My third- (or perhaps fourth-) hand Dino Baffetti melodeon has some pretty impressive basses, but the tonics on the chord side (key-note) have slowly started to sound a bit out of tune. Jon Spiers suggested that whilst it might be a problem with the reeds, the shape of the chamber also affects how reeds speak. It was noticeable that the tuning was fine at low volume, but at higher pressure the tone bent up as much as 50 cents. I took apart the chamber and bellows to see if anything was amiss, but it all looks in order. I did notice that the dust filters on the air vents were looking a bit mucky and that that might be causing the problem:
After a bit of experimentation I found that blu-tack does the job quite well. I put the board flat on a table and pushed blu-tack into the holes. It’s important that the fabric is flush with the table and evenly supported otherwise you will tear it.
Quite a lot of dust came out!
I’m not surprised the basses were having a hard time given all the air-flow constriction they had to deal with!
After doing this, the basses are much, much happier! I’d suggest this as the first course of action if the basses (or right-hand, for that matter) is starting to sound a little off.