I do not think the modulator is (or was) faulty (I had already swapped it) it is simply the lack of Back Porch that makes Black on white unusable (and of course the perfect values for a NO modulator fitted board that stopped it working probably). So the original was probably OK but, I switched it for one on a Spectrum Issue 2 I know is good, that has had a wire Composite bypass (which I reversed) so does not need a working one if the original indeed proves faulty.
Anyway tonight I miniaturised my INS8154 eliminator allowing a switch for Graphics/Text and for Invert video anyway to experiment - I have one spare switch I can use to vary something else (the green one)...
For those who wish to repeat the demo from the video here is the FallingMan as far as I have got - I am intending to replace the jumbled man bits above the horizontal line with a picture or the text Falling Man - 1979 if I can work out the bit pattern....
(just delete the .TXT and restore the last three chars for the file extension)
FALLMANASM.TXT
FALLMANHEX.TXT