From the British Sinclair Amateur Radio User Group Newsletter done by Paul Newman, G4INP. (Reprinted with permission). Here’s a nice system, complete with TU, AFSK, and interface. We are only printing the ZX81 program, send SASE (to W4GHV) if interested in the Spectrum version. A PC board is available from G4INP. The British price is 6 lbs, 50 p (whatever that means), write him for US prices. Note that OZ1BII’s schematic is copyrighted, which means you can build your own for experimenting, but you cannot (legally) make them to sell to others.
The Authoritative Website for Timex/Sinclair Computers