appears defrost heater staying on too long

hi I am a tech. a tech before me had changed the board in this unit. problem was that the unit stays in defrost way to long melting the food in the freezer.new board didn’t fix it. when I looked at it I noticed that the condenser fan was bad, I replaced it, did not solve the problem. I then replaced the evaporator thermistor, did not solve the problem. I watched a video by a ge tech who said if the fan shorts out it might take out the board, he said the way to tell is by checking the 2 resistors under j2 connector. I checked and there not burnt. I then saw a # of poasts where many people are having the same issues, they have replaced all thermistors, board, defrost stat, and still have the issue. I know that if the evap thermistor fails to shut the defrost off then the board should override it and shut it down. do you think their is a problem with these boards?. btw when the stat does open . compressor and both fans come on and it cools fine. this one has me stumped. any ideas?