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Replacement parts for MAYTAG MZD2768GEB Ref - SXS/I&W | AppliancePartsPros.com
Here is a service manual
http://www.servicematters.com/maytag...s/16010154.pdf
Try forcing a defrost cycle, see Page 2-15 in the manual.
With this unit it may be just as easy to check the defrost heater and defrost thermostat with a meter. Then if both are OK assume that the defrost board is the problem.