Abstract: 1. An electrical system for checking the operational state of a plurality transmitters comprising,A. signal generating means,B. signal applying means for sequentially applying the output of said signal generating means to said plurality of transmitters,C. means for radiating the output of each of said transmitters,D. means for receiving the radiated energy,E. a coincidence detector,F. means for connecting said coincidence detector between said signal generating means and said receiving means, andG. an alarm circuit connected to said coincidence detector, whereby said alarm circuit will be actuated in the absence of a coincidence condition in said detector between the signal received from said generating means and the signal from said receiving means.