I have an Atari 2600 and a Sinclair ZX Spectrum, fully working.

I want to connect them to a standard PC monitor, which seems to be a particularly good idea as I have also handy a very good up-scaler:
XRGB-mini Framemeister

So, I only need to convert the "antenna" audio/video output of the two old machines (which is physically an RCA connector) to
either
S-Video + RCA Audio L+R (well, these are mono devices...)
or
RCA Video (yellow) + RCA Audio L+R

How? I can't seem to find such a converter. Yet, that's something any old (and new) VHS VCR could easily do! I really do not want to use an old VCR for that only. I am hoping to simply find a small box that lets me do that.

Do you guys have any suggestion?