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PC power supply

Discussion in 'Troubleshooting and Repair' started by johnfin, Mar 20, 2014.

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  1. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    tested resistors

    Tested resistors and they look ok. Pulled 2) 820uf caps and found a couple .16ohm resistors. (see attachment) very strange resistance but they match the specs. i will put the caps on a couple advanced capacitor testing machines i have and see how they look.
     

    Attached Files:

  2. Jagtech

    Jagtech

    43
    1
    Feb 22, 2014
    I don't know what type of capacitor test machine you have, but I've used numerous types and never found one that I could trust 100%. There are some caps that would test O.K., but not function in circuit til I replaced them.
    Just saying, don't put all of your trust in the cap tester.
     
  3. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    machines

    I have an ESR meter, sprague machine, uf meter and analog to watch the bounce. If it passes all 4 machines and fails in the circuit I need to hang up electronics and try something else.
     
  4. Jagtech

    Jagtech

    43
    1
    Feb 22, 2014
    Put your trust in any of those if you feel comfortable with them. I find the only real 100% test is to measure AC voltage across the cap under normal operating conditions, using a scope.
    That said, I do have and use a cap checker, but I always take the results with a "grain of salt".
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    In a switchmode power supply, rising ESR is often the thing which brings on failure.

    An issue is that it can be impossible to know what the ESR was when new (unless you have the same type of capacitor on hand) so you either go by experience or just replace them.

    The low value resistors may be used to equalise current between separate rectifiers. Assume they're 1.2W resistors, at a power dissipation of 0.25W they would be carrying a current of 1.25A and dropping 0.2V. Unusual, but possible.

    I wouldn't be looking at the output caps for a "failure to turn on" problem.

    First see if the +5V SBY is available.

    Then check to see if the main regulator chip has power.
     
  6. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    The shadow knows

    Where is the main regulator chip located?
    On that domed cap, I put it on the teleohm machine, no shadow, new 470uf has shadow. 1950's technology still working strong.
    [​IMG]

    [​IMG]
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
    2,718
    Jan 21, 2010
    What's a "teleohm machine"?

    The reg chip may maybe the one shown in this image:

    [​IMG]

    What is the chip to the right of your "fan" annotation, the one with the green wire passing over it?
     
  8. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    chip

    You are talking about a voltage regulator right? usually in a to-220 case and heat sinked, not in a dip package. That IC that you are looking at is the ps224 monitoring ic. Yes I believe the green start wire goes to it and if it were dead the unit would not start. I wonder if I could bypass that chip to see if I can get it to turn on.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
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    Jan 21, 2010
    Well, that describes a subset of three terminal regulators, but not the sort of thing I'd be looking for in this case.

    So we're looking for another chip on the board. (something like this -- but don't get hung up on the part number, there's thousands of them. This is just an indication of the type of "non-three-terminal" regulator we're looking for)

    I can't see them, you probably can.

    Have we seen a photo of the underside of the board yet?
     
  10. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    pics

    Here is a pic of the bottom of the board. A/C comes in on top, 2) 820uf caps on left. Also a pic of the only DIP ic's (5 in picture)on the board.This may be a stupid question but will a dead cmos battery keep the power from coming on?

    [​IMG]
    [​IMG]
     
  11. johnfin

    johnfin

    18
    1
    Mar 3, 2014
    FIXED

    It was the small domed cap. Yes the 1950's teleohm fix it. Was looking at a youtube video on power supplies and the guy was explaining how DELL power supplies are not only junk but dangerous to your motherboard
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,271
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    Jan 21, 2010
    I have no idea what you're talking about here.

    And this is why you shouldn't believe everything you hear on youtube.

    Did you know that the blue paint Dell use to print "Dell" on their boxes is made from the blood of dead babies?

    No, neither did I. But if someone told me on youtube, it wouldn't make it any more likely to be true.

    1) the CMOS battery probably won't affect this fault.

    2) OK, there's no large regulator chip on the underside of the board. Please take a photo from the top of the board and mark where all the ICs are located (some may be obscured). Then read what is written on each one of them and tell me.
     
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