only very loud or very silent, or only intermittently, or somesuch problem. It says that the most common reason for these are simply dust and small pieces of trash that have gotten on the electrical connections. And luckily, the poster tells how he did the trick for an old Roland.
I ponder for a few hours, but then decide that at least, I can tell the service centre if I discover some de-solded connections or visibly broken parts. Screwdriver at hand, aided by a single forum post and a couple of YouTube videos (of a totally different model with completely different key mechanism, as I later find out), I plunge into the deep end.
Three hours later, dust and foreign objects have been removed, the keyboard has been put back together and works at least mechanically.
Ignition.
IT'S WORKING! The problem keys sound now as they should, and I just saved myself a couple of hundred euros.