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David Eden WT800 amp, 2010 date, blows mains fuses

Discussion in 'Electronic Repair' started by N_Cook, May 15, 2013.

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  1. N_Cook

    N_Cook Guest

    Sometimes after 1.5 hours originally , now sometimes soon after switch on ,
    sometimes a bit later.
    Amp plate says 5A(T) for 240V use. Inside the IEC, is one non-blown 2A(T) in
    the active position, ah! the simplest of repair jobs, BUT
    No reported noises like magnetisation buzz or speaker noises , no smoke or
    smells. Owner uses the same cab and speaker wire with another amp ok.
    No blown fuses retained by the owner but reported to be blackened. 2A fuses
    passing 3A or so and then failing when asked to pass 4A say, but they will
    have fuse-wire breaks , not vaporisation and blackening.
    Tx torroid 0.8R or so per primary , measured cold, seems reasonable for
    135mm diameter, 65mm high Tx. Not in a position to check the main caps yet ,
    but no bulges.
    Any Eden usual suspects? PbF and RoHS stickers a-plenty
     
  2. N_Cook

    N_Cook Guest

    At least the 25A triac is for mains-on soft start, not some sort of crowbar.
    Still not got to the caps yet. At least most of the schema for a version of
    these amps is out there
     
  3. Phil

    Phil Guest

    A bit like that famous tower in Pisa?
     
  4. N_Cook

    N_Cook Guest

    Blackened pcb under R10 and R11 discoloured bands but reading 3R3 each ,pcb
    charring >30M DVM probing and goes from 20V Tx secondary to 1000uF 10V cap
    so unlikely a mains fuse KO
    Anodes of the valve go via ok MPSA92 to the ps , but may replace the 2x 160V
    10uF caps in that area.
    Just leaves the 2x paralleled 3 , main caps 100V 10000uF. Other than
    separating and heating and measuring ESR (not necessarily related to
    self-curing internal flash-over) , nothing else left to explore?
    If its recovering interlayer discharge inside the Tx then can't do anything
    about that, any sort of tell-tale for that scenario? presumably add fuses
    would 10 amp blow before the 5A mains fuse with failing electro?) to the
    power rail secondaries
     
  5. N_Cook

    N_Cook Guest

    Heavy engineering job as a solid block of snap-in 6 Jamicon , 85 deg C,
    LS, 809K(M) 100V 10,000uF.
    Presumably failing because the contact verticals between are filled with
    filled silicone rubber and hotmelt glue in the 2 central gaps and 85 deg C
    ratings.
    someone's internal view of similar amp
    http://ivanrichards.files.wordpress.com/2012/10/eden-wt800-004.jpg?w=150&h=1
    12
    1500W hot air gun to break the solder and binding of the most amenable set
    of Cs for wedges under then completed with a 100W soldering iron : 4oz board
    , wide traces and both sides of the pcb.
    No oily messes on the pcb but only one cap end is dry, 4 have an oily film
    and one has that plus drops.
    Expensive set of replacement 105deg C ones plus try and get some through
    ventilation, perhaps the replacements set at an angle to form a sort of duct
    and a small fan or as in line with the amp fan perhaps a duct only . And
    hope not an on-going internal Tx problem
    Cold ESR of each is good and warmed to whatever a minute of low setting hot
    air gun gets them to , no noticeable ESR change.
    I suppose the only test would be a current limited 100V supply , resistor
    and voltmeter to monitor implied leakage current and heating each cap .
     
  6. N_Cook

    N_Cook Guest

    Cooked all 6 at 85 deg C, with thermometer in the middle of the block, for
    at least 30min and then applied 110 volt DC to each in turn, all charged up
    and held for 30 seconds or so and discharged normally and equally, while
    still at 85 deg C.
    But not too pleasant electrolyte smell in the room. Orientation of the caps
    was up the other way from useage and no vibration. I wanted one to show some
    obvious problem.
     
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