JPS6364950B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bright spot erasing circuit for an image display device,
In particular, the present invention relates to a circuit for eliminating bright spots that conventionally occur when a video amplification stage and a cathode or grid electrode of a cathode ray tube are coupled with direct current.

Next, the present invention will be explained using drawings.
Figure 1 is the circuit of the present invention, Figure 2 A, B, and C are explanatory waveform diagrams of the same circuit, where 1 is the B voltage application terminal, 2 is the video signal application terminal, 3 is the coupling capacitor, and 4 is the contrast adjustment. 5 is a DC regeneration circuit, 6 is a voltage dividing circuit having voltage dividing resistors 7 and 8, 9 is a variable resistor for
is a video amplification stage having output transistors 10 and 11, 12 is a peaking coil, 13 is a load resistance,
14 and 15 are first and second diodes connected in series in a forward direction to the collector of the output transistor, 16 is a decoupling capacitor, 17 is a first capacitor, and 18 is a second capacitor 19 and a resistor 20. A constant circuit, 21 is a variable resistor for brightness adjustment, and 22 and 23 are heater voltage application terminals for applying voltage to the heater 25 of the cathode ray tube 24.

Next, to explain the operation of the present invention, a video signal is applied to the terminal 2, the output transistor 11 operates with its emitter grounded, and the output transistor 10 operates with its base grounded, and the base of the output transistor 10 is
A predetermined bias voltage is applied by voltage dividing resistors 7 and 8.

At this time, a video signal is applied to the cathode of the cathode ray tube 24 with the DC component being regenerated by the DC regeneration circuit 5, and the output transistor 1
The collector electrode of 0 has a flyback pulse (FBP) derived from a horizontal deflection circuit (not shown).
is applied as a DC voltage via the diodes 14 and 15, the load resistor 13, and the peaking coil 12 (at this time, it is rectified by the diode 14 and the first capacitor 17). A predetermined bias voltage (negative voltage) is applied to the grid of the cathode ray tube 24 by a variable resistor 21 for brightness adjustment.

Therefore, during normal operation, + _B1 as the B voltage of the output transistor 10 is rectified and smoothed by the diode 14 and the first capacitor 17, and is supplied to one end of the load resistor 13 of the transistor 10, and at the same time, the diode 15 is in a conductive state. , the second capacitor 20 is in a charged state via the resistor 19.

The coupling capacitor 3 is also charged with the polarity shown.

Next, when the power switch (not shown) is turned off and switched, the charge in the coupling capacitor 3 is discharged, so that the output transistors 10 and 11 become conductive for a moment, and as the power switch is turned off, Flyback pulse (FBP) is gone;
The terminal 26 is at ground potential and the first diode 1
4 is the cutoff. The charge accumulated in the first capacitor 17 is discharged through the second diode 15 and the output transistors 10 and 11, and when the charge in the first capacitor 17 is exhausted, the anode side of the second diode 15 becomes the ground potential and is cut off. Become.

On the other hand, the time constant circuit 18 consisting of the resistor 19 and the second capacitor 20 starts discharging, but since the output transistors 10 and 11 are cut off after the charge of the coupling capacitor 3 is discharged, the second
Diode 15 is cut off.

Therefore, the cathode potential V _B1 of the cathode ray tube 24 is
The second capacitor 20 is held at the charged potential. Since this potential is set at a potential higher than the cut-off potential of the cathode ray tube 24, no cathode current of the cathode ray tube flows and bright spots are prevented.

To explain this pattern using FIG. 2, t ₀ is the timing when the power switch is turned off, t ₁ is the timing when the heater 25 of the cathode ray tube 24 cools down, V _k is the cathode potential, and V _k0 is the timing when the heater 25 of the cathode ray tube 24 cools down. Figure 2A shows the cut-off potential of the cathode, and as explained above, after the heater of the cathode ray tube cools down after the switch-off, the cathode potential V _k reaches V _k0 .
This is a case where it gradually decreases.

The time constant of the time constant circuit 18 varies depending on the collector potential of the output transistor 10, etc.
As shown in Figure B, when the first grid potential of the cathode ray tube 24 becomes zero but does not rise above the cutoff, the horizontal deflection circuit is operating during a period corresponding to approximately the period t ₀ to _{t 0} ', that is, the raster remains. In this state, after the output transistor is cut off, the cathode potential V _k of the cathode ray tube 24 rises again, that is, the charge in the second capacitor 20 begins to discharge, and then gradually discharges due to leakage. Since the potential V _k is maintained above V _k0 , bright spots can be prevented.

FIG. 2C shows a case where the time constant T determined by the second capacitor 20 and the resistor is smaller than the time required for the heater 25 to cool down, and there is no bright spot between _t0 '' and _t1 . It comes out.

Therefore, the present invention provides a time constant T of the time constant circuit 18 .
As an example, R=22KΩ, C ₂
= 22μF, C ₀ = 1000μF, and C ₁ = 10μF, it takes about 3 minutes for the cathode potential of the cathode ray tube to reach the cut-off potential or less after the power switch is turned off, and bright spots are prevented. I was able to do it.

The time constant circuit for eliminating bright spots is connected to the cathode side of the cathode ray tube in the example shown above, but it is connected to the grid side of the cathode ray tube, and the grid potential of the cathode ray tube is cut off after the power switch is turned on until the heater cools down. If it is maintained below, the same effect as described above can be obtained.

As described above, according to the present invention, in image display devices such as television receivers, video monitors, and character displays, bright spots that occur when the power switch is turned off can be easily eliminated using a diode and a time constant circuit. This can be prevented, and the circuit of the present invention is highly effective for the same device.

[Brief explanation of drawings]

Figure 1 shows the bright spot erasing circuit of the present invention, Figure 2 A,
B and C show explanatory waveform diagrams of FIG. Explanation of main drawing numbers, 9...Video amplification stage, 10, 11
...Output transistor, 13...Load resistor, 14...First diode, 15...Second diode, 17...First capacitor, 18 ...Time constant circuit, 20...Second capacitor, 24...Cathode ray tube.

Claims (1)

[Claims] 1. In an image display device in which a cathode or grid electrode of a cathode ray tube and a video amplification stage are DC-coupled, the output electrode of an output transistor of the video amplification stage is DC-coupled to the cathode or grid electrode, and the load of the output transistor is A time constant having a plurality of diodes connected in series between the flyback pulse application terminal, a first capacitor connected between the connection point between the plurality of diodes and the ground, and a second capacitor between the load and the ground. By connecting a circuit, the time constant circuit maintains the cathode potential at or above the cut-off potential or the grid potential at or below the cut-off potential after the power is turned off, at least until the heater cools down.