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Need help repair dead switching power supply

Discussion in 'Troubleshooting and Repair' started by mikey5791, Oct 5, 2015.

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  1. mikey5791


    Jun 7, 2013
    Sir 73's de Edd,

    Your explanation on the lag response is understandable as you are also taking time to do several assists. I appreciate your detailed writing on the theory side of diode. However, I am not able to do the DC voltage test as you suggested as I had thrown out the "roasted" SB240. It is already shorted on diode test.
    In place of the SB240, I had installed "2A05" a common slower switching diode of 600V 2.0A.

    Does it mean the response time of the 2A05 will be much slower than the SB240? What is the danger or effect (possible fire) in putting in PSU for long term?

    FYI, the 100 ohm series diode(brown,black,brown,gold) is associated to the cathode side of the other diode R1502 (this diode reads 1,468) off circuit and the diode is good.

    I hope I am replying correctly to your question. Many thanks for your kind assist. I look further to your help in making the dead PSU to work again.
  2. 73's de Edd

    73's de Edd

    Aug 21, 2015

    Sir Michael . . . . .

    OK we are being fine as it is, with the old discarded SB 240 being of NO consequence.
    From the information that I read earlier , I was thinking that there was a good SB240 being used as one of the two diodes in the supply and also another one that had gone bad and that you had subbed in the 2A05 in its place.
    In reality, one diode number that I had never known about was the R1502 that you just now mentioned and it is the one associated with the series 100 ohm resistor.
    SO actually the procedure is exactly as I mentioned with the R1502 derived supply ALWAYS being good, but the supply using the SB240 had failed due to its shorting.
    So all you do now is take/read/record the DC output of the R1502 diode's related power supply's cathode.
    Additionally take/read/record the DC output of the subbed in 2A05 diode's related power supply's cathode.
    AND THEN what we want to do is swap the two diodes and do those voltage readings again.
    The main point of interest now, is to see what the voltage on the circuitry that WAS using the 2A05 is being now by using the R1502 in circuit , as compared to it when using the common generic 2A05 diode.
    BTW the R1502 has a Reverse Recovery spec of 150 nanoseconds.
    That's being 3 times slower than the speed of the SB240's 55 nanoseconds.
    But either are MUCH-MUCH faster that the s......l.....o........w 2A05 with its 1250 nanoseconds Reverse Recovery time.

    You are now free to test

    Thank you

    73's de Edd

  3. mikey5791


    Jun 7, 2013
    Sir 73's de Edd,

    Appreciate your great effort to do the explanation in a comprehensive manner. The info will come in handy to those who are eager to learn more on diagnosing dead non working PSU.

    With the common generic 2A05 diode replacing the original shorted SB240, I am doing an initial testing by substituting light bulb series in place of the 5A glass fuse. This is to prevent any surge current to pass thru fully to the PSU thereby blowing other component. Then put wire to short green wire to ground on ATX connector, I switched on main AC power........the light bulb glows,then shuts off, next the PSU fan spins!

    Next, I switched off main AC power, unsolder the light bulb and replaced with the 5A glass fuse. Then, switched on AC main, done voltage testing at ATX connector which at red & black wire(5V rail) DMM reads 5.12, at yellow & black wire(12V rail) DMM reads 11.79 and at orange & black wire(3.3V rail) DMM reads 3.26V. I am a bit reluctant to do voltage test inside PSU esp.cathode of diode R1502 & 2A05 as I have no confidence in not getting touch/shock to high AC voltage which kills......instantly if not careful.

    FYI, it has been a splendid learning journey from a dead ATX slowly being checked part by part with your guidance then transformed to a live PSU. But due to its limited 250W power (or lower) I will use this PSU as a test bench for small equipment and most likely not used in on computer or sensitive equipment.

    Thanks a lot for the great and superb help.
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