Nasty RGB problem

[weezee]

Hi to whoever might still be interested in Dreamcast.
I have a nasty RGB problem: my RGB cable supposedly broke, so I ordered a new one, but when I got it, it didn't work. So I said it to the webshop (Console Plus, UK), and they've sent me another one NOT working. Now I have received a 3rd one still NOT working which they assured to have been manually tested because of my complains. Now, my TV works with RGB cables of XBOX/GC/PS2. So I suppose it is not broken in any way. And when I use the normal cable, my Dreamcast can be seen on screen, so it's not broken either. My TV and my DC are "ok".
Now I wonder: is it possible that my (PAL) Dreamcast is broken in some way that allows me to use normal cable but NOT RGB cable?
Or is it more likely that the 3rd RGB cable is broken too and they did not test a heck?

I have tried this on 2 TVs with same results, normal cable = OK, RGB cable = blank screen/no sound