Is it the receiver or the smartcast head itself? I was an early Smartcast adopter back in the day (at vast expense at the time) and went through several 'floats' as the batteries didn't last long and were sealed in. I dug it out a couple of years ago (with the intention of selling) but replaced the depleted float with an aftermarket version ('Lucky' Transducer) that had a watch battery screw cap access - success - it worked perfectly and I sold the unit.
One like this -
https://www.ebay.co.uk/itm/17474665...mkcid=28&google_free_listing_action=view_item
Would def recommend Deeper these days if using with rod and line / mobile phone (they're sketchy on bait boats however IMO) . . . much better range / casting weight and quality than ye old Humminbird. Would def mount phone on a storm pole rather than the rubber rod mount - you're only one cast away from losing an expensive phone!