Inspection methods for oscilloscope faults often include visual inspection, circuit inspection, and comparison inspection. The visual inspection method is to observe whether the oscilloscope has a broken wire, an imaginary weld, a poor contact, whether the components are burnt with buzzing, whether the solder joints are ignited, whether they have scorched odors, smoke, etc., by watching, listening, and smelling. The circuit inspection method is to inspect by measuring the voltage of the power supply, measuring the voltage to the ground of the complete machine or the local circuit tube, adding the signal of the person, and the like. The comparison inspection rule is to compare the ground resistance, ground potential and waveform of the working point of the faulty oscilloscope with the normal oscilloscope circuit. Now the author has encountered a fault in the DF4211 oscilloscope encountered in the work as a guide, talk about oscilloscope repair methods.
1 Oscilloscope maintenance principles
1.1 Correctly read the circuit diagram
Correct reading is the first step in maintenance work. “Reading” is to fully understand the internal circuit of the oscilloscope based on the electrical schematic diagram, familiar with the working principle and structural characteristics of the circuit, understand the signal generation and transmission process, various changes in the current and voltage, the function of each part and the role of each component The process is as follows:
1.1.1 Build a complete box concept. Divide into integers, use the box structure that the oscillograph work principle described in this text analyzes the electrical schematic diagram, clear the function of every partial circuit, establish the frame concept of the complete machine, grasp the basic structure of the circuit that is analyzed.
1.1.2 Further clarify the signal flow to understand the ins and outs of the signal and various changes. That is, to clarify the relationship between the functional blocks, a clear division of work is performed within the functional blocks. There are two general ways to clean up:
1 forward cleaning method. From the foregoing oscilloscope principle block diagram, grasp certain characteristic components in the circuit and straighten it from the front to the back. Identify the basic structure of the actual circuit, the signal processing process, and define the functions at each level. For example, input from Y1 to the preamplifier, then to the vertical switching circuit, and finally to the Y output amplifier. 2 reverse cleaning method. Starting from the terminal load, clean up from the end to the front. Such as kinescopes, deflection yokes, especially the terminals of X output amplifiers and Y output amplifiers, have distinct features in the circuit and are easy to identify. Therefore, taking them as the starting point, according to the signal processing process required by the theory, step by step forward to clarify the functions at all levels.
1.1.3 Clear the power supply system. It must be clarified which part of the circuit is taken from the regulated power supply, which part of the circuit is taken from the “DC-DC” power supply circuit and marked with a red pen.
1.1.4 Distinguish familiar, rusty, and special circuits. The oscilloscope circuit is actually a comprehensive application of many typical circuits in electronic circuits and integrated circuits. In the oscilloscope circuit, there are some circuits that we are familiar with, although they have new functions in the oscilloscope circuit, but the basic form of the circuit has not changed, and still apply to their overhaul methods; some circuits are relatively unfamiliar; The circuit is unique to the oscilloscope. Differentiating these kinds of situations and using strange and special circuits as the focus of study and study can improve the reading ability of the circuit faster.
1.1.5 Simplify difficult circuits by: First analyzing the DC system. Defining the DC power supply of the circuit under analysis, estimating the quiescent operating voltage at the key point, understanding the nature of the bias and coupling between the stages, and analyzing the role of the components in the circuit.
1.2 Analyzing the Communication System
That is, the AC equivalent circuit is drawn by the analysis method of the AC equivalent circuit, so as to distinguish the nature of the circuit, analyze the working principle of the circuit, and study the input and output waves. Distinguish between the major components and the general components.
1.2.1 The main component is the component that directly affects the basic function of the circuit. The general component is the component added to improve some of the circuit’s performance. In the maintenance, the main component is first examined and then the other component is checked.
1.2.2 Prepare the necessary measuring instruments, tools and spare parts.
1.2.3 Master the method of distinguishing components and understand the principles of component replacement, replacement, and selection. Care should be taken when replacing the oscilloscope with components. The transistor characteristics of the oscilloscopes used in different oscilloscopes vary widely. The characteristics of the substitute model components must not be lower than those of the components used in the original model.
1.2.4 Methods and experience for borrowing TV repairs. Because oscilloscopes and televisions have a large number of similar circuits, such as high-voltage and kinescope circuits, channel amplifier circuits, and small-signal scan trigger circuits, most TV repair methods and experiences are suitable for oscilloscopes and can be borrowed directly.
2 Oscilloscope Common Fault Types and Troubleshooting Methods
1 Close the power switch light is off. Method: Check power supply, power cord, fuse, power transformer. 2 Turn on the power switch light, but there is no trace. Method: Center the vertical horizontal displacement, set the Y mode to “Y1”, set the t/cm to “1ms”, set the scanning mode to “Auto”, set the X mode to “A”, increase the luminance If no trace appears, enter the down One step; set t/cm to “X external” to see if there is a light spot; if not, go to the second step; unplug the Y output socket and internal trigger signal on the board and short-circuit it. If light traces appear at this time that the Z axis amplifier is normal, check the Y output amplifier or X output amplifier, otherwise, check the Z axis amplifier; if t/ cm is set to “X external”, the light point appears and the third step is entered. Check the X output amplifier and the sweep generator that generates the sawtooth wave. 3Y displacement is not normal. Method: Short-circuit the delay line to see if the light trace or light spot can return to the center of the screen. If you can go back to the center of the screen, check the Y preamplifier and delay line circuit. If you cannot return to the screen center, check the Z output amplifier. 4 The horizontal displacement is abnormal and check the X output amplifier. 5 The tested waveforms are not synchronized. Set Y mode and internal trigger to “Y”, trigger coupling to “AC”, trigger source “in”, adjust level, check trigger generator and trigger signal amplifier if it is still not stable. . The 6Y displacement is normal, but the Y mode is set to “Alternate” or “Continue” exceptions, and the vertical displacement switch circuit is checked. 7 The focus is poor or the brightness is too dark. Check the oscilloscope control circuit. In addition, the fault phenomena related to the X output amplifier have large error in the signal cycle test, and the error phenomena related to the Y output amplifier have large signal amplitude error, and some false faults caused by improper use, such as: no trigger selection button There is no scanning baseline when “Automatic” or “Trigger” is selected, there is no test signal or the test signal is bad when the probe of the oscilloscope is in poor contact, and synchronization cannot be synchronized due to wrong selection of the trigger.
3 Faults and Repairs
To repair a faulty oscilloscope, you must be familiar with the working principle of the oscilloscope, so that the specific part of the fault can be determined by the fault phenomenon, whether the power circuit is faulty, or the scan, trigger, amplifier, and display circuit are faulty. Contradictions, faults can be simplified, and then specific to an electronic component, troubleshooting. When we calibrating the DF4211 oscilloscope, we turned on the oscilloscope power and found that the fluorescent screen only showed two bright spots (the DF4211 oscilloscope is double-lined), there is no horizontal bright line, and the coupling mode is DC, which can discharge the power circuit and display the circuit failure may. Turn the vertical deflection sensitivity knob, the fault phenomenon still exists, and then turn the scanning time to deflect the sensitive knob. There is sometimes no fault phenomenon. Initially judge the fault in the scan circuit. Rotation scan time deflection sensitivity knob to 2ms, 5ms, 10ms third gear fault phenomenon still exists, from which to judge the fault in the relevant circuit of these three positions. Turn on the oscilloscope and check it from the simplest circuit. As shown in Figure 1, the sweep time deflection sensitivity knob has a total of 18 files and is converted progressively by 2, 5, and 10. We start by deflecting the sensitive knobs by rotating the scan time and carefully looking for the circuit. We find that 2ms, 5ms, and 10ms (only the third gear is shown in the figure) are connected at the same time in the third circuit with a capacitance of 0. 047μF and 3 000pF capacitors in parallel. One end is connected to a resistor of different resistance, and the capacitor and the resistor may malfunction. No resistance of the measuring resistor, no damage to the capacitor. The resistors and capacitors are re-soldered to the circuit and the fault still exists. With a capacitor of 0.047μF connected to the 0047μF capacitor on the circuit, the fault disappears. An on-line measurement of 0047μF capacitance causes damage to the capacitor. Replace with a good capacitor and troubleshoot. The reason for the failure of the analysis is that the capacitor is damaged, the damage is intermittent, and when the voltage is measured in the circuit, it is damaged; it is good to measure it without voltage under welding.
The troubleshooting of oscilloscopes is a long process—analysis, search, detection, and replacement. Proficient use of oscilloscope three-in-one meters and other testing instruments is required to measure waveform, voltage, current, and other technical parameters. Maintenance of the oscilloscope is a process that constantly denies itself. When a negative reason is not found, it is when the fault is discharged. It is an effective way to improve maintenance technology and improve efficiency by drawing on others’ maintenance experience and working hard.