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Testing two TO220 recifiers...

Discussion in 'Electronic Repair' started by lj_robins, Dec 11, 2006.

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  1. lj_robins

    lj_robins Guest


    I'm trying to repair an ATX power supply that has been dying a slow
    death, turning the power switch on the back off/on has allowed the
    computer to boot up for a couple of months but the supply finally died.

    Shorting the two power wires on the bulk connector that normally make
    the supply come on just make the supply give a little "Ziiiiiit" noise
    then a very faint high frequency squeal can be heard.

    I pulled the heat sink with the three transistors (two transistors and a
    MOSFET) all three tested fine. I then pulled the heat sink with the
    three rectifiers, one tested in the normal range but the other two were
    testing at 0.99x and 0.2xx, I would normally expect these to be in the
    0.4xx or 0.5xx, sometimes a low 0.6xx, is this normal?

    The two rectifiers in question are: SB1040 and SB1640.

    Any help would be appreciated.

  2. lj_robins

    lj_robins Guest

    Hi again,

    Sorry about the typo...

    Here is what they are reading with the DMM on diode tester:

    (negative lead on center pin)

    SB1040: 0.201 on both outer pins.

    SB1640: 0.169 on left outer pin, 0.170 on outer right pin.

    (positive lead on center pin) all readings are infinity.

  3. James Sweet

    James Sweet Guest

    Sounds like they're working fine.
  4. Arfa Daily

    Arfa Daily Guest

    Agreed. They will be Schottky barrier types, which read way different from
    'standard' silicon types. From the symptoms quoted, the most likely bet
    would be a small electrolytic on the primary side gone high ESR, but also
    could be many other things, including resistors gone high. Unless there is a
    particular issue regarding size or physical shape, I wouldn't bother going
    any further than you already have. These PSU's are so cheap now as genereric
    replacements, they don't warrant the labour to fix them. It's also a good
    opportunity to up its rating for the same money.

  5. Meat Plow

    Meat Plow Guest

    It will probably cost you nearly as much in time and effort to repair the
    supply as it would to buy a new one. My guess is you have a dry
    electrolytic somewhere but without the proper meter to test them it will
    be trial and error to find. But hey if you have the time, who am I to
    discourage you? It is very rewarding especially for a newbie to revive
    things like this. Just be carefull though.
  6. lj_robins

    lj_robins Guest


    Thanks to those that have so far replied.

    I figured that those two Schottky rectifiers were fine the way they were
    testing, the numbers just didn't make any sense.

    If I were paying my own technician to diagnose this the supply would
    have already been replaced and the computer would have been out the door
    and back to the customer. But every ATX supply that has died in this
    house acts exactly the same way (gives the ziiiiit and has low output
    voltage) and I'm kind of curious as to what is dying on the circuit boards.

    One supply that was iffy about whether or not it would stay on depended
    on how much was hooked up to it, that ended up being a very obviously
    bulged filter capacitor on the output side. I replaced it with one that
    was double the voltage rating of the old capacitor, two years later that
    supply is still running and ended up being a 60 cent (US) fix.

    But, back to the point, I probably will just replace the supply.

    Thanks again.

  7. Meat Plow

    Meat Plow Guest

    From what I recall what you are hearing is a somewhat common death knell
    for failing switch mode supplies and I have heard it myself.

    Best bet.
  8. James Sweet

    James Sweet Guest

    It's almost always electrolytic capacitors. They dry out internally and
    the ESR goes way up, eventually the voltages sag and get noisy and then
    when it gets bad enough, it won't power up at all.
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