Sounds to me like a setting on the head unit rather then the chip. I'd rule out the antenna per ReelSlim's comments--if you are picking up a signal which it sounds like you are, then the antenna is no longer on the table as a potential problem.
Sometimes it helps to reset all the options to factory settings on the head unit, then re-initiate the zoom function. That's how I solved similar issues with my Furuno. I can now zoom in on individual pieces of seaweed...:]