ATX power supply repair.

Discussion in 'Technical Repair' started by rahul411, Jul 9, 2018.

  1. rahul411

    Thread Starter Member

    Feb 19, 2018
    51
    2
    Hello people,
    Some days back my computer went off and didn't start at all after doing all sort of things from outside, so i decided to open the CPU and check what the actual problem is. Plugging away every pins from motherboard and other components i checked the voltages at every wire that comes out of the PSU and found nothing. After reading some articles on Google i found that after shorting the green wire and any of the ground (black) wire the PSU will turn on, i did that too and checked tge voltages again and found nothing.
    Then i open the PSU and after checking all the components on the PCB found that nothing. So i de-soldered the main switching MOSFET and 3 schottky diodes mounted on the heat sink.
    Voltage drop across the MOSFET was according to what is specified on the datasheet but the 3 schottky diodes behave unusual. They are not completely short but they show very less forward voltage drop.

    Voltage drops are as follows-
    STPS30L45CW
    Vf=0.120V
    STPS20S100CT
    Vf=0.295V
    B30H60G
    Vf=0.170V

    PS. Fuse, rectifier IC and the main big filtering capacitors are okay.
     
  2. rahul411

    Thread Starter Member

    Feb 19, 2018
    51
    2
    I forget to ask the main question...
    Check the datasheet and the values i gave and tell me whether the diodes are faulty or not.
     
  3. dendad

    Well-Known Member

    Feb 20, 2016
    2,091
    539
    Schottky diodes have low forward voltage drop, those numbers look ok to me.
    Do they measure as diodes, pass current one direction and block the other?

    Please be careful when playing with switch mode supplies like these. Lethal volts abound.
    A PC power supply is a cheap item and I would advise you to just purchase another. Make is a bit higher power.
     
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    6,653
    1,556
    Can't do that as we don't know how much current your meter is supplying to the diodes. As long as reverse is much higher than forward then they are probably OK.
     
  5. rahul411

    Thread Starter Member

    Feb 19, 2018
    51
    2
    Thanks a lot...
    I think i must buy a new PSU.
     
    AlbertHall likes this.
  6. rahul411

    Thread Starter Member

    Feb 19, 2018
    51
    2
    Reverse blocking voltage is very high... Meter shows 1 when measured.
     
  7. Janis59

    Active Member

    Aug 21, 2017
    588
    94
    Ordinary methodics for ATX is so:
    1) check the ESR of input caps, those 400V. If any signs of excessive pressure inside, thats it.
    2) Check the `on duty` voltage of TL494 or KAxxx tablet.
    3) Check the forbidding/allowing voltage on this tablet.
    4) check generation on this tablet
    5) check the 5V line and 12V line filter caps for ESR.
     
  8. rahul411

    Thread Starter Member

    Feb 19, 2018
    51
    2
    Actually i have ordered a new one. But if i can repair this one it could be useful in many ways.
    The input caps are good, checked with a ESR meter. All caps are good on output side too, there's no physical marks which indicates that they are in bad health.
    This PSU doesn't have TL494.
    There are two IC-
    TNY277PN and UC3843BN.
     
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