You need to make sure the jumperblockas JP1 and JP2 (these are jumperblocks of 3 on each and is located on the scandoubler) are set correctly dependant on how many monitors you have and how you connect them to the system. The scandoubler manual stats the following:
1 Monitor system with monitor connected to:
Scandoubler: JP1 Closed, JP2 Closed (factory default setting)
Cybervision: JP1 Closed, JP2 Open
Amiga: JP1 Open, JP2 Closed
2 Monitor system with monitor connected to:
Cybervision & Scandoubler: JP1 Closed, JP2 Open
Cybervision & Amiga: JP1 Open, JP2 Closed
Scandoubler & Amiga JP1 Open, JP2 Closed
3 Monitor system with monitor connected to:
Cybervision & Scandoubler & Amiga: JP1 Open, JP2 Open
