Also looking for the schematics so I can repair this board.
The full-volume hum suggests that one of the supply rails was getting through to the speaker. If D4 is a 'catch' diode to prevent inductive emf from damaging the amplifier, it would present this fault if it shorted. Being an all-in-one unit (i.e. no separate speaker/amp), there is little one could do as a user to avoid the problem. It sounds like a simple component fault, and should be warrant-able if the unit is still under warranty.
The unit could have been getting bad power. A surge protector / power conditioner might help prevent another problem.
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