forgot to mention, this should be a PAL signal (Europe/Netherlands)
i have this working now, reasonable picture quality (ok, colours,
sharp enough to use the menu/mp3 player which is what it wanted; no
snow really but does distort a bit)
encountered more of the same probs (bad colours, negative, no sync),
can't reproduce consistently
tried different video outputs on the player (beside 'video' it also
has 'coaxial' and Y, Pb/Cb, Pr/Cr outs) have a scart breakout, video
seems to be the composite signal.
also i wonder if the player was giving the wrong output (NTSC, PAL 60?
or something in the video mode..)

could be inferiour hardware, cables... the cinch cable i use prolly
has been discarded for a reason. or the channel was wrong and it was
tuned to some 'echo' (don't know how to call this)
i really don't have much to test it with (just the player, the tele
and some wires, breadboard, batteries and leds)
my first thoughts where i was overdriving something or that i should
not mix the shieldings/GNDs.. but i'm really just an amateurs that
figured the shiny box in the VCR did what it appeared to do, modulator
video to antenna, and it does

thanks for the comments. i don't think i can improve much on this
without getting measuring devices etc.

(note that the metafilter.com thread is from another guy, though the
modulator is somewhat alike...)

regards, Berend