Burnt Diode in MIDI Keyboard?
Without the back of the PCB I can mostly guess, but from the components and damage it looks like something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

DCHARRED is there to protect against reverse polarity conditions. It is really only effective if there is also a fuse that will blow when something is connected in the wrong polarity, because the diode will then conduct as much as the power supply can deliver, and will quickly burn out if nothing stops the current.

In this case, your power supply was stronger than the diode.

Removing DCHARRED, cleaning the area, replacing with another diode of similar performance (I guess anything 3A or higher should do), should make this work well again.