Scart signal isn't a thing. Scart is just a plug, which can carry Composite, S-Video, and RGB. Putting Composite in a Scart adapter won't magically convert it to a different signal, it just brings your Composite signal to the correct pin on the Scart plug, for your TV to display Composite through Scart.
Do you have a continuity tester? If you have a multimeter you have one. Try to test from the console end of your scart cable to your BNC end. Even better if you can identify the pins.
If you can't do that, you have to assume that the wiring was done wrong, and bruteforce all the different plugging possibilities. I suggest you identify which cable carries the sync by having one plugged to one colour and test each three others on sync. it should get you a monochromatic picture at some point. If none of your cable seem to carry Sync, either the wiring was done horribly wrong or you're having Composite on your Sync (which is fine for TVs but PVM can be finicky).
Obviously I'm assuming your display works fine.
Do you have a continuity tester? If you have a multimeter you have one. Try to test from the console end of your scart cable to your BNC end. Even better if you can identify the pins.
If you can't do that, you have to assume that the wiring was done wrong, and bruteforce all the different plugging possibilities. I suggest you identify which cable carries the sync by having one plugged to one colour and test each three others on sync. it should get you a monochromatic picture at some point. If none of your cable seem to carry Sync, either the wiring was done horribly wrong or you're having Composite on your Sync (which is fine for TVs but PVM can be finicky).
Obviously I'm assuming your display works fine.