JPS6049373B2 - Malfunction prevention device for television receivers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a malfunction prevention device for a television receiver having a rotary mechanical tuner that mechanically switches channels, and its purpose is to prevent malfunctions in automatic frequency fine tuning when switching channels. There is something to do.

Generally, if you switch channels while the AFT circuit (automatic frequency fine tuning circuit) is operating, in the case of a mechanical tuner, the local frequency will fluctuate momentarily, which will cause it to be drawn into the own station or adjacent audio carrier, resulting in malfunction. There is a big possibility.

Particularly recently, the use of IC circuits has progressed, and ICs that integrate VIF and AFT circuits have a pull-in range of approximately 2MH.
In many cases, it is designed as a wide AFT (AFT with a wide pull-in range), and malfunctions are particularly likely to occur in this case. Generally, in order to prevent malfunctions, a default switch is provided that is linked to the switching axis of the tuner to stop the AFT operation when switching channels, but the present invention is characterized in that this is done electrically. This is what I did. A detailed embodiment of the present invention is shown in FIG. 1 below.

In FIG. 1, reference numeral 1 denotes an IC incorporating VIF amplification, video detection, and AFT circuits, and a video detection signal is output to terminal A. In addition, terminal B is a day-to-day terminal of AFT, and when terminal B is OV, the AFT circuit operates and 12
If V is applied, the AFT operation will stop. Now, if you change the channel in the set reception state, the second
A pulse as shown in Figure A is generated.

In the figure, the upper part is a video signal viewed at a vertical rate, and below it a pulse with a width of about 3 mS is generated at the time of switching. The reason why this pulse occurs is that when switching channels, the tuner's local oscillation momentarily stops, resulting in a completely no-signal state with no noise, and the gain of VIF is higher than the M°C voltage when a general weak electric field is input. The voltage changes in the direction of increasing the gain. Immediately after channel switching is completed, the M℃ voltage cannot follow immediately, and the maximum gain is A.
Since the GC voltage is held, until it returns, even though there is a signal input, it is amplified at the maximum gain and reaches the dynamic range. As a result, it is possible to obtain a pulse whose width is approximately 3r11S1 and the DC level at the tip of the pulse is approximately 0.7■ during the period until M° C. follows. Generally, when inputting a weak electric field, there are many noise components, and at the tip of this noise, N
℃, the complete maximum gain state is not achieved, and the level of the leading edge of the noise is approximately at the same position as the leading edge of the sync signal when a strong signal is input. Therefore, even when a weak electric field is input, it is possible to generate this pulse when switching channels, and the state at that time is shown in FIG. If it is necessary to detect a change in the AGC voltage and defect, it is very difficult to detect the change in the AGC voltage when the electric field is weak.

In other words, when a weak electric field is input, the AGC voltage approaches the maximum gain voltage, and the difference from the AGC voltage when switching channels is 0.
．． Since the voltage is only about 2V, detection is difficult, but when the detection output is used as in the present invention, a sufficiently detectable pulse can be obtained as shown in FIG. Now, this detection output signal is applied to the base of a transistor 10 via a resistor 2 and a capacitor 7.

At this time, since the video signal part other than the pulse generated when switching channels is unnecessary, by creating a voltage of about 3 cm with resistors 5 and 6 and passing it through diode 3, it is possible to clip signal components of 3 cm or more. can. This diode 3
The waveform at the intersection of resistor 2 and resistor 2 is shown in FIG. 2B. Furthermore, if the input is applied to the base of the transistor 10 through the cutting capacitor 7 and properly biased with the resistors 8 and 9, the collector output will normally be on as shown in Figure 2C, and will be off only when switching channels. A waveform appears. By applying this pulse to terminal B of the IC, day-to-day operation is obtained. As described above, the present invention operates reliably and stably even in a weak electric field by detecting and defective the pulse appearing in the video detection output when switching channels, and is constructed at a lower cost than a mechanical defective switch. It is something that can be done.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a malfunction prevention device for a television receiver according to an embodiment of the present invention, and FIGS. 2A, B, and 3 are waveform diagrams for explaining the device. 1...■IFIC,. A: Video detection signal output terminal, B: Day-feet signal application terminal, 10: Transistor, 3: Diode, 4:... Capacitor, 5, 6, 2, 8
, 9, 11... Resistor, 7... Capacitor.

Claims (1)

[Claims] 1. A tuner that switches channels by mechanical means, a video intermediate frequency amplifier and a detector connected to this tuner, a switch circuit connected to the output terminal of this detector, and a defective circuit connected to this switch circuit. and an automatic frequency fine adjustment circuit for controlling the local frequency of the tuner, which applies pulses generated at the output terminal of the detector at the time of channel switching to the day-to-day circuit via the switch circuit, A malfunction prevention device for a television receiver, characterized in that the automatic frequency fine adjustment circuit is defeated by an output voltage to prevent malfunction of the automatic frequency fine adjustment circuit when switching channels.