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Vox AC30 , AC30CC2X, 2005

Discussion in 'Electronic Repair' started by N_Cook, May 27, 2008.

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  1. N_Cook

    N_Cook Guest

    Roadie dropped it off the stage , in flight case, worked fine for another
    gig and then next time worked for 3 seconds.
    Sheared metal internal braces and choke looks decidedly drunken, almost
    sheared off, but all big lumps test cold ok so far.
    Could someone confirm that the 1W resistor R188 on the HT ac input should be
    2.2 ohm ? Its not charred or any other indication of excess heating but I
    find the colour bands a bit confusing as red,red, gold,gold, white and is
    nowhere near 2 ohms now. Bit difficult to tell whether it is gold gold or
    silver silver bands until I disconnect the board and can get a better view.
    Is it a possible scenario that this R failed mechanically, but only
    partially, in the shock loading ? without any undue current loading/heating.
    Back-line manager swapped the o/p bottles with another amp and they
    functioned ok in the other amp and the speakers are ok.
  2. N_Cook

    N_Cook Guest

    For HT read LT
  3. N_Cook

    N_Cook Guest

    Any suggestions what to look for on scrapping off the coating of what maybe
    a mechanically failed one. Where to find the fuising bit to check for
    fracture versus fusing ?
  4. N_Cook

    N_Cook Guest

    Removed the grey coating and remnant parts of the metalisation/ MO sum to
    about 1.5 ohm. The joining section is about 1 x 1 mm with a neat hair-line
    crack across it and no sign of any heating there.
    So mechanical breakage, I will dig out my microscope and take a photo of it.

    Before removing coating, was definitely 2.2 ohm markings with white band
    (if) for x16 presumably referring to that 1 x 1 mm section, so x32 violet
    banded one would have a something like 0.7 x 0.7mm intercept region and a
    green band x8 would have more like 1.4 x 1.4 mm fusing area.

    Anyone care to give a reason for shock induced fracture just at that point?
  5. N_Cook

    N_Cook Guest

    Putting my materials testing hat on - it probably suffered torsional
    Wire ended 1W suspended off board, if one end swings one way and the other
    end swings the other then perhaps a torsional twist that is enough to part
    at the weakest point of the conductive strip.
  6. jakdedert

    jakdedert Guest

    Think that was stated in the OP(?).


  7. Well there's irony for you.

  8. N_Cook

    N_Cook Guest
    Is a microscope view of the crack. Now viewing magnified it can now be seen
    the fault developement.
    The just left of centre part of the crack must be part of a crack that
    formed initially but enough current flowed in the other parts making
    touching contact until there was too much localised fusing.
    1 to 1 marks the 1mm or so of conductor between the 2 straight line
    etching/millings ? into the conductor to form part spiral paths.
    The C marks the end of one of these formed breaks and there is another one
    on the other track , off the top left of the image, giving about 1 x 1mm
    central fusible conductor.
    Nowhere on the remaining conductor is there any discolouration due to
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