I'm going to give you a little tough love here, I hope you don't take it the wrong way.
Replace that unit with a new one, don't be foolish and throw good money after bad. I just replaced the 13 year old pair of Raymarine e90's on my flybridge with a pair of garmin 942xs. They are not that expensive and I think you can find the 742xs for under $1,000 if you look around. God what a difference, I should have done it years ago.
I sent parts of my Raymarine network (Radar, DSM, the e120 at the lower helm) back to Raymarine a few times for repairs and probably spent $3,000 keeping them running. It wasn't worth it.
Bite the bullet. Buy new stuff. Or at least barely used recent stuff.
Just my 2 cents.