The dim light is a natural phenomenon that's happening when the backlight gets older.
I got in contact with a electronics supplier and they thought that it would be possible to change the backlight. I will give it a try and at the same time get some info how complicated it will be.
As soon I have tried this I will reply to this forum and let you know what's happened.
I am afraid that this is not a every-mans-thing to do and I will strongly advice someone to do this operation whitout some formal electronics training. The worst thing that can happen is that you damaged the display and whitout the display the synth is useless.
So keep looking and maybe I can get back with some good news for you and maybe also a schematic layout how to fix this thing.

PEO!