A wind turbine diagnostic device for diagnosing mechanical damage to generator components of at least one wind turbine, comprising at least one speed sensor for determining a variation over time of the rotational speed of a generator of a wind turbine, the speed sensor having at least one speed signal output for outputting the determined variation over time of the rotational speed, a frequency analysis module and a frequency spectrum signal output for outputting a frequency spectrum, the frequency analysis module determining a frequency spectrum from the determined variation over time of the rotational speed, and a comparator element for comparing a frequency spectrum with a prescribed standard frequency spectrum and for diagnosing mechanical damage to generator components on the basis of the comparison. This increases the possibility of predicting mechanical damage to generator components caused by vibrations.