Correct me if I am wrong, but wasn't the ZX81 a
UK machine? I was under
the understanding that it was sold in North America as the TS1000
(Timex/Sinclair). I'm pretty sure the ZX81 was made in the UK (at least
that's what mine says on the case).
Several years ago found a ZX81 (marked on the case) at a swap meet in
San Diego, it worked on American tv. I opened it up, it did not look
like anyone made any changes.