Check if the power connection block on the right rear to the AC adapter has a cold solder joint by pressing and wiggling the connector while trying to start up. Also wiggle and really seat the IEC AC cord into the adapter brick, there are a few particular power cords in my house that are a loose fit on the power pins of the brick and fake me out sometimes. If you have two adapters swap in the other and try it. My three blackbirds will start from AC alone with both batteries removed, and even if I fake out the cover closed switch to pretend that it is stuck. If you have test gear hold the open power connector end slot down and see if you get around 14.3VDC across the top pair and 16.6VDC down the right pair. Then if necessary pull the keyboard and check/clean/resecure the flex connections. If still no joy you might consider a keyboard swap from your other blackbird as a test.