Maintenance of abnormal brightness control of the

2022-08-16
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Maintenance of abnormal oscilloscope brightness control

maintenance of abnormal oscilloscope brightness control during the use of electronic oscilloscopes, abnormal "Brightness" control failures often occur, that is, adjust the "Brightness" control knob, and the brightness of the waveform displayed on the oscilloscope screen is very bright and cannot be dimmed, or the brightness of the waveform is dim and cannot be dimmed; The base to display no waveform, that is, it cannot be adjusted. According to the basic circuit structure of the electronic oscilloscope, there are two reasons for the failure of abnormal brightness control. One is the problem of the oscilloscope itself, and the other is the problem of the high-voltage circuit of the oscilloscope. The maintenance methods and steps are as follows:

(1) if the vacuum inside the oscilloscope decreases, that is, there is a slight air leakage problem, the air inside the tube will be ionized by the fast-moving electron beam current, Thus, the current of the third anode (A3) is greatly enhanced, so that the displayed waveform cannot be dimmed. Or an open circuit occurs due to the poor contact of the grid tube base solder of the oscilloscope tube. At this time, the electron beam current is the strongest, and the optical disc or e-mail can be uncontrolled, resulting in the failure to dim the displayed waveform. The difference between the two cases is that the former will show "astigmatism" in the whole screen range, while the latter is only controlled by "focusing". During maintenance, the former can be determined by "device substitution method"; For the latter, when the oscilloscope is powered on, the "measuring resistance method" can be used to detect the path resistance between the grid pin of the oscilloscope tube and the corresponding solder on the tube base, so as to judge it and make necessary repairs

(2) if the cathode emission capacity of the oscilloscope tube decreases, that is, there is an aging problem, the electron beam current in the tube becomes weak, resulting in the dim brightness of the display waveform and can not be adjusted, so that the waveform cannot be displayed. During maintenance, the "change the status quo method" can be adopted, that is, try to increase the supply voltage (V) of the oscilloscope filament, or short circuit the cathode series resistance (RK) of the oscilloscope, so as to increase the electron beam current and improve the brightness of the displayed waveform. But in the final analysis, we still need to update the oscilloscope tube to fundamentally solve the problem

(3) the high-voltage circuit of the oscilloscope tube refers to the positive and negative DC high-voltage power supply used to supply each electrode of the oscilloscope tube, as well as the corresponding voltage dividing circuit. As shown in the figure, the high voltage circuit schematic diagram of the ordinary oscilloscope tube is shown. Here, R1, W, R2, R3, etc. constitute the voltage dividing circuit of "-1500v" DC high-voltage power supply. Adjusting the activity characteristics of potentiometer W1 can make the potential difference between grid g and cathode K of the oscilloscope tube change in the range of "-10v" to "-100v". "-10v" working point is equivalent to the brightest "Brightness"; "-30v" working 3. The maximum force FM is not less than 94.3-384kn; Dot is equivalent to "Brightness" dim; "-40v" to "-100v" operating points are equivalent to dark areas

if the resistance elements behind the voltage dividing resistor R2 have problems such as value change, faulty soldering, damage, etc., that is, the voltage dividing circuit is disconnected, so that the potential difference between G-K cannot be adjusted to the dark area, so there is a fault that the display waveform cannot be dimmed. During maintenance, the "measure voltage method" and "change the status quo method" can be used to detect whether the potential difference between the oscilloscope tube G-K is normal when the power is on. That is, while adjusting the "brightness control" potentiometer W1, the DC electronic voltmeter with high input impedance is used to detect the voltage value between g-k. Or in the case of no power, use the "measuring resistance method" to detect the resistance value and path of each voltage divider in order to find problems

if the partial voltage resistors R1 and W1 have "open circuit" problems such as virtual and sometimes cast structure (cast steel, cast aluminum, etc.) welding or damage, the potential difference between oscilloscope tubes G-K will greatly exceed "-100v", resulting in the fault phenomenon of no wave display. During maintenance, the "voltage measurement method" or "resistance measurement method" can be used to determine. If the voltage dividing resistor R1 changes value or the plug cap of the third anode A3 of the oscilloscope falls off, the fault phenomenon that the display waveform is dim and cannot be adjusted will appear. During maintenance, it can be determined by "observation without power" and "resistance measurement method" belonging to category 1 of equipment accessories

(4) modern electronic oscilloscopes mostly use fast and flexible oscilloscope tubes as display devices, and its high-voltage circuit of oscilloscope tubes is shown in the figure. Here, two groups of negative high voltages "-1100v" and "-1250v" are used as the DC power supply of the voltage dividing circuit of "Brightness" control and "focus" control respectively, in order to reduce the influence of "Brightness" control and "focus" control on the display waveform, so as to improve the stability of the oscilloscope. However, if the voltage value of one of the two groups of negative high voltage changes, or the resistance elements in the two-component voltage circuit have problems such as value change, faulty soldering, damage, etc., it will lead to abnormal "Brightness" control of the waveform

it can be seen from the figure that the maximum potential difference between grid g and cathode K of oscilloscope tube G is "-150v". At this time, there is no electron beam current, that is, there is no waveform display. However, with the help of the voltage divider circuit composed of R4, W2 and W, when the "Brightness" control potentiometer W is adjusted, the potential difference between the oscilloscope tube G-K can change in the range of "-10v" to "-100v", which can be turned on or off, so as to realize the normal "Brightness" control. Here, W2 is the internal voltage divider, which is used to compensate the variable of the voltage divider resistance; Rk is the series resistance of the cathode of the oscilloscope tube, which is used to limit the size of the electron beam current. If the output of the "-1250v" negative high-voltage power supply is low (that is, the absolute value of the voltage is less than 1250v), or one of the resistance elements in front of W4 in the voltage divider circuit has problems such as value change, faulty soldering, ring loss, etc., the potential difference between the oscilloscope tube G-K may not be adjusted to the "dark area" (vg-k "-30v), and the oscilloscope will have the fault that the" Brightness "control cannot be dimmed. During maintenance, the "change the status quo method" can be used first, that is, to change the resistance value of W2 to observe the improvement of its "Brightness" control, or under the condition of no power supply, the "measurement resistance method" can be used to detect the leakage resistance of G in the "-1250v" filter circuit, as well as the resistance value and path condition of each partial voltage resistance, so as to find out the cause of the fault and eliminate it

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