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Autumnal musty fruity smell

Discussion in 'Electronic Repair' started by N_Cook, Jun 9, 2009.

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  1. N_Cook

    N_Cook Guest

    Inside a Marshall MG50DFX. Someone must have thrown a pint of cider into the
    fan vent, inwards blowing fan. Heatsink protected the TDA7293 from being
    spattered but 6 faults found (so far), dotted around the amp. Washed boards,
    redid solder points where there was sediment etc. So far working with no
    reaction to twizzle- sticking, I wonder if it will come bouncing back.
     
  2. N_Cook

    N_Cook Guest

    one fault but by different method. The effects loop yes , problem in the
    ribbon.
    Not crimping , assumed corrosion in the recess of a socket, cleaned the pins
    , put alternating set on the pins , both ends, and swapped end for end the
    connectors.

    The second time I've come across one of these with a loud buzz at switch on,
    sometimes !!. I suspected a leaky path via the supposed isolted heatsink and
    have added extra isolation and its not returned. One time I monitored
    the -38V rail , when powering up and it dropped to -28V until lightly
    tapping and went to -38V and no buzz. -rail is on the tab of the TDA7293.
    I've powered up numerous times since , and no return, but I had that happen
    before - is this a known problem? Problem with the electros around the o/p
    board or internal problem in the TDA ? How to force the problem if its
    hiding for the moment?
     
  3. N_Cook

    N_Cook Guest

    "but I had that happen before"
    on another MG50DFX that is, thought I'd cured the problem, tested soak, but
    not repeated on/off and that problem returned intermittently, owner did not
    return so real problem not diagnosed
     
  4. N_Cook

    N_Cook Guest


    so far switched on/off with up to hours between and no return.

    Previous time I convinced myself it was a problem on the muting signal line
    as there was oscillation on it. I thought it was at the preamp end there was
    a problem but it was probably a problem associated with the TDA feeding
    forward. It was only for 2 or 3 seconds at start up but very large amplitude
    buzz, independent of vol/gain settings , ie setting all at zero made no
    difference.
     
  5. N_Cook

    N_Cook Guest

    Have you tried cooling it for 24 hours in a refrigerator and then
    testing it, or is it too big for that?

    *********8

    The output section of these is demountable and could be placed in a fridge
    quite easily but I'm not sure it would tell me much as this fault is not
    consistent, does not always happen at switch on.

    I only have a 7294 sitting around as a spare rather than 7293 , will have
    to order one for next time. Must have switched on 20 to 30 mtimes by now and
    the problem not returned once so will return it to owner and order a 7293
    for if it should bounce back. After that then caps problem is the next
    recourse
     
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