Try putting it into TAPE mode and check if anything shows in the screen!
Mine had this problem some time ago..!
To: korgpolyex@yahoogroups.com
From: wrezal@...
Date: Wed, 21 Aug 2013 18:08:10 +0000
Subject: [korgpolyex] Re: Have I fried my Poly 800 mk1?
Just thought I would add the power supply I am using is: 300mA unregulated AC/DC adaptor. Also checked I was using the right polarity, which appeared to be working fine. Also works on other devices I have such as a small little casio keyboard and a bigger yamaha one.
Thanks again