I took it apart completely.
The defrost system is similar to some commercial equipment used today. There is a probe that attaches to the coils, when that is satisfied, it releases a solenoid to allow the hot liquid to circulate through the evap.
It's quite simple, but the problem is once you have a problem, you need to open up the system. In my case with my compressor failure, I couldn't even swap out the compressor. It is very specialized. All I could have done was convert it to a more modern system but I simply did not have the time to do all that.