Patent GB-1107959-A (GB)
Title: Improvements in and relating to arrangements for simultaneously recording and/or reproducing different signals
IPC: H03F1/54
Filed: 1966-06-02
Abstract: 1,107,959. Automatic change-over of faulty circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 2 June, 1966 [5 June, 1965], No. 24576/66. Heading H3T. In an arrangement for simultaneously recording and/or reproducing different signals on or from tracks of at least one record carrier, in which each signal is recorded or reproduced on or from a track through an amplifier associated with that track and wherein one of the signals provides time information, upon failure of any of the signal amplifiers another one of the amplifiers handles both the time information signal and one of the other signals. Amplifier V1 normally handles timing signals and amplifiers V2-Vn information signals. A pilot tone Hs is additionally applied to the input of each amplifier and is derived from the output by means of resistors Ra1-Ran in series with the corresponding recording heads K1-Kn. It is applied to the bases of transistors TrA1-TrAn biased to act as rectifiers: provided the pilot signal appears across each of the resistors Ra the corresponding transistor TrA is conductive and its collector potential low, so biasing a following transistor TrB non-conductive. If, however, pilot tone fails to appear, e.g. across resistor Ra2 the corresponding transistor TrA2 is non-conductive and transistor TrB2. biased conductive, so that the corresponding warning lamp L2 is lit. Since the emitter of transistor TrA2 is fed via resistor Rb2 it is driven negative by the current through transistor TrB2 so that at a reappearance of the pilot signal rectification does not occur, transistor TrB2 remains conductive and the lamp L2 lit. The collector of each; transistor TrB is connected to the positive supply via a potentiometer comprising resistors Rd, Re and photo-resistor FwA, from which is biased the base of a transistor TrC whose emitter circuit includes an incandescent lamp A. In normal operation, the value of each photo-resistor is high, so that the base of the corresponding transistor TrC is biased negatively and it is non-conductive: if any lamp, e.g. L2, is lit, however, the collector potential of the corresponding transistor TrB2 falls sufficiently for transistor TrC2 to be biased conductive. Lamp A2 is then lit and illuminates photoresistor FwA2, the resistance of which falls and so biases transistor TrC2 that it remains conductive even if lamp L2 is extinguished. Lamp A2 also illuminates a second photo-resistor FwB2 which connects the input of the faulty amplifier V2 to that of the time information amplifier V1. When any of the transistors TrC becomes conductive, it causes a common lamp C to be lit; this illuminates a photo-resistor FwC, whereby an attenuator R1, R2 is introduced into the time-information input to amplifier V1. If any amplifier, e.g. V2, fails and the corresponding transistor TrB2 becomes conductive, its collector becomes more positive and renders the base of transistor TrD slightly more positive, via resistor Rf2 and diode Db2; the impedance of base resistor Rg is high so that this transistor remains just non-conducting. If, however, a second amplifier, e.g. Vn fails, the current through resistor Rg and the base potential of transistor TrD becomes so high that the latter becomes conducting and its collector goes negative, so that diodes Dc2 and Dcn become conductive. Transistor TrC2 is unaffected and remains conducting since its base potential remains positive due to the low value of photoresistor FwA2; the base of transistor TrCn, however, becomes negative so that it cannot become conductive, since photo-resistor FwAn is not illuminated and is high impedance. Thus the change-over of a second defective amplifier is prevented. Diodes Dc1-Dcn prevent the potential at the junctions of resistors, e.g. Rc2 and Rd2, when e.g. amplifier V2 fails, from being transmitted to the junctions of corresponding resistors, e.g. Rc1 and Rd1 as otherwise transistors TrC1 &c. might become conductive. The out-off of transistors TrA and TrC is eliminated by reversing switch S; this transiently connects the base of e.g. transistor TrB2 via a diode Da2 to the positive supply, so that it becomes non-conductive and the negative emitter bias of transistor TrA2 is removed. The current through photo-resistor FwA2 is also interrupted, lamp L2 extinguished and resistor Re2 attains substantially full negative potential so that transistor TrC2 is cut off and lamp A2 extinguished. If the time-signal amplifier A1 fails, photoresistor FwB1 is illuminated by lamp A1, so that its input I1 is connected to the input I2 of amplifier V2: in this case also lamp C illuminates photo-resistor FwC so that the time signal is attenuated.