Oscilloscope brightness control is not normal maintenance - Database & Sql Blog Articles

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When the oscilloscope's brightness control is abnormal, the maintenance electronic oscilloscope often has an abnormal phenomenon of "luminance" control, that is, the "luminance" control knob is adjusted, and the brightness of the waveform displayed on the oscilloscope screen is very bright. Can't dim, or the brightness of the waveform is dim, can't be brightened; the base can't show the waveform, that is, it can't be brightened. According to the basic circuit structure of the electronic oscilloscope, there are two reasons for the abnormality of the luminance control. One is that the oscilloscope itself has a problem, and the other is that the oscilloscope has a problem with the high voltage circuit. The maintenance method and steps are described separately. as follows:
(1) If the degree of vacuum inside the oscillating tube drops, that is, there is a slight air leakage problem, the air inside the tube will be ionized by the rapidly moving beam current, thereby greatly enhancing the current of the third anode (A3), resulting in display. The waveform cannot be dimmed. Or, due to poor contact of the gate tube soldering lugs of the oscillating tube, the current of the electron beam is the strongest and uncontrolled, and the waveform of the display cannot be dimmed. The difference between the two is that the former will exhibit "astigmatism" throughout the screen, while the latter will only be controlled by "focus". During the inspection, the former can be determined by the "device replacement method"; for the latter, when the oscilloscope is powered, the "measuring resistance method" is used to detect the gate pin of the oscillating tube and the corresponding soldering piece on the socket. The path resistance is judged and the necessary trimming is performed.
(2) If the cathode emission capability of the oscilloscope is reduced, that is, there is a problem of aging, the beam current in the tube becomes weak, and the luminance of the display waveform is dim and cannot be brightened, and the waveform is not displayed until the waveform is displayed. During the overhaul, you can use the "change status method", that is, try to increase the supply voltage of the oscilloscope filament (7-8V), or short-circuit the oscilloscope series resistance (RK) to increase the beam current and make the display waveform The degree has improved. But in the final analysis, it is necessary to update the oscilloscope 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 supplying the electrodes of the oscillating tube, and the corresponding voltage dividing circuit. The schematic diagram of the high voltage circuit of a conventional oscilloscope is shown. Here, R1, W, R2, R3, etc. constitute a voltage divider circuit of the "-1500V" DC high voltage power supply. Adjusting the activity characteristics of the potentiometer W1 allows the potential difference between the gate G of the oscilloscope and the cathode K to vary from "-10V" to "-100V". The "-10V" working point is equivalent to the "brightness" is the brightest; the "-30V" working point is equivalent to the "luminance" dim; the "-40V" to "-100V" working point is equivalent to the dark area.
If there are problems such as variable value, soldering, damage, etc. of the resistor elements behind the voltage dividing resistor R2, that is, the voltage dividing circuit is turned off, so that the potential difference between the GKs cannot be adjusted to the dark area, and thus the display waveform cannot be dimmed. phenomenon. During maintenance, the "measured voltage method" and the "change current status method" can be used to detect whether the potential difference between the oscilloscope tubes GK is normal or not. That is, while adjusting the "luminance control" potentiometer W1, the voltage value between the GKs is detected using a DC electronic voltmeter with a high input impedance. Or, in the case of no power, use the "measurement resistance method" to detect the resistance and path of each voltage divider element in order to find the problem.
If the voltage dividing resistors R1 and W1 have "opening" problems such as soldering or damage, the potential difference between the oscilloscope tubes GK will greatly exceed "-100V", resulting in a malfunction phenomenon without wave display. The voltage method "or" measuring resistance method is determined. If the voltage dividing resistor R1 is changed or the cap of the third anode A3 of the oscillating tube is detached, a malfunction phenomenon in which the waveform is dim and cannot be brightened appears. When repairing, it can be determined by "non-powered observation method" and "measurement resistance method".
(4) Most modern electronic oscilloscopes use a fast high-intensity oscilloscope as a display device, and its oscilloscope high-voltage circuit is shown in the figure. Here, the "-1100V" and "-1250V" negative voltages are used as the DC power supply for the "brightness" control and "focus" control of the voltage divider circuit, respectively. The purpose is to reduce the "luminance" control and "focus". Controls the effect of each other on the displayed waveform to improve the stability of the oscilloscope's operation. However, if the voltage values ​​of one of the two sets of negative high voltages change, or if the resistance elements in the two components of the voltage circuit have variable values, solder joints, damage, etc., the "luminance" control of the waveform will not be controlled. normal.