Alesis QS7 issue

[craigw]

I just purchased an Alesis QS7 on eBay. When I received I hooked it right away. The unit produced a kind of static howl that slowly disappeared within a minute or so. During this initial minute I was able to get the selected program to sound, but it was very soft and barely audible through the static like sound. Once the initial minute passed, the keyboard seems to work properly. I was able to reproduce the sound later when I returned to try again. It also was produced via the main outs and the headphones.

Any ideas? I don't want to return it because this particular eBay seller has not been the easiest to deal with, but I will if necessary.
Thanks for any help.