Resurrecting A ZX81, ZX Printer & Spectrum

[1024MAK]

If you want to prove that the problem is the TV not liking the non-standard signal from the ZX81 / ZX Spectrum, and you have, or have access to an oscilloscope, then that will show you the video signal waveform.

Alternatively, you could just monitor the composite video signal with a multimeter on the 2V DC range. But keep in mind that unless it is a good quality (meaning expensive model) type, it will only give an average indication, as it will be far too slow.

Some TVs are far more tolerant of out of specification signals on the aerial input (VHF/UHF) input compared to the composite input

Mark