Improvements in or relating to radio and like receivers


523,697. Muting circuits for wireless receivers. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and KEALL, O. E. Jan. 12, 1939, No. 1106. [Class 40 (v)] In radio receivers which are normally held inoperative by a muting bias applied to one or more valves in the circuit, which bias is automatically overcome on reception of signals above a predetermined strength by an opposing bias set up by the "striking" of a neon or like glow discharge tube in the anode circuit of the muting control valve, the part of the I.F. signals which is tapped off for rectification in order to obtain the control voltage applied to the muting valve is passed on its way to the rectifier through a channel containing a piezo electric crystal resonant at the I.F. frequency. The arrangement may be used in conjunction with receivers provided with automatic frequency and volume control circuits. I.F. valves 1, 2 of a superheterodyne receiver are coupled through transformer 9, 10 to the demodulating diode 11 of a double diode pentode valve 3, the other diode 12 of which is used to produce A.V.C. voltages applied via lead 13 to grid of valve 2. Normally the diode 11 is back biassed by a resistance 14. Part of the signal from the valve 1 is tapped off through condenser 18 and line 19 to the grid 20 of a coupling valve 4 in the anode circuit of the control valve 5 feeding a discriminator circuit 22, 23 of known design. The anode of the valve 5 is coupled through a piezo electric crystal 25 to the diode circuit of a diodetriode 6 acting as the muting control valve. The anode of this valve is connected to a source of H.T. potential and to a neon or other gas discharge tube 7 so that on receiving a signal of above a certain strength the rectified voltage applied by the valve 6 causes the neon tube to "strike" and a voltage is applied through the lead 26 which overcomes the bias 14. In a modified form (Fig. 2, not shown) the crystal 25 is included as part of a balancing crystal network 27 constituting a coupling between the valves 4 and 5. The crystal may be incorporated in a capacitative or bridge type balancing circuit. The light from the neon tube may be employed to provide additional and distinctive illumination of the tuning scale.




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