This low cost plastic musical keyboard worked on batteries but not using the external power adaptor.
The fault was the small printed circuit board which holds the audio amplfier and several connectors. An extrnal force applied to the power connector had broken a corner off of the PCB, and detatched two of the plastic mounting posts. The siolder connectors for the power connector were also defective.
The damaged PCB resulted in one of the speakers not working, and left the PCB loose in the case.
Repairs involved gluing the PCB back together, adding a wire to repair the damaged track, and resoldering the connections for the power connector.