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Estimation of faults in the power supply section or decompression plate. Open cover, and the power output of the test power is normal. The head can reset and have normal focusing action, indicating that the DSP is normal. Take out the decompression board, the board uses CIA84 decompression chip, CH7201 video encoder. External +5V DC power supply, the power of the test separately. Using the oscilloscope observation found CH7201 3, 4 feet 14.318mhz normal waveform, 41 feet in the output to the CL484 40.5MHz waveform of the normal, decompression chip 3.3V voltage is normal, normal external decompression board CPU MHz crystal oscillator waveform, reset is also normal.HUpservice manual download site
Will decompression plate loaded back to the machine, further analyzing the fault phenomenon was estimated to be operating panel display drive the CPU does not receive the host CPU data flow signal and decompression chip did not provide any data signal to the video encoder. When considering the replacement of decompression CPU board, when used with three tables measuring 20MHz crystal pin voltage found the pin voltage is 2.5V normal and display screen is partially lit, and immediately replaced a 20MHz crystal and then boot, display the normal light, the control panel of normal. In the disc, everything is normal. Why 20MHz crystal wave normal but decompression CPU board does not work? Will take the crystal and crystal cheer a waveform comparison, observed waveform of the original machine crystal containing heavier "burr" component, initially thought to be oscilloscope probe grounding caused by poor, resulting in overhaul waste a lot of time.
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