501F freezer has ice accumulating beh...

It can be one of three things. Unplug and use an Ohm Meter to measure the resistance of the defrost heater that is mounted on the evaporator coil in the freezer compartment. Then disconnect the defrost heater and measure with ohm meter. It should measure in the 20 ohm range. If it measures as "open" replace the coil.

Next suspect is the defrost timer mounted in the front of the compressor compartment. It's tough to test, so I'd just replace the timer.

Finally, there is a sensor mounted on the top of the defrost heater that shuts off the defrost heater when it reaches a high temperature. If the coil tests OK, and you are going to replace the timer, you may want to replace the sensor too.
It is awkward to test, the temperature of the sensor needs to be be below room temperature to close. The cost of the sensor is low enough that you may not want to replace the timer, then test the system, and then have to go back in to replace the sensor later.