I bought this keyboard second hand, it's a pretty cheap generic plastic keyboard. It was demoed to me briefly and seemed ok. But when I took it home I realised there's quite a high pitched whine coming from the speakers.
I plugged some headphones in but the same problem (only now through the headphones).
I've tried attaching a ground loop isolator to the headphone port, and tried a different AC adapter, but neither fixed it.
Is there anything I can do about it?