This PCB provides composite video for the Sinclair ZX81 / ZX80 / Timex 1000, it replaces the internal PCB from the modulator. 

For ZX81's with earlier type ULA's it generates a back porch signal.

It will work with the later 2C210 ULA. However, this ULA does provide a backporch signal, so the 555 timer chip is not needed and can be removed.