Just some comments from the guys on Facebook who watch this feed:
"The 75 ohm vs 300 ohm situation shouldnt be a problem, this is just referring to the 'impedance' of the feed line and system, worst scenario is that you will have a mismatch and would really only affect weaker signals. As the signal from the modulator is pretty strong, and generally stronger then the average TV signal you shouldnt have issues. Im reading between the lines here and wondering if you are trying to view the picture on the wrong frequency? Hence the snowy picture. If you have a UHF modulator in the Sinclair, that would have an output impedance of 75 ohms generally, and if you were trying to view the picture on one of the American VHF channels, American systems are generally 300 ohms impedance. Now the modulators will generate harmonics and yes you could receive a very weak signal on a VHF tv from a UHF modulator, and you could also receive a signal from a VHF modulator on a UHF frequency, but being a harmonic, it would be weak and give the results you are getting. You need to ascertain what the modulator is and its number, and what band the tv is tuned to VHF or UHF.....Hope that helps
.... little bit of a search later...The Timex version was Ch2 VHF, the Sinclair version was Ch36 UHF...I think youve tuned to a weak harmonic and not the main signal."
Personally my bet is that the ZX81 is missing the back porch latch and so the TV cannot find a strong enough signal (this is what normally results on the black text on a black screen) - older black & white TVs did not have a problem with this, but modern TVs do. If so, the composite video mod will not help and you need a ZXVid Module, or a ZXCCB.
