Fault is the front panel power indicator is not lit, upon enquiry, was informed that failure occurs due to the mains line does not work after, and appears to be caused by electrician mistook the junction voltage failures.

Open the Cabinet, Visual inspection found the fuse had fuses, +300V main filter capacitor has a slight bulge. For ease of maintenance, its schematic plot (as shown in the figure).

In the picture you can see that the power supply using common PWM controller KA3842 as its core, with the effect of n-channel Dojo tube 01 switching power supply, stable 3.3V output voltage to the switch on the motherboard.


Circuits are defined by: General circuit are sent out through the optical coupling of the error signal to control KA3842 II foot (error input voltage) purpose of the reach regulation, and this circuit of optocoupler send the error signal is amplified by the 02 to control KA3842 of black-foot (external oscillator signal input), so as to achieve the goal of stable output voltage. The benefits of doing so are:

Error control signals from making subprime response faster. In addition, secondary output 3.3V voltage rectifier using multiple diode STPS2045, 3.3V output currents are therefore quite large.

Fuse has a fuse. Suspect components in a circuit short circuit, digital examination revealed a rectifying diode Dl and D2 breakdown, replace it using the same model components. Considering voltage rise is likely to penetrate the field effect tube Ql welding examination was normal, so I decided that KA3842 should be normal, main filter capacitor Cl has a slight bulge.

Experience estimating capacity decreases, other aspects should not be much of a problem, so replaced fuse power up test, can be blown up again. Seems to have a place to short circuit check 01 is normal, this description short circuit should be at the front of the circuit, the rectifier diode D1~D4 all, it inadvertently check Cl has a short circuit and its renewed power and no reaction, what then? With a digital instrument Cl ends without voltage 300V checked forward along the lines and found a PIN on the wire and Ll until this feet broken, is due to excessive current. Carefully connect power test, she found a stable 3.3V output, connected to the switch Board and cable work, thus, troubleshooting.