Re: Hopefully hopping
Probably the "ear" socket, they had an odd "ear→ear, mic→mic" convention. (Least I thought it odd, I normally think of Line Out → Line In)
I have a Timex Sinclair 2068 which is basically a ZX Spectrum with some other bits, and a NTSC video encoder (since it was for the US market), and I recall this being mentioned in the handbook (which I also have somewhere).