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Samsung Syncmaster 740BF repair

Discussion in 'Troubleshooting and Repair' started by bushtech, Apr 20, 2018.

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  1. bushtech

    bushtech

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    Sep 13, 2016
    I found a swollen cap (replaced) and a FET (DMO565R) split open.
    The negative of the cap is connected to the ground of the FET so I hope it's the failed cap that popped the FET.

    Board:
    Board.jpg

    The big cap at an angle is the one that I have replaced

    FET:
    FET.jpg

    My problem is the FET
    Both Mantech and RS show no stock, ditto farnell and mouser further afield. (although I'm not very au fait with their search engines)

    Ali express shows 4: https://www.aliexpress.com/w/wholesale-dmo565r.html

    What are my chances that aliexpress FET will work? Are there any other places I can try. Stuck on the sharp end of Africa so shipping very quickly becomes a deal breaker.
    Any advice/ wisdom appreciated, equivalent maybe?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    As you can see, a DMO565R is not just a FET.

    The AliExpress versions are not particularly expensive.

    I'd be cautious that there isn't another failure which killed the device. Check for shorted rectifiers on the output, or a shorted turn in the transformer (that's harder to check).
     
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  3. bushtech

    bushtech

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    Thanks *steve*

    By rectifiers do you mean the diodes?

    Regarding the rectifiers/diodes: A diode is normally marked D on the pcb and zener is marked ZD. But what on earth is a BD? Looks like an overweight diode without any markings. You can see one of them between the two staggered diodes and the FET
     
  4. dave9

    dave9

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    Last edited: Apr 21, 2018
    bushtech likes this.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yes, the rectifier diodes for the low voltage output.

    BD may refer to a ferrite bead. This is just a small lump of ferrite that is slipped over a piece of wire (or even a component lead) to provide a small amount of inductance. They will measure as a dead short.
     
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  6. bushtech

    bushtech

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    Thanks dave9 and *steve*

    When Mantech show no stock or price they are out of stock.

    The BD do indeed show very low resistance both ways, DMM shouts, so I think they are probably ferrite beads. New component to me, dr. google will be consulted.
     
  7. 73's de Edd

    73's de Edd

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    Herr bushmeister . . . . .

    Indeed . . .a Fairchild / ON FSDMO365 is being a " switch mode PS within a chip ".
    I would have expected that line rectifier capacitor to be the last cap suspected, with those 3 green ones to the left to be the most likely suspect.

    Seems like your Mantech is not stocking that "obsolete for new production" unit, but just might get a copy for you when they make their next milk run to THEIR supplier.

    I would just be absolutely SURE to use a series incandescent lamp in the AC power line when initially powering up the monitor after the new part is in it.

    73's de Edd
    .....
     
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  8. bushtech

    bushtech

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    Thanks St. Edd, to the rescue as always. Yes, all caps will be tested, once I get a new ESR tester. Rookie mistake with the old one, connected a charged cap, poof, magic smoke escaped. Sigh! You live and learn.
    This monitor got hit by lightning, so I suppose normal rules of deduction won't apply.

    Query in to Mantech.

    Have now built myself an incandescent lamp tester which runs everything I work on. Just have to get my tester connected to that lol
     
  9. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    ESR Meter . . . . .

    You might check across the input probes internal connecting circuitry . . . . . .usually they have a set of back to back diodes installed there just for THAT protection of encountering a charged cap boo-boo.
    You might just find that one or both of those diodes changed to a direct SHORT junction, to protect the circuitry there on inwardly.

    This monitor got hit by lightning, so I suppose normal rules of deduction won't apply
    In that case validating the small Reddish-brown round case line fuse might be in order along with te units FWB that your just replaced line cap connects directly into.
    # 2000 down the list of suspect parts will be that " BD " parasitic RF suppressor bead.
     
    Last edited: Apr 23, 2018
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  10. bushtech

    bushtech

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    Sep 13, 2016
    ESR Meter

    Afraid the Chinese were not that diligent. I can't see any relevant diodes

    Esr Tester.jpg

    But I think I found that magic elusive place where the smoke escaped

    Magic smoke1.jpg

    But I can't see any damage. Pulled the chip and absolutely no damage.

    But thank you again St. Edd. Now I know what that small Reddish-brown round thingymabob is (Get continuity through it so not blown) Will start checking the bridge rectifier as soon as I figure out how it fits together.

    Great news is that Mantech can order the DMO565R's without my credit card running away whimpering:)
     
  11. bushtech

    bushtech

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    Sep 13, 2016
    Replaced the DMO565R and set still dead, pilot light doesn't even come on. I want to check some voltages on the board, always nervous when working on a live board powered by mains.

    My question is, where can I connect my negative probe?, Actually want to solder a lead on there so I can check one handed. And then will that neg point serve for measurements anywhere. The board is split in two with a broad black line. (see photo below) Denoting high and low voltage?

    Bottom.jpg

    Please help me not blow something on the board or my DMM. I am running through a dim bulb tester
     
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