The soundboard of our 2007 Yamaha C2 appears to be in good condition; I do not see any cracks, nor any separation between the glued individual boards that run diagonally. Is there a problem when you can see a very slight indication in the finish as to where the boards are joined? It's hard to keep the humidity from fluctuating when it bounces between 75% - 99% outside for long periods of time during the summer in Connecticut. We have central air, but it doesn't run constantly. We keep the temperature at about 76-78 degrees F. We also have a Dampp-Chaser humidifier/dehumidifier installed. This degree of concern is probably just my paranoia. ;-)

My 40 year old Kawai GS30 also has what I'll call pressure ridges where the boards join. Crown is still good and no cracks. Basically don't worry, it is what it is and your doing all you can to minimise things.

Sydney Australia
Retired part-time piano technician