Oscilloscope luminance control abnormal maintenance

When the oscilloscope is not properly controlled during the maintenance of the electronic oscilloscope, the "brightness" control abnormal phenomenon often occurs. That is, the "brightness" control knob is adjusted, and the waveform of the waveform displayed on the screen of the oscilloscope tube is bright. Can not be dimmed, or the brightness of the waveform is dim, can not be bright; base to show no waveform, that can not be brightened. According to the basic circuit structure of the electronic oscilloscope, there are two aspects of the cause of the malfunction caused by the abnormal brightness control. One is that the oscilloscope itself has a problem, and the other is that the high voltage circuit of the oscilloscope has a problem, and its maintenance method and steps are described here. as follows:

(1) If the degree of vacuum inside the oscilloscope decreases, ie, there is a slight air leakage problem, the air in the tube is ionized by the rapidly moving electron beam current, thereby greatly enhancing the current of the third anode (A3), resulting in display The waveform cannot be dimmed. Or because the oscilloscope tube has a poor contact with the solder joint of the gate socket, the electron beam current is the strongest and is not controlled, so that the displayed waveform cannot be dimmed. The difference between the two cases is that the former will exhibit "astigmatism" across the entire screen, while the latter will only be controlled by "focusing." In the case of overhaul, the former can be determined by "Device Substitution Method"; for the latter, under the condition that the oscilloscope is powered on, the "Measurement Resistance Method" is used to detect the gate pin of the oscilloscope tube and the corresponding pad on the stem. The resistance of the channel is judged and necessary trimming is performed.

(2) If the cathode discharge capability of the oscilloscope decreases, that is, there is a problem of aging, the electron beam current in the tube becomes weak, causing the display waveform to be dimmed and unable to be brightened, until no waveform appears. When overhauling, “change the status quo method” can be used, namely, to increase the supply voltage of the oscillating tube filament (7-8V), or to short-circuit the oscilloscope series resistance (RK), so as to increase the beam current and make the displayed waveform brighter. Degree has improved. But in the final analysis, the oscilloscope must be updated to solve the problem fundamentally.

(3) The high voltage circuit of the oscilloscope tube refers to the positive and negative DC high voltage power supplies for the electrodes of the oscillometric tube, and the corresponding voltage divider circuit. The schematic diagram of the high voltage circuit of the ordinary oscilloscope is shown in the figure. Here, R1, W, R2, R3, etc. constitute the "-1500V" DC voltage divider circuit. By adjusting the active characteristics of the potentiometer W1, the potential difference between the gate G and the cathode K of the oscilloscope can be varied from "-10V" to "-100V". The "-10V" operating point is equivalent to "brightness" being the brightest; the "-30V" operating point is equivalent to "darkness" dimming; the "-40V" to "-100V" operating point is equivalent to the dark area.

If the resistance elements behind the voltage dividing resistor R2 have problems such as variable value, imaginary welding, damage, etc., that is, the voltage divider circuit breaks, so that the potential difference between the GKs cannot be adjusted to a dark area, and thus a failure that the displayed waveform cannot be dimmed appears. phenomenon. In the case of maintenance, it is possible to detect whether the potential difference between the oscilloscopes GK is normal by using the "Measured voltage method" and "Change the status quo method" under power-on conditions. That is, while adjusting the "luminance control" potentiometer W1, the voltage between the GKs is detected using a high input impedance DC voltage meter. Or, in the absence of power, the “measuring resistance method” is used to detect the resistance value and the path condition of each voltage divider element in order to find the problem.

If the voltage divider resistors R1 and W1 have a problem such as “failure” such as welding or damage, the potential difference between the oscilloscope tubes GK will greatly exceed “-100V”, resulting in a failure phenomenon of no wave display. The voltage method or "measurement resistance method" is determined. If the voltage divider resistor R1 is changed or the cap of the third anode A3 of the oscilloscope tube falls off, a faulty phenomenon that the waveform is dim and cannot be brightened will appear. During inspection and repair, the "non-energized observation method" and "measured resistance method" can be used to determine.

(4) Most modern electronic oscilloscopes use high-speed oscilloscopes as display devices, and their oscilloscope high-voltage circuits are shown in the figure. Here, "-1100V" and "-1250V" groups of negative high voltages are used, respectively, as DC power supplies for the "brightness" control and "focus" control of the voltage divider circuit. The purpose is to reduce "brightness" control and "focusing." Control the influence of each other on the displayed waveform to improve the stability of the oscilloscope. However, if the voltage values ​​of one of the two groups of negative high voltages change, or if the resistance components in the two components of the voltage circuit have problems such as variable value, imaginary welding, and damage, the "brightness" control of the waveform will not be caused. normal.

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