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If you have a Main Board Error and gross symptoms like that, try disconnecting everything from the Roland D20.
Turn it Off and unplug it for at least 2 hours, or longer.
Then at the end of that time, plug it in again, and turn it On with nothing attached to it.
If the loud sounds repeat and you again cannot initialize, then the Roland device is badly 'broken', has severe problems in control circuitry and initialization.
It will require repair (unless you would rather replace it.) So how old is it? and, what is your local zip code?