I have that manual. Its copyright is 1990 ... so that should be about right for the "age" of your unit.
If you are going to keep the unit and try to make it work correctly - I'd be more than happy to send you the manual. Just click on my screen name (crappiepappy) on this post ... then click on "send e-mail to crappiepappy" ... my e-mail address is listed there. Just click on it and you're good to go. Send me your mailing address, and I'll get the manual in the mail to you, in the next couple of days.
NOTE ... if the screen is cluttered with pixels, regardless of the sensitivity setting (& especially on low sensitivity setting) -- or you have "vertical lines" of "missing" or "washed out" pixels (looks like someone "erased" a line or two, or more, of the screen picture) ... you're unit has probably seen its better days. In which case, you'd be better served in getting a new unit (and don't forget to check with Lowrance and see if they'd take the old one in on trade for another Eagle or Lowrance model ... 1-800-324-1356 -- and you might tell the "rep" you talk to at this number - what problems your unit is experiencing ... they might be able to help you solve the problem. They would, also, probably be able to tell you if they would take the unit in on trade and how much discount it would bring. I doubt they are still servicing these units.) ....... luck2ya .......cp