[setarip_old]

1) Have you done this testing only WITH the switchbox included, or have you tried excluding the switchbox and directly connecting with what, if I remember correctly, is a composite connector (that connects to the switchbox at the top)?

2) (Unrelated to 1) above) Can't you have it set up "normally", so that the Sinclair's output is connected to your television and your television's composite output (If it has this) or RF output is connected to your VCR's composite input or RF output?

3) (Also unrelated)

When the VCR RF pass through switch is set to TV, the ZX-81signal appears on the TV set, but when it is set to Video, the black image appears instead of the ZX-81 output.

Did you set BOTH the television and the VCR to Ch.3?