My switching PSU gone silent dead. There is voltage at DC side of diode bridge rectifier so the input filter and protection are in good condition. Where should I look next to check which component are broken? Note: this model using KA7500B as PWM controller.

Update 1: there is no voltage supply to PWM controller ICs. Diode and transistor seem fine when I check with multimeter in diode mode (see some forward voltage and not short-circuit)

Update 2: for referencs I found some reverse engineer work for similar product here enter image description here I'm not sure how it's work at standby supply part. what to do when there are no voltage output? enter image description here enter image description here enter image description here

  • 3
    \$\begingroup\$ Most likely to fail is the switching transistor which is most likely the pair fixed to the chassis on the top side. It could also be transformer winding. At the cost of those, buying a new PSU is probably the best option. \$\endgroup\$
    – Damien
    Nov 27, 2020 at 7:28
  • \$\begingroup\$ Measure all power semiconductors with your multimeter in diode mode. Anything open circuit or shorted? \$\endgroup\$
    – winny
    Nov 27, 2020 at 7:51
  • \$\begingroup\$ @Damien I need to buy new one for sure but maybe I will try to fix it for educational purpose. \$\endgroup\$
    – M lab
    Nov 27, 2020 at 10:32
  • \$\begingroup\$ Good! That shouldn't be too hard to fix. Find out what's either not supplying Vcc or what's draining it. \$\endgroup\$
    – winny
    Nov 27, 2020 at 13:15
  • \$\begingroup\$ Electrolytic capacitors are the weakest parts in power supply due to their lifetime. Have a look around this as well. \$\endgroup\$
    – Delphesk
    Nov 27, 2020 at 13:55

1 Answer 1


Common problems with these power supplies I've encountered are:

  1. high value (100k+) resistors going open circuit. There is normally 2 resistors in series to give adequate voltage rating, but some have only one resistor. These rsistors can be difficult to measure in circuit - unsolder one end and measure.

The resistors in question are the 'kick start' - they give enough voltage to the switcher ic to start up. Once started, a winding on the main transformer is used along with a diode and capacitor to supply the switching ic from then on. The capacitor (usually 47-100uF 25-50V) can dry out. The power supply normally hiccups in this case.

  1. Output diodes gone short circuit. This will shut the supply down quick sticks. Measure the diodes. Usually these are TO220 pkg dual diodes for the main high current output.

  2. Main input capacitors drying out. If this happens with a psu topology like yours, then the switching transistors vapourise. You can replace the transistors but they keep on blowing until you replace the electrolytic caps.

Note that some power supplies don't like starting into no load. Some big resistors or some automotive light bulbs can suffice for a dummy load.

Sometimes you get a random failure. In this instance it a matter of testing each component. You can test the switcher ic by powering from a bench supply - read the chip's datasheet to see what voltage it requires - usually 12-15V. Use an oscilloscope to see if the ic is outputting pwm to the transistors. Make sure the power supply itself is NOT powered by mains voltage!

  • \$\begingroup\$ I got some update please check it out. \$\endgroup\$
    – M lab
    Nov 27, 2020 at 11:06
  • \$\begingroup\$ where is kick starter in my reference schematic? \$\endgroup\$
    – M lab
    Nov 27, 2020 at 13:35
  • \$\begingroup\$ The 150k resistors around Q1 and Q2. \$\endgroup\$
    – Kartman
    Nov 27, 2020 at 21:02
  • \$\begingroup\$ Can you explain how this circuit startup? I don’t understand how the first pulse go to main tranformer? \$\endgroup\$
    – M lab
    Nov 28, 2020 at 8:26
  • 1
    \$\begingroup\$ Consider it black magic. But if those resistors go open circuit, the magic don't work. \$\endgroup\$
    – Kartman
    Nov 28, 2020 at 11:19

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