Open cover observation, when the failure occurs when the spindle motor speed will suddenly become slow, and then return to normal; when the spindle is completely stopped when the crash. According to the judgment of the fault and the main axis servo circuit. This machine adopts servo circuit Philips digital servo movement, the spindle servo control loop is shown in Figure 1. Voltage values for each pin. It can be known that the SAA7345GT{22} pin output of the main shaft is controlled by the pin at the stop time. The change of the output of the main shaft is decreased to the abrupt change of the 0V, which caused the speed of the spindle motor to be changed slowly or stopped at the moment. According to the principle of the spindle servo control, the servo control voltage of the {22} pin is normal or not and the amplitude of the input RF signal. With an oscilloscope observation RF signal SAA7345GT{8} input pin, found the waveform instability, eye is not clear, and the amplitude of the only 0.8VP - p; RF signal waveform in the head assembly to elicit soft row line {4} feet observed is only 0.8VP - P, which shows head assembly output RF signal is too weak. The VR potentiometer on the head, the amplitude of the RF signal is changed. So far, the bald head aging, a new CDM12.4 after the replacement of a new, troubleshooting. The measured RF signal amplitude is greater than 1.5VPP, and clear eye diagram, stable. The machine head and CDM1201 different between the two leads to the cable is also different, Philips CDM12.4 / 05 bald head and Philips CDM1201, 1202 head circuit shown in Figure 2, figure 3, each other can't swap.GbHservice manual download site