Hi.
I'm not any repair guru or engineer, but just thinking loud when see your post.....
Just to eliminate all other possible causes outside the BK.
Did you disconnect all other equipment and plugs totally from the BK, and play only the keyboard, and use headset to see if it is the same situation?
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Cheers 🥂
GJ
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"Success is not counted by how high you have climbed
but by how many you brought with you." (Wil Rose)