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Black foam padding for toroidal mains transformer - thermo-degradation?

Discussion in 'Electronic Repair' started by N_Cook, Feb 6, 2013.

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  1. N_Cook

    N_Cook Guest

    I've only ever seen rubber disc padding to the chassis or shaped-top clamp
    to torroids. Amplifier smells smokey when run hard. This Tx is 650W and the
    smokey smell is only when the amp is run hard. Amp had a ventillation
    problem so could easily have generally overheated including the Tx, no
    mention of internal fuse on the Tx label, but has full V and A ratings and
    name etc , otherwise. The nut to the coachbolt was slack because this foam
    had compressed.
    I needed to warm up the foam to remove cleanly as intentional sticky-back
    glued to the tape binding of the toroid, the central hole for the bolt (not
    stuck down in that area as the epoxy core is recessed) was oval but
    otherwise I would not say it looked/felt overheated. This foam , and only
    the foam. smells of that distinct tarry smell that lingers around burnt-out
    buildings for months. Anyone come across such foam used in potentially hot
    circumstances and noticed this smell . There is not the slightest trace of
    smoke emenating through the gaps of the Tx winding tape or high magnetising
    current from shorted turns of the Tx .
    I think its that ordinary black foam with sticky backing that is used to
    keep wiring looms etc in place in chassis .
  2. N_Cook

    N_Cook Guest

    I've come to the conclusion that this foam was deliberately added by the
    manufacturer, perhaps instead of a buried thermal fuse. The minimum setting
    of my temp controlled soldering iron is too high for this but blipping on
    and off the mains showed that smoke comes off this foam starting at 150 deg
    C and copiously by 160 deg C.
    So it is going back under the Tx clamp , snipping off a bit for my
    It is closed cell from viewing under x30 and certainly does not act as a
    sponge with water, with gas/smoke unknown. At this stage I'm pretty
    confident that the foam was preloaded with tar or something like that ,
    rather than a malfunction of the Tx despite high temp running. The coachbolt
    nut was probably loose from creep of the windings at this elevated temp as
    much as foam compression
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