This usually happens when you use the wrong power adapter.
My laptop and Uniden adapters look almost identical, so a couple times I've accidentally plugged the laptop one into the 396T. It will either say "Illegal Voltage" when you try to use the right adapter, or not work at all, and runs fine only on batteries.
You will have to send your scanner to Uniden Repair. You will be down without your scanner for a month or less, but it's a flat rate repair service and it's done right.