VGA<-->Y/G PbCbB PrCrR HD VD, RCA/Cinch NOT HAPPENING?

Hello,..

I have a VGA signal I want to feed into an older Sony plasma KZ-42TS1E.
The Sony has Y/G PbCbB PrCrR HD VD, RCA/Cinch rca inputs.
I bought a VGA->RGB cable and hooked that up but no cigar,.. Sony won't display the signal.
Can anyone suggest my next step to try?

thanks.
-shawn

p.s. I'm trying to display an ATV (3), via a Kanex ATV Pro converter (converts HDMI into VGA), on the Sony. The ATV and converter are working great (if I hook up the VGA out to a VGA projector it displays fine). Googling VGA to RGB HD VD isn't giving me useful answers sniff.