Volt Ohmmeter test indicated defrost thermostat and heater were OK. Defrost voltage measured ~25 VAC. I ordered all the most likely repair parts, but thought the control board had failed. I replaced the control board and the freezer has been working properly for several days. I haven't decided whether to keep or return the unused parts. I damaged the paint removing the electrical control. There must be a trick to protect the paint from the putty knife and screwdriver.