Open circuit detector for multipoint recorder

Apparatus for detecting an open input circuit in a multipoint recording system having a plurality of input circuits which are sequentially switched in a predetermined time phase relationship with an a.c. power source to sequentially connect said input circuits to a servo-operated measuring circuit through a low-pass input filter, comprising:

• a pulse generator for generating a train of short pulses at the frequency of said power source including,
• means for connecting said generator to said a.c. power source so that said pulses are phased to occur only when one of said inputs is connected to said measuring circuit,
• a clipping circuit for providing limited amplitude square wave having the frequency of said power source, and
• means for differentiating said square wave to produce said short pulses;
• means connecting said pulse generator across said input circuits when said input circuits are connected to said measuring circuit so that the magnitude of the pulses generated at the output of said generator increases in response to an increased impedance across the generator output when the connected input circuit is open circuited;
• and
• a pulse magnitude detecting circuit connected to said pulse source, said detecting circuit being operative to produce an output signal when detecting said increase in said pulse magnitude indicative of an open circuit in the input circuit connected to the measuring system.