These units had a problem with the defrost element being too small. After a while the element burns out and there is no defrost.
There is also a thermal switch on the evaporator (which acts only as a safety) that could have opened up; but, most likely the heater is burned out.
The frequency and duration of the on the defrost cycle is actually determined by the control PCB.
There is a retrofit element available for these units. The new heater actually has two heaters replacing one. This will, most likely, solve your problem.
maddie580, November 2008