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We have experimentally investigated on the effects of optical feedback on the performance of monolithic colliding-pulse passively mode-locked semiconductor lasers operating at 60 GHz, designed to be efficient sources of millimeter-wave electrical signals. The characteristics of the optical-to-electrical converted signal are investigated for a long and a short external cavity by means of an external photodiode and by using the saturable absorber of the device as an intracavity photodetector. For a power feedback ratio larger than 10-3, the linewidth of the millimeter-wave signal is severely broadened with respect to the value of 230 kHz measured in unperturbed conditions. We also report that optical feedback causes central frequency shift and instability, and a reduction of the useful colliding-pulse mode-locked operating region in the biasing parameters of the device.