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Dual output supply dead

Discussion in 'Electronic Repair' started by Don, Oct 1, 2006.

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  1. Don

    Don Guest


    I use a lot of external disk/tape/etc. enclosures or
    my computer peripherals. These typically have ~60W,
    dual output (5/12) supplies in them.

    They are a "popcorn part" so, when one fails, it is usually
    easier to grab a spare out of my junk pile and replace the
    entire power supply than waste time trying to repair it.

    However, I have one such supply that is dead AND has an
    atypical form factor (long and skinny). So, finding a
    suitable spare in my junk pile has been a problem :<
    (the supply in question is a Autec UPS65-2002) It is
    functionally equivalent to the dozens of other supplys
    that I have so I have temporarily replaced it with one
    of those.

    I figured it might be worth a few minutes to poke around it
    and see if I can steal some components off another supply
    to breathe new life into this one. (If not, I'll just pull
    the disks/tape out of the enclosure and throw the whole
    thing out in favor of another enclosure).

    A quick look at the types of components on the PCB leads me
    to make some guesses as to what the circuit topology is.

    It looks like mains power passes through an input filter
    (choke, snubber, etc.) and then rectified. Switching
    transformer primary is chopped and controlled by a sense
    winding on the xformer. "Center" tapped secondary (no doubt
    assymetrically tapped) two legs are rectified and then LC
    filtered to provide the final output. I assume some sort
    of OVP watches one or both outputs as I see optoisolator
    feedback across the transformer no doubt to shutdown the

    Can someone confirm my guesswork to be correct?

    Nothing is visibly toasted. And, assuming this topology,
    I'd guess the chopper is "opened". Of course, it is possible
    that there is a failure elsewhere on the board/load but I
    suspect it may have just opened from fatigue, etc.

    I.e. is it worth hunting down a replacement part (out of the
    scrap bin) for a "quick fix"? Or, are these a bit more
    complicated to repair (in which case, tossing it all in the
    hazardous waste recycling bin is more expedient).

    Recall I am *not* particularly "attached" to this enclosure... :>

  2. Guest

    Circuit sounds about right, though there will likely be overcurrent
    protection as well. As far as repair goes, just bin it, theyre often
    sods to do.

  3. Check the fuse? :)
  4. Might also be as simple as an open startup resistor.

    But as someone else suggested, may not be worth the trouble. For now,
    if you need to get your system back up, move the drives to another
    enclosure. Then, later if you want a challenge, try to repair it.

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  5. Don

    Don Guest

    Nope on both counts. When I removed it from the enclosure,
    I made the mistake of picking it up a short time later and
    sitting down on the couch to have a look at it.

    Quite annoying to get "bit" from a small device with no
    wires attached! (of course, the input filter was still
    charged :< )
    Unfortunately, this enclosure handles *3* half height drives
    (instead of the one or two typically supported) so I have to
    look for a more creative packaging option.

    Or, leave the replacement supply sitting atop this enclosure
    until I find something else, suitable.
  6. Arfa Daily

    Arfa Daily Guest

    You can't say " nope " for the contention of open startup resistor, as the
    main primary filter cap remaining charged ( often for hours ) is a totally
    typical symptom of this failure. Couple of quick checks would be to follow
    the print from the " + " terminal of the primary filter cap, and see if a
    high-ish value resistor is hooked to it on its way to one of the chopper
    tranny pins. Another possibility which is very common on all switchers, and
    will also leave the main filter cap charged due to the chopper oscillator
    not running, is defective primary-side electrolytics. Look for any small
    electros, particularly if they are positioned close to the heatsink or
    anything else that gets hot, and ESR them. If you haven't got an ESR meter,
    just replace them. It's worth going that far, but beyond that, commit it to
    landfill ...

  7. Don

    Don Guest

    I repeat, "Nope". :> The fuse is known good (since the input
    filter would never charge through an open fuse).

    And, this topology does not employ the sort of startup resistor you
    are expecting. I.e. there is no DC path to the SMPS controller's
    "power" pin from the input filter. (I assume you are expecting a
    ~1W dropping resistor between the input filter and the SMPS

    To be precise, there are exactly 5 connections to the input filter:
    - the input filter cap's + terminal
    - the bridge's '+' output terminal
    - one side of the transformer primary
    - one side of an RC snubber across the xformer primary
    (which is *two* components/connections) in series with catch diode

    I'll drag out a DVM and verify power is available at the SMPS
    controller, though...
    Aside from the input filter (180uF 400VDC) and secondary output
    filters, there are exactly two electrolytics in the design. The
    first serves as a filter on the sense/feedback coil. I haven't yet
    tracked down the role of the second...
    This looks tiny enough that I can probably trace the circuit
    topology in half an hour. I'll photocopy the foils and see
    how quick i can reconstruct the basic design...
  8. Bob Urz

    Bob Urz Guest

    I assume you checked ALL solder connections for bad joints?
    And put your meter on all higher current film resistors and compared the
    measured value to the color code?

  9. Mike

    Mike Guest

    ok question. we may have to step up a notch now.

    What kind of SMPS controller does it have? is the controller and chopper
    transistor seperate, or is it an all-in-one such as a VIPIR or STR
    regulator. those are notorious for dying out.
  10. Don

    Don Guest

    3842 driving a discrete FET (can't see the P/N on the FET since it
    is hiding behind the input filter).

    And, to answer "Bob's" questions, below (his post hasn't made it
    to my news server):
    - first thing I did was reflow all the solder joints (since it is
    so easy to do and I've seen problems with fatigue in the past)
    - there are only two "higher current" (wattage) resistors on
    the board: one in the snubber and the other in the current
    sense leg. The current sense R was verified as "approximately
    the right value"; the snubber was ignored.

    I made a photocopy of the foil patterns last night and will trace
    it out this evening if I have time. In the late evening quiet, I
    also was able to notice a "ticking" sound -- as if the switcher
    was starting up and then shutting down again. I'll pay closer
    attention to the current sense circuit when I am tracing the
    foil patterns tonight -- along with the OVP shutdown (as those
    are the two things that come to mind that could cause this
    cyclic start-shutdown behaviour).

    But, tomorrow brings another surplus equipment auction so I
    may stumble on a suitable replacement and just pitch this... :-/
  11. Arfa Daily

    Arfa Daily Guest

    If it uses a 3842 / FET circuit, then it just abiut *must* have a startup
    resistor and supply pin decoupling cap somewhere, otherwise, how does it get
    a supply to the IC to start it up ? If it's ticking, my money is still on a
    cap ...

  12. Don

    Don Guest

    Trust me, I can eyeball a 1.5" length of foil with 5 holes in it! :>

    As I said in a previous post, only two R's larger than 1/10W:
    one as the current sense R; the other as the snubber (it's REALLY
    easy to see an RC-D across the primary ... you don't even have to
    look hard! :> )
    My first guess would be the current sense resistor being open
    and immediately shutting the controller down. But, I haven't
    yet traced the foils to see what else is going on.
  13. Arfa Daily

    Arfa Daily Guest

    Did you get to the bottom of it yet, Don ? I'm always interested in SMPS
    faults as I do a lot of work with them, and any info that saves a few
    minutes, is extra money in the bank ...

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