It's not a standard wall-wart by any means, and might even be a faulty electrolytic
smoothing capacitor inside the mixer rather than a problem in the PSU.
don't get any joy sorting out the problem, and since repairs can sometimes cost more than
the value of the gear with old stuff like this, I've got a Spirit Folio 12/2 that worked
perfectly last time I used it, but that's currently stored in its original box, surplus to
requirements, and not likely to get used again.
The Spirit Folio 12/2 saw me
through many years as a keyboard mixer before I upgraded.
Drop me a PT if
you're interested in it N*Joy.