Updated 8 Feb 2019
This document only relates to the ZX Spectrum 128K ‘toastrack’ and the grey model ZX Spectrum +2.
THIS DOCUMENT IS NOT RELATED TO THE BLACK ZX Spectrum +2(A/B) or +3!
Underneath is the currenly best way of creating a proper signal for RGB BLANKING (SCART pin 16) from a ZX Spectrum 128K (toastrack) or ZX Spectrum +2 (grey model).
RGB BLANKING will make TVs automatically switch to the RGB input of the SCART connector.
Here is the page of the ZX Spectrum 128 manual with the RGB connector pinout (click).
Here is the current version of my ‘ZX Spectrum video solutions’ document (click).
Best method of getting a proper signal for BLANKING from the RGB socket
The best option is to use the Composite Video (CVBS) signal available on the RGB connector, with an added 220uF capacitor between pin 16 and pin 18 (ground) of the Scart connector, to stabilise the signal, making it suitable for BLANKING on many TV’s.
NB: most of the times with this cable solution there is NO INTERNAL MOD required inside the ZX Spectrum!
Some people modify the composite video output pin of the ZX Spectrum RGB connector to carry a voltage that is suitable for RGB blanking. like described on this page (click).
I don’t think it can do any harm, but as said, most of the time it’s not needed anymore.
Where to find a correct cable?
The best RGB cables can be obtained at Retro Computer Shack (click).
Credits go to Ian Priddey from Retro Computer Shack (click) for updating me with this solution!