Thank you very much everyone for your answers. The truth is that I was surprised by the sound quality of this A5000. I really like how it sounds despite this quality issue.
As I am a restless person and have some experience after 35 years using synths, I have studied various youtube videos of people opening the SX900 and A5000 and have decided to investigate myself the cause of the problem. I don't want to return this keyboard and have to wait months again for it to be in stock. This was clear to me. Either a mounting fault or the key was heat warped or whatever.
It's easy to open and I haven't had to desolder any wires to isolate the keybed. Just pull a connector and set aside a cable route. I have done the process by very carefully disassembling the screws because they are very small and taking photos of each step to reassemble it exactly as it was.
The cause of the problem is what I suspected and I have seen it right away. The worker forgot the screw that holds the B low and adjacent key. It's a human error and I have nothing against it. What seems absolutely inadmissible to me is that in the final quality control of Yamaha Indonesia they let a 2,000 euro keyboard pass with a key like this.
Fortunately, the screw is the same as the ones used to close the case and I have been able to add it without any problem. I have already ordered the missing screw from amazon to complete the closing of the instrument, although this is not important since it has many closing screws.
Happy ending despite everything. The keyboard is perfect now and I'm going to enjoy it a lot.
Greetings.
