It is possibly the modulator, yes, but could be other things.
Do you have an extrenal modulator (A520) from an Amiga 500 you can connect up and try?
Failing that you could try the composite out jack, though I am unsure if this is just B&W on an A600.
If this proves your modulator is faulty, I would get a SCART cable and connect up to TV that way (if you are in Europe)
Good luck.
