JP3326947B2 - CRT electron beam landing correction value measuring device and second grid cut-off voltage measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the measurement of various parameters of a CRT (cathode ray tube), for example, a method and apparatus for measuring a landing state when an electron beam reaches a phosphor in a CRT.

[0002]

2. Description of the Related Art Conventionally, in a CRT inspection process, CRT landing data is measured at a plurality of points on a tube surface, and a pass / fail judgment is made.

[0003] An example of this conventional apparatus will be described with reference to FIG. In the figure, 1 is a CRT as an object to be measured, 2 is a sensor, 3 is an AGC circuit, 4 is a left and right signal separation circuit, 5 and 6 are peak detection circuits, 7 is a differential integration circuit,
10 is a capacitor of the integrating circuit, 11 is a switch for short-circuiting the capacitor, 8 is a signal source for wobbling, 9 is an amplifier provided in a feedback loop for feeding back a signal to the gun center coil 13, and 12 is a measurement. A data output terminal 13 is a gun center coil.

Next, the operation of the apparatus shown in FIG. 10 will be briefly described. Here, in order to make the explanation easy to understand, the explanation will be given with the following conditions. (A) The amount of misrun of the CRT to be measured is 0 [μm]. (B) It is assumed that the peak difference of the AGC circuit 3 is integrated. (C) The gain of the differential integration circuit 7 is set to 1. (D) Photo sensor 2 and AGC circuit 3
The luminance distribution obtained by the above is symmetrical with respect to the Y axis as shown in FIG.

(E) A coefficient for converting a luminance change with respect to a beam movement amount into a voltage is represented by α [V / μm].
(F) A coefficient for converting an input voltage of a GCC (gun center coil) amplifier into a beam movement amount is β [μm / V]. (G) The width of WOBB (wobbling) is l [μm]
And (H) Let the maximum value of the AGC output voltage be k [V].

Considering the luminance distribution under the above conditions, FIG.
As shown in FIG. In the figure, the horizontal axis indicates the amount of movement of the beam, and the vertical axis indicates the output voltage of the AGC circuit 3.

As is apparent from FIG. 1, when the beam is at the center position, the AGC output voltage takes the maximum value k, and when the beam is swung right and left, the AGC increases with distance from the center.
The C output voltage decreases. Here, the AGC output voltage is CR
It corresponds to the luminance distribution of the T screen.

This luminance distribution is linear under the above condition (d). In FIG. 11, Y = αX + k and Y = −αX +
given by k. Now, X at point A in FIG.
_{When 0} [V] is added, the center of wobbling moves by βX ₀ [μm] as shown in FIG.

[0009] When the swung by the same amount the beam to the right and left in this state, AGC output voltage as shown in FIG at Y _l0 and Y _r0 is obtained. This AGC output voltage is set to L /
The signals are separated right and left by an R separation circuit, the respective peaks are detected, and the difference is obtained by a differential integration circuit 7, whereby an output Y _r0 −Y ₁₀ is obtained at a point C.

This situation is described in more detail as follows. By opening the feedback loop by disconnecting the connection between the points A and C in FIG. 10, X ₀ [V] is applied to the point A,
When the voltage is converted into a beam movement amount by a GCC amplifier, β
Since X ₀ [μm], the center of the beam wobbling in FIG. 11 moves to βX ₀ [μm]. Therefore, when the beam is swung to the right, the amount of movement of the beam is βX ₀
+ L / 2.

When this movement amount is converted into a voltage, -α (βX
₀ + l / 2). Assuming that the maximum value of the AGC output voltage is k [V] as shown in the above condition (k), when the beam is swung to the right, Y _r0 = −α (βX ₀ + l / 2) + k The beam is swung to the left It was when _{Y l0 = α (βX 0 -l} / 2) + voltage at point C in FIG. 10 from k to become the _{Y 0 = Y r0 -Y l0 =} -
2αβX ₀ .

Next, when the feedback loop is closed and the signal output to the point C is added to the point A and recirculated to check the convergence, the signal output to the point C in the above description is Y _0. The new input at point A is X ₁ = X ₀ + Y ₀ .

In response to this input at point A, it appears again at point C.
Output voltage is Y₁= -2αβ (X ₀+ Y₀) =-2α
β (1-2αβ) X₀It is. Similarly, this behavior
Again, in general, Y_n= -2αβ (1-2αβ)ⁿX₀ Becomes

This situation is shown in Table 1. In the table, A
It is shown in the form of an open loop showing the relationship between the voltage applied to the point and the voltage at point C.

[0015]

[Table 1]

Then, when a condition for the voltage at the point C to converge to zero is obtained, Y _n = −2αβ (1-2αβ) ⁿ X ₀ converges to zero, so that −1 <1-2αβ <1 And therefore, 0 <αβ <1 ‥‥ (1).

From equation (1), it can be seen that the circuit of FIG. 10 converges if the inverting amplifier has an open loop gain smaller than 2. 12 to 14 show the relationship between the value of αβ and the convergence waveform at the point C.

As is apparent from a comparison of FIGS. 12, 13 and 14, when αβ = 時間, the convergence time is minimized. In other words, the circuit is balanced by two samplings on each side (a total of four samplings).

Therefore, as shown in FIGS. 12 to 14, if the sampling frequency is 60 Hz and the maximum value of the luminance distribution is within the wobbling width, the measurement may be completed in 66.7 msec. I have.

In this way, when the center of the wobbling signal converges to a certain stable point, measurement data of the landing value is obtained at the output 12.

In the measuring apparatus described above with reference to FIG. 10, an external magnetic field or an AG (aperture grill)
In most cases, the noise component makes the output of the integrating circuit 7 unstable due to the fluctuation of the noise component (however, there is a case where there is no influence depending on the frequency and phase of the noise component).

Also, positive feedback may be applied to the circuit due to the reset timing or the influence of an external magnetic field, and oscillation may occur. Further, since the measurement device shown in FIG. 10 does not output a signal indicating that the balance has been achieved, the measurement is performed at the estimated time with a margin in the measurement time. Therefore, the measurement takes time.

This will be described with reference to FIG.
In this measurement, sensors such as the photosensor 2 shown in FIG. 10 are arranged at nine points on the screen of the CRT, and they are divided into three columns, a left column, a middle column, and a right column. This is performed by arranging three sensors in the direction and inputting measurement values from those sensors.

Referring to FIG. 15, the measurement is started in step S1, the previous measurement value is reset in step S2, and the left column is balanced in step S3.

Next, in step S4, the input from the upper left photo sensor is measured. Next, the input from the middle left photo sensor is measured. Next, the input from the lower left photo sensor is measured. These measurements are taken twice.

After the measurement in the left column is completed, step S7
To reset the previous measurement value, and go to step S8.
To balance the middle row. When the balance is obtained, the input from the middle and upper photo sensor is measured in step S9. Next, the input from the middle and middle photo sensor is measured. Next, the input from the middle and lower photo sensors is measured. This measurement is also performed twice.

After the measurement of the middle row is completed, step S1
Proceed to 2 to reset the previous measurement value, and
At 13, balance the right column. After the balance is obtained, the input from the upper right photo sensor is measured in step S14. Next, the input from the middle right photo sensor is measured, and then the input from the lower right photo sensor is measured. As described above, when all the measurements are completed, the measurement is completed in step S17.

Assuming that the time required for this measurement is 1 second per reset, the time required for balancing is 3 seconds for 3 rows, the time required for balance is 3 × 3 = 9 seconds, and the measurement time is 0.7 ×
(6 + 6 + 3) = 10.5 seconds. Therefore, the total time is 22.5 seconds (about 23 seconds). As described above, in the conventional measuring device, the time required for the measurement was long.
In addition, a capacitor having a large time constant is required to hold the peak-detected signal, and there is a drawback that data has ripples. SUMMARY OF THE INVENTION It is an object of the present invention to provide a measuring method and an apparatus capable of overcoming the drawbacks of the conventional CRT landing correction value measuring circuit and performing highly accurate measurement in a short time. Another object is to apply the method to applications other than the landing measurement.

[0029]

In order to solve the above problems, the present invention provides a CRT electron beam landing correction value measuring apparatus provided with the following means. That is, CRT
A signal source to be measured using a luminance signal generated by an electron beam emitted from the electron gun landing on the tube surface, and a tube of the CRT for detecting the luminance signal from the signal source to be measured. A sensor provided on the surface, a signal processing circuit for processing a signal detected by the sensor and outputting a measurement output, and a wobbling signal applied to a gun center coil of the CRT for causing the electron beam to oscillate at a constant period. An electron beam landing correction value measuring device comprising a wobbling signal source to be supplied and a feedback circuit for feeding back the output of the signal processing circuit to a gun center coil in order to converge the center of the deflection of the electron beam to one stable point. Means for detecting that the center of the deflection of the electron beam has converged to a stable point and outputting a balance completion signal. To provide an electron beam landing correction value measuring apparatus RT. Further, a signal source to be measured configured to apply a predetermined voltage to the second grid of the CRT to be measured to detect a cathode current of the CRT, and convert the cathode current detected from the signal source to be measured into a second grid voltage. I
/ V converter, a logarithmic converter for logarithmically converting the output of the I / V converter so that the cathode current is linearly proportional to the second grid voltage, and a logarithmic converter for swinging the cathode current. A wobbling signal source for generating a wobbling signal to be applied to the two grids, a closed circuit for detecting an error in the value of the left and right swings of the cathode current and feeding back to the second grid, and a stable center for the cathode current swings Means for detecting a convergence to a point and outputting a balance completion signal; after the balance is completed, the predetermined voltage applied to the second grid is reduced to detect a cathode current corresponding to the amplitude of the wobbling signal. Also provided is a CRT second grid cut-off voltage measuring device for detecting the second grid voltage at the time of the detection as a cut-off voltage.

[0030]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing one example of the CRT landing measurement circuit of the present invention.

In FIG. 1, reference numeral 101 denotes a cathode ray tube (hereinafter referred to as C).
A photosensor 102 is usually arranged at a plurality of points on the screen of the CRT to detect a luminance signal. 103 is an AGC (automatic gain control) circuit, 10
Reference numerals 4 and 105 denote peak hold circuits that hold the peak values of the left and right swings of the electron beam, and reference numeral 106 denotes a differential amplifier that determines the difference between the peak values held by 104 and 105.

Reference numeral 107 denotes a sample and hold circuit which samples and holds the output signal of the differential amplifier, 108 denotes a decompression circuit, 109 denotes an integration circuit, and 110 denotes a measurement data output circuit.

Reference numeral 111 denotes a wobbling signal source for generating a signal for causing the electron beam of the CRT to swing left and right.
2 is a GCC amplifier provided in a feedback loop for returning an output to the gun center coil, and 113 is a gun center coil.

A holding circuit 114 holds the output of the AGC circuit 103, a comparator circuit 115 for detecting an abnormality described later, and a detection circuit 116 for detecting when the output of the decompression circuit falls below a predetermined value and balancing the output. This is a window comparator circuit that outputs a completion signal.

Next, the operation of the CRT landing measurement circuit will be described with reference to FIG. First, the brightness of the CRT screen to be measured is detected by the photo sensor 102 and the AGC circuit 103
To supply. The output from the AGC circuit 103 is supplied to peak hold circuits 104 and 105, where the peak value of the left and right swing of the electron beam is held as a voltage.

The peak hold circuits 104 and 10
5 is supplied to the input of the differential amplifier 106, and the differential voltage of the peak value appearing at the output of the differential amplifier is supplied to the sample-and-hold circuit 107 and held therein.

The output of the sample and hold circuit 107 is supplied to a decompression circuit 108. This decompression circuit is constituted, for example, by a circuit as shown in FIG.

That is, the luminance difference signal applied to the input terminal 201 is transmitted to the diodes 202 and 203 and the resistors 204 and 20.
5 is applied to the negative input of the operational amplifier 200. Thus, the positive input of the operational amplifier 200 is connected to ground. Output 20 of operational amplifier 200
A resistor 206 and a series circuit of zener diodes 207 and 208 are connected in parallel between the resistor 9 and the negative input.

The expansion circuit configured as described above functions to prevent the output of the integration circuit 109 from becoming unstable due to noise components. That is, the loop gain is reduced by performing non-linearization of the loop gain by the diodes 202 and 203, and the balance of the left and right beam fluctuations is slowly adjusted to stabilize the operation of the integration circuit 109 (FIG. 1). It is.

The diodes 202, 203
When the L / R (left / right) luminance difference is large, the gain is increased to increase the convergence speed, and when the L / R luminance difference is small, the gain is reduced to improve the measurement accuracy.

The diodes 207 and 208 function as a limiter, and serve to limit the loop gain and prevent oscillation. Returning to the description of the circuit of FIG. 1, the output of the integration circuit 109 is taken out to the terminal 110 and used as measurement data. For the CRT 101, the gun center coil 11
A wobbling signal from the wobbling signal source 111 causes the electron beam of the CRT to swing right and left.

In the CRT landing measurement circuit of FIG. 1, the signal from the output terminal 110 is superimposed on the wobbling signal, and the feedback is applied so that the electron beam converges to the point where the right and left vibrations become the same. . Note that a GCC amplifier 112 is provided in the feedback loop.

In the measuring circuit of one embodiment of the present invention shown in FIG. 1, a hold circuit 114 and a comparator 115 are provided to ensure the reliability of the measured value.

In the conventional measuring circuit, the measured value is captured even when the CRT is not illuminated, or the measured value is captured even when the phototransistor 102 is broken, so that the reliability of the measured value cannot be ensured. .

In the measuring circuit of this embodiment, the output of the AGC circuit 103 is held in the hold circuit 114, and the held value is compared with a predetermined reference value in the comparator 115. It has become. Therefore, when the output of the AGC circuit is smaller than a desired value, it can be detected as abnormal.

In the circuit shown in FIG.
Although only one is shown in FIG. 2, a plurality of photosensors are provided to measure a plurality of points on the CRT screen.

Accordingly, it is necessary to provide a whole or a part of the circuit of FIG. The detection of the completion of the balance in such a case will be described with reference to FIGS.

In FIG. 4, reference numeral 401 denotes a programmable
A logic device, to which inputs I1 to I9 are input a balance completion signal indicating that balance has been completed for nine points on the CRT screen.

Programmable logic device 40
One output 402 is applied to the D input of flip-flop 403. The flip-flop 403 latches a signal supplied to the input terminal D when a clock signal is input to its CK input, and outputs an output to a Q output 406.

On the other hand, the output terminal 402 is connected to the delay circuit 404, and the output of the delay circuit 404 is
05 is connected to one input terminal. A horizontal synchronizing signal is supplied to another input terminal of the AND circuit 405,
The AND operation with the output from the programmable logic device 401 delayed by a predetermined time, for example, 1 second, is obtained by the delay circuit 404, and the result is supplied to the CK input terminal of the flip-flop 403.

Accordingly, the circuit of FIG. 4 detects that the balance of nine points on the CRT screen is completed, and outputs a detection signal to the output 406.

FIG. 5 is a waveform diagram showing this state. As shown in FIG. 7A, when nine points are balanced, the programmable logic device 401 outputs an output (a). The output signal is turned off as shown when the balance is lost for some reason, and then turned on when the balance is restored again.

The signal (a) in FIG.
3 is latched by a clock signal applied to CK and applied to CK, so that an output as shown in FIG. That is, a data capture instruction signal is output with a margin of one second after completion of the nine-point balance.

As is clear from the figure, when the nine-point balance is lost, the capture is stopped, and when the balance is regained, a data capture instruction signal is issued with a margin of one second.

Next, an operation flow for performing a CRT landing measurement using the measurement circuit of FIG. 1 will be described with reference to FIG.

In this measurement, a total of 9 photo sensors 102 shown in FIG.
So that the same row is processed at the same time,
For three columns, the data is sampled in sequence and
The number of samplings per row is 20 times / second.

First, when starting in step S1, the program proceeds to steps S2, S3, and S4 for performing left-row balancing, middle-row balancing, and right-row balancing. In these three rows of balance operations, as shown by the output terminal 118 in FIG. 1, a balance completion signal is output when the left and right swings of the beam are balanced.

Step S5 is a step in which it is detected that all of the three columns have been balanced, and a balance completion signal is issued.

Next, in step S6, the above nine points are simultaneously measured. This measurement is repeated twice. When this is completed, the measurement ends in step S7.

The time required for this measurement is as follows: the operation for balancing is 1.5 seconds in 0.5 seconds × 3 rows, 1 second with a margin for issuing a balance completion signal, and 2.5 seconds for measurement. In seconds, the measurement is completed in a total of 5 seconds. It can be seen that taking the measured values according to the present invention can greatly reduce the time in comparison with the conventional measurement flow that took about 23 seconds.

Table 2 shows a comparison between the above-described measuring system of the present invention and a conventional measuring system.

[0062]

[Table 2]

Next, an embodiment in which the measuring circuit of the present invention is applied to the measurement of a CRT cut-off voltage will be described with reference to FIG.

In the figure, 601 is an input terminal connected to the cathode circuit of the CRT, and 602 is a cathode current I _k
I / V converter for converting into a second grid voltage V, 603
Is a logarithmic converter for performing logarithmic conversion on the output of the I / V converter.

Reference numeral 604 denotes a sample and hold circuit for sampling and holding the output of the logarithmic converter 603, reference numeral 605 denotes a differential amplifier for calculating the difference between the output of the logarithmic converter 603 and the output of the sample and hold circuit 604, and reference numeral 606 denotes a differential amplifier. This is a sample and hold circuit that samples and holds the difference signal from the device.

Reference numeral 607 denotes an adder, which is a sample hold circuit 6
06 is added to the DC voltage E. 608 is an expander,
609 is an integrator, 610 is a wobbling signal generator, 6
11, an amplifier, 612, a second grid power supply, 613, a power supply terminal for the second grid, and 614, a balance completion signal output terminal.

Next, the operation of the circuit of FIG.
This will be described with reference to FIGS. First, referring to FIG. 7, when the cathode voltage E _{k of the} CRT is fixed, the relationship between the current I _k flowing through the cathode and the voltage applied to the second grid is an exponential relationship as shown in FIG. It has become.

Now, assuming that the signal from the wobbling signal source 610 is applied to the second grid and fluctuated between E1 and E2, the cathode current changes between I1 and I2.
Since this change is non-linear, the output is not symmetric when wobbling is applied around E.

The logarithmic converter 603 is provided in the measuring circuit of FIG. By doing this,
The relationship between the cathode current and the second grid voltage can be represented as shown in FIG.

In FIG. 8, the relationship between the cathode current and the second grid voltage is linear, and when the voltage of the second grid is swung between E1 and E2 by the wobbling signal, the cathode current falls between I1 and I2. Vary between.

The voltage corresponding to the difference between the currents I1 and I2 is obtained by the sample hold circuit 604 and the differential amplifier 605 shown in FIG.

The adder 607 converts the difference voltage into a DC power
, And the result is supplied to the expansion circuit 608. The output of the decompression circuit 608 is integrated by the integrator 609 and is superimposed on the wobbling signal from the above-described wobbling signal source.

As shown in FIG. 8, when the wobbling signal voltage is superimposed on the DC bias voltage E and the second grid voltage is swung between E1 and E2, the cathode voltages become I1 and I2.
2 and if there is a difference between I2-I and I-I1, it becomes an error signal and becomes a sample-and-hold circuit 60.
6 and the second grid G2 via the integrator 607
As a result, the second grid voltage converges to a point which is symmetrical about E, that is, E2-E = E-E1.

Next, when the DC voltage E is lowered, as shown in FIG. 9, the cathode current becomes zero at the point of the second grid voltage E _C2C0 , so that the cathode current difference (I2−I) +
(I-I1) = I2-I1 no longer becomes zero but remains.

[0075] However, since the relationship between the second grid voltage EC2 and cathode current I _k as described above has a linear, one deflection in balance state I1-I = K
Then, the other swing is also I2-I = K, so
By using this relationship, the cut-off voltage of the CRT can be obtained.

As shown in FIG. 9, one of the shakes (I1-
If I) is zero, that is, if the other swing becomes I2-I = K in a state where I1-I = 0, the voltage E of the second grid is obtained, and the value is obtained as the CRT cut-off voltage E of the second grid.
_C2C0 .

As described above, according to the present embodiment, it is possible to measure the second grid voltage E _{C2 at} the point where the cathode current becomes zero (I _k = 0), which has been conventionally difficult.

[0078]

According to the measuring method of the present invention, the measuring time can be reduced and the measuring accuracy can be improved. In addition, the measurement method of the present invention significantly improves the reliability of measurement data as compared with the conventional measurement method, and in the case of multi-point measurement, it is possible to monitor a convergence value at those multiple points almost simultaneously. Become.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a CRT landing measurement system according to one embodiment of the present invention.

FIG. 2 is a circuit diagram of a decompression circuit used in the system of FIG.

FIG. 3 is a flowchart showing the operation of balance completion and measurement when multipoint measurement is performed by the measurement system of the present invention.

FIG. 4 is a circuit diagram of a balance completion detection circuit.

FIG. 5 is a waveform diagram of the circuit of FIG. 4;

FIG. 6 is a block diagram of a circuit for measuring a CRT cutoff voltage according to another embodiment of the present invention.

FIG. 7 is a characteristic diagram showing a relationship between a second grid voltage and a cathode current of a CRT.

FIG. 8 is a characteristic diagram showing a relationship between a second grid voltage and a cathode current of a CRT after logarithmic conversion.

FIG. 9 is a characteristic diagram illustrating a relationship between a second grid voltage and a cathode current for explaining a CRT cutoff voltage.

FIG. 10 is a block diagram showing a conventional CRT landing measurement system.

11 is an operation explanatory diagram of the circuit in FIG. 10;

FIG. 12 is a waveform diagram showing how the measured voltage converges in the circuit of FIG.

FIG. 13 is a waveform diagram showing how the measured voltage converges in the circuit of FIG.

FIG. 14 is a waveform diagram showing how the measured voltage converges in the circuit of FIG.

FIG. 15 is a flowchart showing a measurement flow of the circuit of FIG. 10;

[Explanation of symbols]

 Reference Signs List 101 CRT (cathode ray tube) 102 Photosensor 103 AGC (automatic gain control circuit) 104, 105 Peak hold circuit 106 Differential amplifier 107 Sample hold circuit 108 Expansion circuit 109 Integration circuit 110 Measurement output terminal 111 Wobbling signal source 112 Amplification Instrument 113 Gun center coil 114 Hold circuit 115 Comparator 116 Round comparator 118 Balance completion signal output terminal

Claims (7)

(57) [Claims] 1. A signal source to be measured having a luminance signal generated by an electron beam emitted from an electron gun of a CRT landing on a tube surface, and a luminance signal from the signal source to be measured is detected. C above
A sensor provided on the screen of the RT, a signal processing circuit for processing a signal detected by the sensor and outputting a measurement output, and a CRT for oscillating the electron beam at a constant period
A wobbling signal source for supplying a wobbling signal to be applied to the gun center coil, and a feedback circuit for feeding back the output of the signal processing circuit to the gun center coil in order to converge the center of the deflection of the electron beam to one stable point. An electron beam landing correction value measuring device, comprising: means for detecting that the center of the electron beam shake has converged to a stable point and outputting a balance completion signal. Value measuring device. 2. The measuring apparatus according to claim 1, wherein a loop gain is controlled in a closed circuit including the signal source under test, a signal processing circuit, and a feedback circuit, and when the gain is large, the convergence speed is increased. An electron beam landing correction value measuring device for a CRT, comprising: an extension circuit which serves to increase the measurement accuracy when the value is small. 3. A measurement apparatus according to claim 2, wherein said expansion circuit includes a limiter for controlling signal amplitude to prevent oscillation. apparatus. 4. The measuring apparatus according to claim 1, wherein a plurality of sensors for the signal to be measured are provided, and a center of the deflection of the electron beam from a signal processing circuit corresponding to each sensor. Detecting that convergence to a stable point and outputting a balance completion signal, and outputting an overall balance completion signal when the entire balance is obtained. measuring device. 5. A signal source to be measured for applying a predetermined voltage to a second grid of a CRT to be measured to detect a cathode current of the CRT, and a cathode current detected from the signal source to be measured in a second grid. An I / V converter for converting a voltage to a voltage, a logarithmic converter for logarithmically converting the output of the I / V converter so that the cathode current and the second grid voltage are linearly proportional, A wobbling signal source for generating a wobbling signal to be applied to the second grid, a closed circuit for detecting an error between the left and right swing values of the cathode current and feeding back to the second grid, A means for detecting that the center of the swing has converged to a stable point and outputting a balance completion signal; after the balance is completed, the predetermined voltage applied to the second grid is reduced; Second grid cut-off voltage measuring device of the CRT for detecting a second grid voltage as a cut-off voltage when the cathode current is detected which corresponds to the wobbling signal amplitude. 6. A measuring apparatus according to claim 5, wherein the expansion circuit controls the loop gain during the closed circuit to increase the convergence speed when the gain is large, and to increase the measurement accuracy when the gain is small. CR characterized by having
T second grid cut-off voltage measuring device. 7. The second grid cut-off voltage of a CRT according to claim 6, wherein said expansion circuit includes a limiter for controlling the amplitude of a signal to prevent oscillation. measuring device.