How can i fix it?
you need to provide above 1 and below 3 Volts on pin 16, on TV SCART socket.
usually, people use simple voltage divider, with resistors. Amiga video socket has both +5 and +12 Volts, so you can choose, but be careful.
I'd first check the cable if it already has something on pin 16.
its used to automatically switch TV to RGB (component) instead of CVBS (composite).
older TVs didn't need it, as you could select RGB with remote.
perhaps you're lucky, and can select it with remote too, check TV manual.
here is a nice explanation of blanking signal:
http://www.worldofspectrum.org/forums/showpost.php?p=717712&postcount=8oh, there is a possibility that your TV simply doesn't have RGB SCART, only CVBS.
if so you're SOL.