Start from raster is a keystone to check, log pillow 27V School Board power supply voltage is 19V, check 27V power supply circuit components are normal, test -27V power source only about -19V. Because the voltage is provided by the output circuit, indicating output circuit is not working properly. Measuring output circuit supply voltage + b-86V, and unstable, while working at 115V. From this analysis, fault causes are probably in switching power supply circuit and grating trapezoidal distortion, image swings, different steps are likely to be caused by +B power supply voltage drops and unstable. + B voltage measuring switching power supply, standby 56V 86V and unstable second boot time. Adjust the potentiometer RP851 monitor +B no significant change in voltage, optocoupler NQ426{3} pin voltage is always -7V also remained unchanged. Check key elements NQ426, RP851, and VQ827 were normal, disconnect the load, connected to 70W iron false load, power on the test +B voltage is 115V; with two 150W lamp parallel false load boot + b voltage measuring and significantly lower, indicating that switching power supplies with load capacity variation. Combined with +B voltage instability, and should concentrate on examining the current constant current drive circuits.
The power constant current driver circuit as shown in Fig. Figure VQ820 for the drive pipe, provide positive feedback control signals by T803{9} feet. VD820 and C821 as constant current power supply rectifier element R822 drive current limiting resistor, resistance size determines the size of drive current, current the bigger, stronger load-carrying capacity. Point a in the diagram provided by the VD820 constant-current drive voltage, it is subject to standby to control. Normally, a voltage in standby for 3. 5V, 8 second boot time. 5V. Measuring a voltage in standby mode is 0. 1V, the boot time is 0. About 4V, suggesting that constant-current drive circuit is not working properly. A low may have two reasons: (1) a fault in the load circuit that follows; (2) the rectified filter circuit has a problem. Check the load circuit is not short circuit, checks the rectifier circuit VD820, C821, ZP01 found no abnormalities, T803 questions? In the boot state logging T803{8} AC voltage 5V or so feet, why after the rectifier voltage becomes low, then? Welding VD820 double check again, discovery and its forward resistance value is too large compared to normal diode. Replaced VD820, a voltage to return to normal, + b voltage to return to normal, these symptoms ruled out. Analysts believe that is VD820 to the resistance value becomes Ambassador of constant-current power supply internal resistance becomes larger, resulting in the rectifier with load capacity variation, so that the total switching power supplies with load capacity variation. A seemingly complex faults, and its cause was a small diode!