I have never seen any place advertised that works on finders. When I have talked to folks it's because they can't buy the parts to repair. You might go over to BassBoatcentral and ask. If there is anyone out there they would know. They have a dedicated Humminbird page. And maybe Rickie will see this and he might have some info. I know you don't want to hear this but if they did repair them you could probably take that money and a little more and buy a good latter model unit the way prices are falling with all this new stuff coming out.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979