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Smoothing capacitor replacement. Value unknown.

Discussion in 'Troubleshooting and Repair' started by Don1972, Jan 28, 2019.

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  1. Don1972


    Sep 15, 2013
    Hello all!

    I am attempting to repair my Behringer Multicom Pro-XL MDX4600. When I opened the case, the (apparently) non-polarized ceramic capacitor at the FBR output was blackened and cracked open. I could not read the value at all, and when attempting to gently remove it, it quite literally disintegrated into little chunks and powder.

    Now I'm assuming it's a smoothing capacitor, since it sits between the FBR and the voltage regulators. Without a schematic (I couldn't find one anywhere online), I don't know how to calculate what I need, or if it was indeed non-polarized (the silkscreen on the PCB is no help here.)

    The power consumption of the unit is a max of 18W @ 60Hz,120v.

    Also - this happened suddenly - the unit was working normally - I went to work, came back home and it wouldn't power on. I also can't provide pics, as the component in question is currently dust and tiny chunks ;)

    So two questions:

    1) What would a safe value be?

    2) How does this happen? The mains power comes in, goes through the toroidal transformer, into the FBR and through the cap to the power regulators. I'm not sure how a spike (maybe?) could've gotten through enough to burn this cap to a crisp.

    Any info and/or discussion (I'm still learning every day) would be appreciated and welcomed :)
  2. davenn

    davenn Moderator

    Sep 5, 2009

    Hi, have you tried contacting the company for service info or maybe their dealer ?

    We have one lad here who is a gun at digging up service manuals .... @73's de Edd lets see if he can help
  3. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir Don1972 . . . . .

    Sorry . . . . don't have any data on that one unit.

    From your limited info given, so far, I would venture to say that your damaged component is going to be a surge limiter / NTC thermistor that is being in series with the AC line input.
    It should be electrically located on the AC hot line just after the AC line fuse, but before the FWB.
    And with a complete powdering up of this component, I would fully suspect a section(s) of the 4 part FWB to be totally crunched as well as having a "hard" blown fuse.
    If you can provide a close up photo of the related components side of the board, as well as the foil side, in order to " read its circuit connectivity path", more info could then be forthcoming.


    On the above sample, the case coverings are usually being black or green.
    In accordance to the thickness of the surrounding conformal coating . . . depends on how well the one angled leads on each side fare.
    If the coating is just a light paint coating . . . or it having none at all . . . the two vertical leads will be left sticking up while the slug/disc between them has multi-fractured free and fell down to the pcb below.

    73's de Edd . . .
    . . . . . . . . . . .

    If nothing ever sticks to teflon, how do they make teflon stick to the pan?
    Last edited: Jan 29, 2019
    Tha fios agaibh likes this.
  4. Don1972


    Sep 15, 2013
    Thank you for the replies 51477571_415142789241586_3918876478976032768_n.jpg !

    I do (finally) have more info -- apologies, as I (and my eyes) were rather tired last night: I was finally able to get a good pic using a high-powered flashlight, and I am now able to see the "100n" label for the capacitor -- so that's getting ordered.

    I'm still unsure how this happened - The main AC fuse is fine - but I bought another just in case.

    Here's how it's laid out (really simplified, sorry):

    AC --> Fuse --> Switch --> Toroidal Transformer --> Full Bridge Rectifier (discreet components, not a FBR package) --> The blown capacitor --> Power regulators (2) --> to the rest of the unit.

    After work I can try to get a detailed view of the other side of the circuit board if you all would like :)

    Thank you again for the replies!
  5. Don1972


    Sep 15, 2013
    I have not done this, as I finally was able to see the component value. I did try searching online (for quite some time) for the schematic, only to find sites that were dubious at best. However, I think I will contact Behringer for a service manual, just in case something else happens in the future.
  6. Don1972


    Sep 15, 2013
    Interesting !!! I took a quick pic with The Flashlight(tm) ;-) before getting ready for work; This is the other side of the PCB: The AC & common lines are coming off the transformer. 51214401_1913822602079199_8951957660257746944_n.jpg
    davenn likes this.
  7. davenn

    davenn Moderator

    Sep 5, 2009

    100n = 100nF = 0.1uF and you would want something rated at 250V or better
  8. Tha fios agaibh

    Tha fios agaibh

    Aug 11, 2014
    Id check out that diode in series with it and reflow the solder joints as some look corroded and fractured.
  9. TallPaul


    Oct 5, 2014
    You have to figure out caused the capacitor to burn up because it will happen again. probably something failed open
  10. Don1972


    Sep 15, 2013
    Thank you for the replies all, I appreciate it. Sadly I'll have to do more testing and digging -- I replaced the cap and still a no-go. It's a slow process for me, as I'm at work most of the time ;-) Thank you again! I'll post updates if/when I find the culprit.
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