This is an old post, preserved for reference.
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What do you get if you cross a ZX81 with a ZX Spectrum? One of these:
TS1000. That was just a rebadged ZX81, the same issue 3 ZX81 board in a ZX81 case (with a bit of extra RF shielding). This is much more than that. It was only released in the United States, so is a but of an unusual sign here in the UK.
I thought about it for a while and decided the best option was to remove the MC1374. That would disable the rest of the circuitry in there, as well as providing convenient points to pick up the 5V supply and video inputs. It also leaves the option to revert to RF at a later date, should anyone want to.