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240V conversion

Discussion in 'Electronic Repair' started by Phil Allison, Oct 13, 2012.

  1. Phil Allison

    Phil Allison Guest

    ** Hi,

    mainly due to the popularity of Ebay, electronic gear originally sold in the
    USA and Japanese markets is now regularly turning up in Australia. USA items
    are intended for a 120V, 60Hz supply while Japanese ones are for 100V,
    50Hz.

    Conversion to 240V mains operation involves internal inspection and
    modifications OR simply determining what VA rating step-down tranny will be
    suitable. The modifications involve removing or shifting PCB links, altering
    QC connectors or rewiring the supply side leads to the AC tranny - PLUS
    changing fuses and the AC plug to the Aussie version.

    I had an EVENT active studio monitor (twin port model) in for such
    conversion this week and it looked easy. There is an IEC inlet on the back
    and markings that indicate the possibility of 120V and 230/240V operation.

    The unit has a toroidal tranny fitted inside with four supply side wires, so
    it was of twin primary design allowing 120V and 240V operation. I
    reorganised the four QCs according to a pattern drawn on the PCB and used a
    variac to raise the voltage.

    At about 30V, the ( T4A ) PCB fuse blew - indicating the two primaries were
    wired out of phase. No real harm done and a few minutes later I had the four
    wires the right way around.

    Variaced the unit up again ( to about 130V ) and this time it started
    working, I verified the woofer and tweeter were both OK. Next I wound the
    variac up in gradual steps to 240V and suddenly and there was a very loud
    BANG !!

    All sound stopped. WTF !!!

    When I opened the unit again there was a smoke inside and black deposits
    visible on the PCB and the nearby acetate filling used in the box. The
    source was a 20mm varistor on the PCB that had exploded.

    But why?

    Markings on the part showed that it was rated for 270 volts ( 20K271) - but
    that is a DC voltage rating. So when exposed to a 240VAC supply with 340V
    peaks it was grossly overloaded.

    The stupid " Ying Tongs " had fitted the WRONG varistor.

    The correct part is a 430 volt rated varistor OR one marked as being 275VAC,
    as is the case with the class X2 cap wired in parallel.


    ..... Phil
     
  2. Jamie

    Jamie Guest

    Just about every device I've ever hand my hands on that allowed such
    configuration had 2 MOV's, each rated for the lower voltage. When you
    properly configure the connections on the board for 200+ volts for
    example, the 2 MOV's end up in series, otherwise, they end up in
    parallel for low voltage configuration.

    Maybe some one has serviced that unit in the past and replaced them
    incorrectly.

    Jamie
     
  3. Phil Allison

    Phil Allison Guest

    "Jamie"

    ** There was ***ONE** only MOV wired across the pins of the IEC inlet.

    Fuckhead.


    ..... Phil
     
  4. N_Cook

    N_Cook Guest


    Ever come across Crate (not some mickey mouse operation) USA "made" amps ,
    for export to 240V countries , with mains switches + neons?
    Neons glowing like lighthouses and melting all around, from the charring of
    the droppers which are set for 110V including stating so on the switch
    bodies
     
  5. N_Cook

    N_Cook Guest

    Yesterday had a somewhat related 110/240V repair but this time not safety
    issue.
    Little PV practise amp , 20W, original PV , properly set up for 240V , mains
    fuse inside had "blown" at switch on.
    1.25 inch F rated 250V 500mA . Maybe all right for 110V areas but repeatedly
    switching on with the random occassional 330V surges and it will fail
    mechanically (filament laying nicely diagonally in the barrel, fixed at one
    end only).
    T 500mA one went in there
     
  6. Phil Allison

    Phil Allison Guest

    "Nutcase Kook"

    ** Surge and running currents are DOUBLED with 120V supplies.

    The max peak ( switch on) surge can be estimated by measuring the resistance
    of the primary winding of the AC tranny and dividing that number into the
    supply voltage.

    Eg:

    The PV " Encore 65" tranny has a 7 ohm primary, the amp draws 0.25A rms at
    idle and 1A rms at full tilt. The max inrush surge is however over 30
    mps - so the AC supply use needs to be rated at " T3A " merely to survive
    the surge.

    In virtually essential to use a "delay" or " slo-blo" fuse with transformers
    and SMPS. Unfortunately, the fuckheads at PV and Fender do not comprehend
    the issue and refuse to be informed on the matter.


    .... Phil
     
  7. I've noticed quite a bit of dubious pilot lamps on equipment that should
    allow 240volt export use. Luckily it's 120 here, so I've not have the
    pleasure of experiencing what really happens with those.
     
  8. N_Cook

    N_Cook Guest

    Neoned rocker switches. After being on for ten minutes or more, the switch
    is too hot to keep a finger on it. The little neon is glowing as bright as a
    1W filament lamp, and the lens over it has a brown patch. Go inside inside
    and melted plastic around the neon and the dropper which by now is a
    blackened cylinder with a helix, certainly no colour markings
     
  9. N_Cook

    N_Cook Guest

    Neoned rocker switches. After being on for ten minutes or more, the switch
    is too hot to keep a finger on it. The little neon is glowing as bright as a
    1W filament lamp, and the lens over it has a brown patch. Go inside inside
    and melted plastic around the neon and the dropper which by now is a
    blackened cylinder with a helix, certainly no colour markings
     
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