What is the maintenance method of switching power supply
STR—F6454 STR-F6454 is a special thick film circuit for switching power supply, which contains oscillation circuit, overcurrent, overvoltage, voltage stabilization control and switching pulse amplification circuits. The application circuit of this thick film block in Changhong CN-11 core color TV (H25K6O, 25Kl8, 29Kl9, C-29E6 and other models) is shown in the figure. The maintenance steps of the color TV with this movement repeatedly burning STR-F6454 are as follows:
1. Check the rectifier and filter circuit. If the inside of the fuse is black and the STR-F6454 (NQB21) is burst and damaged, check the rectifier diodes VD801A~VD8O4A, surge absorbing capacitors C805~C808, +300V filter capacitor C810, peak absorbing capacitors C811, C827 (Some models do not have C827, R823, R824, UB822).
2. Check the external circuit of STR-F6454 (4) pin STR-F6454 (4) pin is the power supply terminal for power start, and the normal voltage is 18.2V. If the voltage of this pin is less than 11V and stable, check the electronic voltage regulator circuit with components such as V801, VD808 (18V), VD803 (22V) as the core; if it fluctuates between 11V~15V, it means that the switching power supply has started to oscillate , then check the switching transformer secondary rectifier, filter components and load circuit for short circuit, especially the row circuit.
3. Check the voltage stabilization control loop. If the +B voltage drops from 160V to zero at the moment of power-on, check the voltage stabilization control loop components. You can first replace the optocoupler N830, and then replace the error amplifier N831. It is best to check whether R807 is open.
Note: Schottky diode AKO3 is externally connected between pins (4) and (5) of this IC. In practice, this diode may not be installed, or RU2, RU4, 55295 and other diodes may be used instead.
