Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I think that you should try powering it On with nothing but the power hooked up to it, after giving it 10 minutes Off to cool down (or longer).
Then, see if, with nothing hooked up to it, it does or doesn't show the lines over the display.
- if it does, you have a defective Raymarine C120 display - the LCD or support circuitry or maybe video memory has a defect. After 12 years small wonder - that's a good lifetime for a device.
- if it doesn't... leave it On 10 minutes, long enough to heat up. Then if it still doesn't show lines, shut it Off briefly, and hook up one of the disconnected connections, and try it again On for 10 minutes... and so forth, until you either determine that one component is at fault or one hookup causes the problem, or else you determine... there's no real problem present, it was just temporary? well I hope it turns out that well. But go through that procedure, please.
Let me know how it turns out.
(And if this answers your question in full, please rate my answer - though I would like to see this case through to a good conclusion and am in no hurry with it.)