How to use an oscilloscope for home appliance repair

　　The use of oscilloscopes in the process of home appliance maintenance is very common. Through an oscilloscope, you can visually observe the waveform of the circuit under test, including shape, amplitude, frequency (period), and phase. You can also compare two waveforms to quickly and accurately find the cause of the fault. Correctly and skillfully using an oscilloscope is a basic skill for novice maintenance personnel.

　　Although there are many brands, models, and varieties of oscilloscopes, their basic composition and functions are similar. This article introduces how to use a general-purpose oscilloscope.

　　1. Panel Introduction

　　1. Brightness and focus knob

　　The brightness adjustment knob is used to adjust the brightness of the light trace (some oscilloscopes call it "brightness"). The brightness should be appropriate when using it. If it is too bright, it is easy to damage the oscilloscope tube. The focus adjustment knob is used to adjust the focus (coarseness) of the light trace. It is better to use it when the image is clear.

　　2. Signal input channel

　　Commonly used oscilloscopes are mostly dual-trace oscilloscopes, which have two input channels, namely Channel 1 (CH1) and Channel 2 (CH2). They can be connected to oscilloscope probes respectively, and then the oscilloscope casing is grounded and the probes are inserted into the part to be measured for measurement.

　　3. Channel selection key (vertical selection)

　　Common oscilloscopes have five channel selection keys:

　　(1) CH1: Channel 1 is displayed separately;
　　(2) CH2: Channel 2 is displayed separately;
　　(3) ALT: The two channels are displayed alternately;
　　(4) CHOP: The two channels are displayed intermittently, which is used for dual-trace display when the scanning speed is slow;
　　(5) ADD: The signals of the two channels are superimposed. In maintenance, channel 1 or channel 2 is often selected.

　　4. Vertical sensitivity adjustment knob

　　To adjust the vertical deflection sensitivity, the position of the knob should be adjusted according to the amplitude of the input signal. Multiply the value indicated by the knob (such as 0.5V/div, indicating that the amplitude of each vertical grid is 0.5V) by the number of grids occupied by the measured signal in the vertical direction of the screen to obtain the amplitude of the measured signal.

　　5. Vertical movement adjustment knob

　　Used to adjust the vertical position of the measured signal light trace on the screen.

　　6. Horizontal scan adjustment knob

　　To adjust the horizontal speed, the knob position should be adjusted according to the frequency of the input signal. Multiply the knob value (such as 0.5ms/div, indicating that each horizontal grid time is 0.5ms) by the number of grids occupied by one cycle of the measured signal to obtain the period of the signal, which can also be converted into frequency.

　　7. Horizontal position adjustment knob

　　Used to adjust the horizontal position of the measured signal light trace on the screen.

　　8. Trigger mode selection

　　Oscilloscopes usually have four trigger modes:

　　(1) NORM: When there is no signal, there is no display on the screen; when there is a signal, it cooperates with the level control to display a stable waveform;
　　(2) AUTO: When there is no signal, a light trace is displayed on the screen; when there is a signal, it cooperates with the level control to display a stable waveform;
　　(3) TV: Used to display TV field signals;
　　(4) PP AUTO: When there is no signal, a light trace is displayed on the screen; when there is a signal, a stable waveform display can be obtained without adjusting the level. This mode is only used in some oscilloscopes (such as CALTEK CA8000 series oscilloscopes).

　　9. Trigger source selection

　　There are two types of trigger sources for oscilloscopes: internal trigger source and external trigger source. If you choose an external trigger source, the trigger signal should be input from the external trigger source input terminal, which is rarely used in home appliance maintenance. If you choose an internal trigger source, generally select channel 1 (CH1) or channel 2 (CH2), which should be selected according to the input signal channel. If the input signal channel is selected as channel 1, the internal trigger source should also select channel 1.

　　2. Measurement Method

　　1. Amplitude and frequency measurement method (taking the calibration signal of the test oscilloscope as an example)

　　(1) Insert the oscilloscope probe into the channel 1 jack and set the attenuation on the probe to "1";
　　(2) Set the channel selector to CH1 and the coupling mode to DC;
　　(3) Insert the probe tip into the small hole of the calibration signal source. A light trace will appear on the oscilloscope screen;
　　(4) Adjust the vertical and horizontal knobs to stabilize the waveform displayed on the screen and set the vertical and horizontal fine adjustments to the calibration position;
　　(5) Read the number of grids occupied by the waveform in the vertical direction and multiply it by the indicated value of the vertical attenuation knob to obtain the amplitude of the calibration signal;
　　(6) Read the number of grids occupied by each cycle of the waveform in the horizontal direction and multiply it by the indicated value of the horizontal scan knob to obtain the period of the calibration signal (the inverse of the period is the frequency);
　　(7) The frequency of the general calibration signal is 1kHz and the amplitude is 0.5V, which is used to calibrate the internal scanning oscillator frequency of the oscilloscope. If it is abnormal, adjust the corresponding potentiometer of the oscilloscope (internal) until it matches.

　　2. Oscilloscope application example (taking the measurement of 13MHz clock pulse of 788 mobile phone as an example)

　　The normal 13MHz clock signal in the mobile phone is a necessary condition for starting up, so when repairing, you should often measure whether there is a 13MHz clock signal. The steps are as follows:

　　(1) Turn on the oscilloscope and adjust the brightness and focus knobs so that a horizontal bright line with moderate brightness and good focus appears on the screen;
　　(2) Calibrate the oscilloscope as described above and then set the coupling mode to AC;
　　(3) Clamp the grounding clip of the oscilloscope probe to the grounding point of the mobile phone circuit board and insert the probe into the CPU pin of the 788 mobile phone;
　　(4) Turn on the power of the mobile phone, press the power button, adjust the vertical scan level and the horizontal scan knob, and observe whether a stable waveform appears on the screen. If not, it generally means that there is no 13MHz signal.