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Bestec ATX-300-12Z with NO bad C36

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    #1

    Bestec ATX-300-12Z with NO bad C36

    Hi,
    I have a Bestec ATX-300-12Z that has what seemed "the usual problem" at first sight. When plugged in but not connected, the led (connected to 5VSB) is on. 5VSB measures okay at ~5,1 volt. As soon as I try starting the power supply (I just connect PS ON to GND) the led goes off. When I put a ~1A load on the 5VSB, the voltage drops and the led starts flashing. You'd think C36 (the 2200 uF smoothing capacitor for 5VSB) is bad. But it isn't. Neither is C50 (100 uF, 50V) or C60 (10 uF, 50V). I tested them all with an ESR meter, and even though the ESR meter showed they were fine, I replaced them anyway. No change at all.
    I haven't started measuring voltages on the live power supply yet (feels a bit tricky...) but if all capacitors are fine, what other components could be suspect?
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    #2
    Re: Bestec ATX-300-12Z with NO bad C36

    I have seen this before. There was a short...Mine just had a solder ball that just got stuck between two leads lol. Can we see some pictures of the unit? It is also possible that you have brown conductive glue causing problems.

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      #3
      Re: Bestec ATX-300-12Z with NO bad C36

      Here are a few pictures. I took quite a few bits out already, hence the different kind of solder used on some components.
      Attached Files

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        #4
        Re: Bestec ATX-300-12Z with NO bad C36

        Check the 5VSB output diode. I had a unit once that would give ~5V on the 5VSB but would not power up. Pulled out that output diode and it tested bad. Believe it or not, I've also had a Bestec unit not power up due to failed caps on the secondary side.

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          #5
          Re: Bestec ATX-300-12Z with NO bad C36

          I checked the diode, but it's ok. I even tested it by running 2 amps through it and it worked fine. As the 5VSB can't supply 1 amp, even if I don't try to power on the supply, I doubt that it's related to any of the secondary caps.
          I still don't understand what's happening with the power supply

          Comment

            #6
            Re: Bestec ATX-300-12Z with NO bad C36

            Originally posted by SpecialK View Post
            I still don't understand what's happening with the power supply
            We don't either without a schematic - do you have one? It is possible your standby circuit is barely operating, just enough to charge up the secondary filter, but that's all. Some d.c. readings and component checks are the next step.
            Is it plugged in?

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              #7
              Re: Bestec ATX-300-12Z with NO bad C36

              how did you check them with the ESR meter? In or out of circuit? Also there are some cheap ESR meters, that are good for nothing. That can be another factor. That happened here before.

              I guess we need some schematics and better pictures too, yours are slanted and then not all of the board showing.
              If the PWM has a start up resistor, change that. I had an instance on a PSU, that would power, but shortly after it turned of like dead. Found out that the startup resistor went high in Ohm value when under load, but went back to correct value after sitting for a minute.
              Question right now is, does the PWM for the 5VSB rail get correct Vcc when the PSU breaks down?
              Can you identify that IC?

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                #8
                Re: Bestec ATX-300-12Z with NO bad C36

                O.k. I found something on a ATX-250-12Z. Verify that circuit if it is the same or close enough?
                Attached Files

                Comment

                  #9
                  Re: Bestec ATX-300-12Z with NO bad C36

                  The closest matching schematic I found is attached. I've measured the caps out of circuit, and I even replaced the critical caps but that didn't change anything.
                  The 5VSB IC is an STR-A6351. I haven't done any measurements yet when it's plugged in (always a bit tricky to measure on the primary part I think) but I guess I have no other choice anymore
                  Attached Files

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                    #10
                    Re: Bestec ATX-300-12Z with NO bad C36

                    It won't get you anywhere to look at pins 7 and 8, since you know that the circuit works half way. A good place to check is pin 3 of IC8 - the Vcc terminal. Vcc is supplied by a separate winding on the transformer through D7-C1. That voltage has to be there right away after startup and free from switching hash. Also check R24 for a change in value.
                    Is it plugged in?

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                      #11
                      Re: Bestec ATX-300-12Z with NO bad C36

                      Must have been a bad solder joint or a problem in the circuit board after all. After putting the power supply back together to do some more measuring, it suddenly started working again. I hate when that happens...

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                        #12
                        Re: Bestec ATX-300-12Z with NO bad C36

                        Reflow all the joints in the 5vSB section and hope it doesn't happen again.
                        "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                        -David VanHorn

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