There are various forms of sine waves running in nature, such as ocean waves. Earthquakes, sound waves, sounds that travel through the air or the natural rhythms of the body, energy, vibrational particles and invisible forces are everywhere in the physical world. Even light has its own fundamental frequency and appears different colors depending on the fundamental frequency.

There are various forms of sine waves running in nature, such as ocean waves. Earthquakes, sound waves, sounds that travel through the air or the natural rhythms of the body, energy, vibrational particles and invisible forces are everywhere in the physical world. Even light has its own fundamental frequency and appears different colors depending on the fundamental frequency. ?

With sensors, these forces can be converted into electrical signals that can be observed and studied with an oscilloscope. With an oscilloscope, scientists, engineers, etc. are able to observe events over time. ?

An oscilloscope is a must-have for anyone designing, manufacturing or servicing Electronic equipment. In today’s fast-changing world, engineers need the best tools to quickly and accurately solve measurement problems. In the eyes of engineers, oscilloscopes are naturally a key tool to meet today’s various measurement challenges.

Oscilloscopes quickly repair CPU and bus-related faults

As we all know, the basic working conditions of the CPU are mainly that the power supply (CPU power supply VCC), the clock (the clock oscillation circuit of the CPU), and the reset (the reset circuit of the CPU) must be normal. With the circuit changes of new products, the bus circuit, keyboard interface and other auxiliary signals (such as TV line and field reverse pulse) will also affect the normal operation of the CPU. This lecture will explain how to use an oscilloscope to quickly repair CPU and bus-related faults with examples.

It is impossible to judge whether the clock oscillation of the CPU is established with an ordinary multimeter. The clock frequency cannot be measured, nor can it be intuitively measured for bus signals and other auxiliary pulse signals. Therefore, it is often used to make rough judgments and replace them one by one. The maintenance efficiency is naturally not high. If it encounters difficult faults, it will also consume a lot of time. But using an oscilloscope to detect, we will find that the detection time is greatly shortened. The speed and accuracy of fault location is much higher than with other traditional methods.
 

Oscilloscopes quickly repair CPU and bus-related faults

The basic detection points for overhauling CPU and bus circuit faults are shown in the attached figure (A~N is the detection sequence).

The attached drawings more comprehensively reflect the relationship between the color TV CPU and other circuits, but a specific TV will not include all the circuits in the block diagram. For example, some TVs have only one pair of read and write EEPROM buses. Another example is that some TV sets are all controlled by bus, without PWM control signal, and the current single-chip microcomputer does not have an independent character oscillation circuit.

Brief description of the inspection points of each inspection point

(1) The working power supply of the CPU mainly observes the power supply ripple. If the failure of the power filter capacitor or the abnormal voltage regulator circuit occurs, the CPU will work abnormally. Slow start-up may occur. Or the CPU resets repeatedly, causing the standby control output voltage of the CPU to repeatedly switch between standby and power-on, or the relay vibrates and makes an abnormal noise. The VCC voltage value can be directly read by the DC coupling of the oscilloscope. The method is to “count the grids in the vertical direction” and then convert the voltage value, instead of using a multimeter to measure the voltage.

(2) The reset circuit generally only needs to measure the DC voltage. If you need to observe the reset process, you need to use a dual-trace oscilloscope to observe. Since the reset time is in the order of milliseconds, it is difficult to observe.

(3) The observed signal frequency of the clock oscillation circuit is the nominal value on the quartz crystal. We should learn to count the horizontal grid, and after reading the period of the signal, we can calculate its frequency. Note: During measurement, the probe attenuation switch of the oscilloscope should be set to the 10:1 position to increase the input impedance of the probe (10MΩ), otherwise the clock circuit will stop oscillating due to the load effect of the probe.

(4) The bus signal SDA and SCL, at the moment of booting, the CPU must read the data in the EEPROM through the bus. Determines the power-on working state of the TV (program position, volume and other analog quantities are too small). The magnitude of its amplitude is the power supply VCC. For example, the CPU is powered by 5V, and the SDA and SCL waveform amplitudes are 5Vp-p (peak-to-peak).

(5) The second pair of buses is used to control other circuits except EEPROM. When starting up, the CPU needs to transmit the control data to the corresponding circuit units through this bus, such as raster geometric distortion correction circuit, AV/TV conversion circuit, sound effect processing circuit , frequency synthesis tuner, etc.

(6) Keyboard control circuit The early TV sets adopted the keyboard scanning control method, and it was very convenient to measure the signal with an oscilloscope. Its waveform is also a pulse with an amplitude of VCC. Most of the current TV sets use the voltage comparison input method. You can also use an oscilloscope to measure and observe the amplitude and stability of the DC voltage. If the circuit is abnormal, there will be fluctuations, such as button leakage and other faults.

(7) The pulse of the remote control signal amplitude equal to VCC.

(8) PWM signal, that is, pulse width modulation signal whose amplitude is equal to VCC.

(9) The character positioning pulse signal is input into the shaped line and field pulses respectively, and the amplitude is approximately equal to the pulse of VCC.

(10) The character oscillating signal generally oscillates only when the character needs to be displayed and the character positioning pulse signal is input at the same time. If the above conditions are met, there is still no oscillation. It is the CPU or peripheral circuit failure.

(11) The character output by the CPU shows the pulse whose signal amplitude is equal to VCC.

(12) It is helpful to observe the change amplitude and change range of the AFT input signal when overhauling the fault that the automatic search cannot save the program.

(13) Program identification signal There are generally two ways for CPU program identification, namely pulse counting mode (detecting the number of line synchronization signals per unit time) and voltage identification mode. It is very convenient to observe the signal waveform or the voltage variation range when overhauling the fault that the automatic search cannot save the program.

(14) The protection control input terminal is usually a high level input, and when a fault occurs, a low level is input. Measure the DC voltage to analyze whether it enters the protection state (Note: the working power VCC of common CPUs is 5V or 3.3V).

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