Patent GB-1249160-A (GB)
Title: Television receiver
Inventor: FUJIWARA TETSUYA
IPC: H03J7/26
Filed: 1968-12-10
Abstract: 1,249,160. Automatic tuning arrangements. MATSUSHITA ELECTRIC INDUSTRIAL CO. Ltd. 10 Dec., 1968, No. 58749/68. Heading H3Q.  A television receiver is automatically tuned by means of varactor diodes, final tuning being under control of a frequency discriminator, and means are provided to prevent tuning being effected to the sound carriers. Separate " low " and " high " band tuners A are provided, tuning being effected by means of varactor diodes 5 biased from terminal 6: the H.T. supply at terminals 3, 4 respectively is derived from the collectors of a two stable state circuit L, whereby the two tuners are alternately operative. The tuning is swept by the charging of a capacitor C1 through a constant current source comprising transistor TR3 the voltage across the capacitor being applied to terminal 6 via emitter-follower TR2. When the entire sweep has been completed, the voltage across resistor R7 causes triggering of S.C.R. TR6 whereby the capacitor O is discharged: the discharge causes a differentiating circuit to apply a pulse to the two stable state circuit L which reverses it to bring the alternative tuner into operation for the next tuning sweep. If, during the tuning sweep, a signal carrier is intercepted branch I.F. amplifier TR7 supplies an output to discriminator J, which if the correct tuning point is passed, applies a bias to transistor TR1 which causes, it to become conductive and hold the voltage across capacitor C1 constant, so terminating the tuning sweep. The input to transistor TR1 is however shunted by transistor TR4 which is biased from monostable circuit K; normally transistor TR4 is held conductive to prevent the application of bias to transistor TR1 and so maintain the sweep in operation. If, however, a video carrier is intercepted, synch pulses from synch separator F are applied over resonant circuit M to diode D2 whereby they are rectified to apply a D.C. bias to monostable circuit K; this then reverses and holds transistor TR4 non-conductive, whereby transistor TR1 may be biased conductive and the sweep terminated. The arrangement may alternatively be such that the presence of an audio carrier prevents the interruption of the tuning sweep. A switch SW is provided whereby the sweep may be continued if it is desired to tune to another station, its closure applying a pulse to diode R10 which causes it to become conductive and back biasing transistor TR7 and so removing the input to discriminator J. To prevent the sweep being reinitiated by momentary fading of the signal or loss of synch pulses a delaying circuit comprising monostable circuit H may be provided. This normally holds conductive a transistor TR5 in series with charging transistor TR3; when, however, monostable circuit K reverses on the termination of synch pulses, a pulse is applied to monostable circuit H which causes it to reverse and bias transistor TR5 non-conductive for a period determined by its time-constant. For this period, charging of capacitor C1 is interrupted and sweeping inhibited. To prevent delay occurring when switch SW is operated, closure of this switch causes a bias to be applied via diode D3 to monostable circuit H which prevents it from reversing. The Specification describes alternative circuits (Figs. 12, 13, not shown). The tuning point is indicated by means of ammeter M which is fed via resistor R8 from terminal 6, the lower half of the meter scale being used to indicate the low-frequency band; on the high-frequency band a further fixed current is applied to the meter from terminal 4 via a resistor, so that it indicates on the upper half of its scale. The Specification also describes arrangements for tuning over three bands using three tuners, bi-stable circuit L being replaced by a tri-stable circuit (Fig. 10, not shown); suitable tuning indication arrangements are also described (Figs. 14, 15, not shown).