Some more ideas:
- Emulator(s). There must be one for ZX81. For spectrum, i believe a program existed that digitized audio casettes through sound card.
- (Very unlikely) Some years ago, PCI cards appeared that included whole 8-bit computers on them - but I guess they would be difficult to find/expensive...
- In light of AVIL's post, maybe the RF output of the ZX81 changes impedance under load (when connected to non-TV input) and overloads the poor Z80? Yeah, a longshot, but you seem experienced in electronics and maybe you can check...
GL
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