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Testing MOSFETs on my dead motherboard with a DMM

Discussion in 'Electronic Repair' started by alpha_uma, Aug 1, 2004.

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  1. alpha_uma

    alpha_uma Guest

    On my dead motherboard's VRM circuitry, I have six MOSFETs with three of
    each of the following types:

    NEC K3296: These are probably equivalent to 2SK3296S (20V, 35A, N-channel).
    JS214A (B550S03): Someone from another newsgroup said it might be equivalent
    to 2SJ214A (60V, 105A, P-channel).

    Would the standard routine of MOSFET testing with a digital multimeter (DMM)
    (in the "diode" setting or as an ohmmeter)work while the MOSFET is still in
    circuit on the motherboard (but motherboard not energized)? I'm referring to
    the routine as described, for example, on the webpage

    http://www.4qd.co.uk/serv/mostest.html

    Also some webpages are saying that the DMM needs to supply 3.3V in order for
    the MOSFET test to work. My DMM has a "diode" setting, but its manual says
    the range is automatically set to 3V. Is 3V enough voltage to turn on a
    MOSFET for this test to work? Is this test procedure independent of whether
    the MOSFET is an n-channel or a p-channel device?

    Any advice will be appreciated.

    Al-U
     
  2. Tim Perry

    Tim Perry Guest

    use an oscilloscope with a 10X (10 megohm) probe.

    follow standard test procedures.

    first check power supply.

    then start at one end of the signal chain and work forward or backward.

    mosfets tend to be touchy about how they are treated when out of circuit
    (see dire warnings about anti-static handling procedures on the replacement
    parts package)


    if it is a power transistor it MIGHT survive testing with an ohmmeter. the
    MFR of certin high power industrial equipment which use banks of these (
    @55$ ea) have recomended building a test jig. dont forget to add a bit of
    heat sink compound when replacing the dead one(s).
     
  3. alpha_uma

    alpha_uma Guest

    use an oscilloscope with a 10X (10 megohm) probe.
    Unfortunately, I don't have access to an oscilloscope. I know, I know. I
    probably should start investing in one.
    Yes, power supply is confirmed good. I tested it with a 12V system fan
    connected directly to the PS, and confirmed it with my digital multimeter.

    I'm still hoping and trying to determine if the MOSFETs are shot or not
    while they are still in circuit. (Yes, I know. I need an oscilloscope to do
    that, but I was hoping for some "miracle" advice from some troubleshooting
    gurus here!)

    Three 3300uF caps around the MOSFETs were bulged at their tops, so I removed
    the suspect caps and replaced them with good ones having identical ratings.
    The three suspect caps are confirmed BAD after removing them from circuit
    (the resistance does not rise STEADILY to infinity in the standard testing
    routine, it rises, drops and then rise again). I also replaced four other
    3300uF caps near the CPU socket just in case. After testing these four caps
    out of circuit, I found them to be still "good" (not like those earlier
    three I mentioned).

    But the motherboard still won't power up.

    The problem is definitely on the motherboard. It just died on me one day. I
    have subsequently bought another motherboard, and all other components
    (including CPU, RAM and other peripherals) have been reused and are working
    fine. It just that I want to find out what went wrong on this dead
    motherboard.

    Here is the test I'm performing and the symptoms:

    Motherboard alone is placed on a wooden table top.
    All peripherals removed. CPU removed. All RAM removed.
    A 12V system fan is plugged into the system fan socket on the motherboard.
    As a make-shift power switch, I modified a spare HDD LED jumper kit by
    cutting the LED away and stripping the ends of the two wires still connected
    to the jumper. This leaves me with a jumper with two separate wires coming
    out of it. It is then plugged into the power switch header of the
    motherboard.
    The ATX power supply is then connected to the motherboard.

    When I switched the power on (by shorting the two wires of my make-shift
    switch), the power supply turned on for a very brief moment (probably no
    more than 1 second) and then shut itself off. This is confirmed by the brief
    operation of the system fan (which has red and green LEDs on the fan
    assembly). It started to spin and the red and green lit up, but then it shut
    down. My digital multimeter also said the power was briefly on (the voltage
    readings on the 12V and 5V rails never got more than 1 volt). Using the
    ON/OFF button on the ATX power supply itself, I reset the power supply. And
    the test is repeated several times. Identical results every time.

    It seems to me that there is a "short" somewhere on the motherboard, right?

    I heard that when electrolytic caps go, they usually go "open", not "short".
    A cap going "short" is rare, right? So, I'm just wondering whether I should
    now start replacing the other still-not-so-tiny-for-my-soldering-iron caps
    (three 1500uF caps, and six or seven 1000uF ones), or suspect and check on
    the MOSFETs instead.
    Yes, but I don't want to replace the MOSFETs yet unless I ran out of
    troublshooting options.

    Al-U
     
  4. Tim Perry

    Tim Perry Guest

    is this an older A-bit mobo? we got a lot of those in where half the caps
    blew up.

    turns out there was some issue with the electrolyte formula and a zillion
    bad caps got installed and shipped all over the world.
     
  5. alpha_uma

    alpha_uma Guest

    No, it is an ECS P4VMM2. With regard to the "bad caps" fiasco, I have now
    read many articles on it ever since my board went south a couple of months
    ago, and I spotted the bulging caps on it. The death of my motherboard may
    have something to do with those three caps with bulged tops, but I'm just
    hoping that the problem did not get to the MOSFETs. It would be a lot
    messier to replace the MOSFETs.
    Al-U
     
  6. me

    me Guest

    Here is the test I'm performing and the symptoms:
    It sounds like you do not have enough of a load on the power supply.
    Attach an old hard drive to it when you do the test. The mosfets are
    likely ok. I have repaired a few boards with similar bad caps, they are
    usually the only problem.
     
  7. Hi!
    That's a normal reaction from most ATX power supplies when the power is
    first connected to them. You might double check to be sure the supply is
    loaded enough (old drives are good for this purpose) and that you're going
    about the right way of getting it to turn on.

    You haven't come far enough yet to know if your board is good or bad. It
    might be alright. I've fixed a few here with bad caps and all of them have
    usually come out fine afterward, even if the bad caps exploded.

    William
     
  8. alpha_uma

    alpha_uma Guest

    While it is true that most (if not all) newer ATX PSUs require an external
    load in order to start properly, I have a couple of older ATX PSUs that are
    exceptions. It is not an "insufficient load" problem for this particular ATX
    PSU of mine that I'm using to test the dead motherboard. In fact, I like
    using this ATX PSU as a piece of test equipment precisely because when its
    green wire is shorted to "ground", it always turns on itself without fail
    and with the correct voltage levels (as confirmed by my DMM)--with or
    without any external load.

    I've also tried loading the PSU with a spare CD-ROM drive and a floppy
    drive. No difference. This socket 478 motherboard of mine has a short
    somewhere. The MOSFETs are the most likely candidates at this point.

    For comparison's sake, I have a spare socket 370 motherboad in known good
    condition. When I connected the system fan ALONE to this motherboard--no
    CPU, no RAM, nothing else exept the system fan--and then the same ATX PSU to
    it, the system fan spins perfectly (with the fan LEDs flashing as usual) as
    soon as I started the PSU.

    (For a related thread that I posted at the sci.electronics.components
    newsgroup, see
    Thanks

    Al-U
     
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