Merc.....When I purchased my first Lowrance LMS 520C unit, it had a small problem reading very shallow depths in thick weeds....I went on the BBC Board Forum site( http://www.bbcboards.net/zeroforum?id=13 )and everyone there said to contact Linda Colt...I did via email and she sent me out a Brand New Unit with NO money exchanged or even my credit Card for deposit, only on trust. When I received my new unit I put the original purchased unit in the box that came with the replacement which also has a postage free sticker back to Lowrance via UPS....now if anyone can beat that I would love to deal with that company also....Humminbird screw me years ago with a flasher unit...but that's another story all together.
Edited to add:
Try this site....they are VERY helpful for electronic, etc....this one is specifically for "Birds"...
http://www.bbcboards.net/zeroforum?id=279