This site has many requests about Samsung models with same problem - refrigerator not working (warm, icing) and freezer OK. But there most haven’t reported that problem is solved.
I have replaced a control board since the defrost timer is part of it. I have ohmed every sensor, heater, compared resistance values between sensors in both sections (refrigerator and freezer) at room temp and when 38 deg C.
The refrigerator still continues to build up ice on rear wall. It got a bit better since there more defrost cycles taking place but they are short in duration ( few sec each). When cycles starts, it clicks loud and producing a humming noise for few seconds. Sometimes ice on rear wall turns into water and the surface is wet. This water ends up on bottom floor. The fan is working right.
Next thing is to try to change switching relay.
Any further suggestions in troubleshooting?