Ill stick to PS1 troubles for now, when i hooked up my ps1 with rgb scart some solid colours had a very annoying chequerboard like stitching pattern to it like pic related. I thought switching to a special luma sync scart cable would help but occasionally the patterns remain. Switching to composite eliminate the problem but the overall sharpness and quality suffers. Anyone know a way around this?
A break in a connection/wire. Something not being grounded properly for example. Pretty common especially with modern 3rd party cables that are cheaply made and put together. You can test this out with a multi tester
Pretty easy to Google but they are used to check current so of I put the multi tester points at either end of a wire I should be getting a voltage reading if not there if a short in the wire.
I wouldn't use an HD monitor to gauge whether an rgbs signal is working as they will scale the image on the fly and crap it up even if it's perfect. Unless you're using some kind of upscaled box which could also be the issue
when composite is used as sync for rgb, if it's badly shielded it bleeds into other signals, producing that pattern.
shield the composite wire, or use luma as sync, as it's black and white and is less poluted for sync purposes.
oh wait, you already have a lumasync cable.
If it's PS1 then it migh be PS1 general dithering.