A system for diagnosing the condition of mechanical equipment, and particularly the condition of bearings in rotating machinery. Vibrations from the mechanical equipment are detected by a wide range acoustical detector. A sweep voltage generator causes a voltage controlled oscillator to scan through a bandwidth of interest. A synchronous detector receives inputs from the acoustical detector and the voltage controlled oscillator, and produces an output whenever the frequencies of the two signals match. A recorder having inputs from the synchronous detector and the sweep voltage generator charts the amplitude of the detected vibrations as a function of frequency. After a period of time has elapsed and it is desired to check the condition of the bearings, a second recording of amplitude of detected vibrations versus frequency is taken. The first and second recordings are then visually compared for significant differences which would indicate changing operating conditions of the bearings.