Fixing the defrost circuit is relatively simple. It made of three things, the defrost heater, the thermostat and the timer. The usual parts that fail are the heater and the timer. The first thing to check is the defrost timer. It looks like this:
It can found either near the compressor or in the little box inside that controls the temperature. What it does is that it powers the compressor for about 8 hours and then turns off the power for 45 minutes to allow the defrost heater to melt the frost buildup. The timer is run by a little motor. Often the motor stops working and the freezer is either stuck running the compressor continously or the defrost heater.
On the side of the timer is a little cam that you can turn with a straight blade screwdriver. Turn it until it clicks. When it does, the compressor should shut off and the heater should start to heat up. If you hear the crackling of ice melting, then the problem is with your timer not working. If the ice is not melting then the problem is either with the heater/thermostat. Check to see if the heater is burned out or use a meter to check continuity between the heater and the thermostat. If it is open then replace the heater and thermostat.
Any of the defrost parts should be available from an appliance supply house or online. Hope this helps you.