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Hi I'm Brian and I'm here to help!
Let me find the defrost board for that unit and I'll be right back.
Ok, just had to refamiliarize myself with that board. Of course the first thing to do (and I'm sure you've done it) is to make sure the refrigerant charge is at proper level. If you've replaced the defrost board with the OEM board, it is a true demand defrost control and should defrost properly if all other components are ok. Did you replace both sensors? If the outdoor ambient sensor is bad or way out of range, the defrost board will default to time defrost mode and that could be your issue. Check the sensors, make sure they're reading accurately. This Manual shows a temperature/resistance chart in figure 5 on page 5. You can also set the termination temperature jumper on a higher setting and that may help. The default position is 50, you could go up to 80 or 90 and see if that corrects things.