Thanks.
I would rather not acquire a game console to test out the converter, though, but that is a good way rule out my *Amiga* as a source of the problem.
In addition, everything I read about game consoles says they use a lower resolution signal than the Amiga. To me that would seem to introduce even more incompatibility.
At this stage, I have asked the seller for a return. If the replacement unit does the same, not sure what to do next.