Use an oscilloscope to detect VCD machine waveforms

　　There are also many key points for maintenance and testing in the circuit of the VCD machine. The oscilloscope is the most powerful tool for repairing VCD machine circuits. As long as the fault waveform measured during maintenance is compared with the normal waveform, it is easy to compress the fault scope and find out the original location of the fault.

　　When testing the signal waveforms in the VCD machine circuit, it should be noted that some waveforms can still be measured when the VCD machine is in a paused state, and some waveforms can only be measured when the VCD machine is in a playing state. The normal waveforms of some key points given in this article are mainly measured on the Xinke Super VCD (SVD201MP) machine, which is a single-disc machine, as shown in Figure 1.

　　2. Optical head pick-up signal waveform The optical head pick-up signal waveform always exists after the computer is turned on and the disk is read normally. Therefore, the waveform can be tested in both the playback state and the pause state. The waveform is stable when stationary, making it easier to observe.

　　Figure 3 shows the 1st pin LD terminal 1. Clock waveform of the RF signal processing integrated circuit CXA2549M. The microprocessor transmits addresses and data with various clock beats. The normal operation of the clock system is the prerequisite for the normal operation of the entire machine. In the SVD210 machine, an oscillation waveform with a frequency of 16.9334 MHz can be measured on the external crystal oscillator lead of the digital signal and digital servo processor CXD3008, while a frequency of 27.000 can be measured on the 49th pin oscillation terminal of the MYEG decoder and microprocessor SVD1811. MHz oscillation waveform, an oscillation waveform of 27.000MHz can be measured at the 71st pin crystal oscillator output terminal of the audio/video processing circuit SVD1810, as shown in Figure 2.

　　Measured waveform. Set the vertical amplitude switch of the oscilloscope to "20 mV/div" and the scan speed switch to "0.2μs/div"

　　files.

　　3. RF signal test point waveform When the VCD player is in playback state, a clear "eye diagram" can be observed on pin 17 (RF signal amplification output terminal) of the RF signal processing integrated circuit CXA2549M, as shown in Figure 4. The signal amplitude is generally 1V. A clear eye diagram reflects that the power system, control system and focusing function of the whole machine are basically normal. During the test, set the vertical amplitude switch of the oscilloscope to the "0.5V/div" position and the scanning speed switch to the "0.5μs/div" position. This waveform can also be tested at the test point marked RF on the decoding board.

　　4. Servo loop The input end of the servo loop drive amplifier is also a key point for detection. For example, the input terminals of focusing, tracking, coasting and capstan servo drive amplifiers. On the decoder board of Xinke Super VCD (SVD210) machine, there are some points marked with "TE" (tracking error signal) and "FE" (focus error). Signal) and other words can be measured directly on these points, which is easy to operate. The "TE" (tracking error signal) waveform is a fuzzy waveform and can only be measured in the playback state. "FE" (focus error signal) is available in both the playback state and the static state (a) Waveform measured at the "TE" test point (b) Waveform measured at the "FE" test point Figure 5 "TE" and " FE” signal waveform (focus error signal) and other signals are shown in Figure 5.

　　The ③, ④, ⑤, ⑥, ⑧, and ⑨ pins of the RF signal processor CXA2549M are all photoelectric signal input terminals. The waveforms are similar, and they are all fuzzy "eye diagrams". Figure 6 shows the CXA2549M The actual waveform of pin ⑥ in Figure 6 is the measured waveform of pin ⑥ of CXA2549 (there are test points on the lead line of pin ⑥, which is easy to measure). This waveform can only be measured in the playback state.

　　5. Signal channel waveform There are two types of signal channels in the VCD machine: digital signals and analog signals. When observing the waveform with an oscilloscope, the digital signal is an irregular pulse waveform with an amplitude close to 5V. The analog video signal is similar to the signals in color TVs, such as: full TV signal, brightness signal, chrominance signal, etc., these signals vary with the content of the program changes, but it can still be measured in the paused state after powering on. In the paused state, the screen has the word "Total Track" on the blue background, the amplitude switch is set to the "1V/div" file, and the scan speed switch is set to the "2ms/div" file.

　　6. Display system waveform The LCD device used in this machine, MPEG decoding and microprocessor SVD1811 related pins output the LCD driving voltage. Figure 9 is the VFD-DATA waveform measured at the 169th pin of the SVD18111 at the measurement position (b) Figure 9. The VFD-DATA waveform at the 169th pin of the SVD1811. This waveform can be measured in both the static state and the playback state. The waveform in the static state is relatively Stablize.

　　7. Driving signal waveform focusing, tracking, capstan and pallet rack driving signals can also be measured with an oscilloscope. The image waveform of a still video signal such as a super VCD machine is very stable. The key point of detection is the input/output port of each channel mentioned above. Figure 7 shows the waveform measured at the relevant test points on the power supply and output board. During the test, an ordinary disc was used instead of a test disc. During the test, the disc player was turned on and then in a paused state (that is, playing the signal of prompt characters on a blue background). The vertical amplitude switch of the oscilloscope is set to "0.5V/div", and the scan speed switch is set to the audio signal waveform. It can also be measured on the 7th pin of the dual-channel operational amplifier 4558 on the output board. (SVD21OMP) uses a four-way BTL driver BA6392FP .

　　8. When other signal waveforms are operated and transmitted by the remote control, the remote control encoding waveform can be observed at the output terminal of the receiving amplifier with an oscilloscope. When measuring and observing with an oscilloscope, keep pressing or releasing the buttons on the remote control to make it continuously emit waveforms.

　　In microprocessors, there is also a terminal called the reset terminal. When powered on, under the action of the reset signal, the system's random access memory is cleared and the microprocessor enters the initial state. The reset voltage is a delayed voltage. Observe it with an oscilloscope. When you turn on the computer, you can see the jitter of the baseline, which is the rising process of the reset voltage.