Hi shneurslonim,
is your SX900 a NEW device or a USED one?
If it is NEW, I would not "experiment" with the AC power adapter, but simply return or exchange the device (the complete keyboard).
The original power adapter of the SX900 is the Yamaha "PA-300C". It provides a stabilized voltage of 16V and can deliver a maximum output current of 2.4A (= 2400mA).
Of course you can use another adapter, but it must have the same specifications (output voltage 16V stabilized and must can deliver at least 2.4A output current). Very important is also the polarity of the DC plug: The positive (+) pole must be in the CENTER. (Please see also the attached pic of the "PA-300C" specs.)
If you are not shure if the original power adapter works properly, you should ask a technician to check that the adapter provides the correct 16 volt output voltage.
Best regards,
Chris
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