First I would check the three colors.
Make this test :
1. Remove for example the blue and the red wires... Do you still have a picture?
Yes -> It means that the signal is correct on that wire
2. Remove two others colors ... same question?
If the three colors wires are sending a signal (I think it's the case), I would say your TV cannot fully understand that she have to switch in RGB.
Check the voltage on that specific pin (pin 16, 5V to inject). Note that you can force your TV to switch in RGB by pressing several time the external boutton [->1] on your remote control.
If you get a stable picture, it means that the synchro is fine, don't check that.
If your TV has two scart connectors, only one can understand a RGB signal (most of the time), try two plug on the other connector.
You can also remove the wire that force the TV to switch (pin 8, 12v). On some TV, that make the external channel going mad.
Don't hesitate to try on different TV, you'll be surprised
But don't try on modern TV flat screen... because these new tb TV are able to understand the input signal, and if something go wrong (not exactly a precise voltage expected for example), it displays... a black screen
Not easy for investigate of course
Each time I got problem while using PCB to Scart (or VGA to Scart) connector, I'm doing a mix of these tests ...and guess what... I works ALL the time
I've done this more than 20 times, maybe more.
Good luck
