I had previously replaced the defrost heater in the Samsung refrigerator compartment when it iced up, but 5 months later a repeat occurrence. i called an appliance repair man--not the one from Samsung, who didn't have any parts anyway. He took a look and suggested that the thermo fuse might be shorted. It wasn't, but he noted water vapor (very small drops of water) inside the clear seal tube. BTW, the new fuse came wrapped in mylar, so you can't see inside!
He said he always replaces the fuse when he replaces the defrost heater, and this is a common samsung repair.
Since he did not have the right fuse for this model, he said I might as well order and install it myself. If it doesn't solve the problem long term, I'll call him back. So far so good, but it hasn't been long enough yet to tell if the problem will reoccur.
Notes: Directing a fan at the bottom of the evaporator cover panel accelerates defrosting. There are six screws, including two quite recessed and two under the light panel covers. Once the panel is defrosted enough to remove, directing the fan at the evaporator until it is totally defrosted helps to ensure that the defrost cycle resets properly when the refrigerator restarts. Pouring a little water down the drain helps ensure that the drain was not clogged. Since these units seal around the evaporator coils, they should not normally have clogging in the drain unless the refrigerator has been in a situation where mold can grow.