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Discussion in 'Electronic Repair' started by G, Aug 30, 2005.

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  1. G

    G Guest

    Does anybody have some info or know where I can get a schematic on a
    panasonic microwave model nn-s543bfw?
    It runs for a 23 seconds or so then shuts off?
    Thanks in advance
  2. Graham

    Graham Guest

    I don't recognise that model (I am in the UK)
    Is it an "inverter" HV PSU?

    If so, have a close look at the magnetron magnets.
    I bet one or both of them are cracked from overheating.

    If you just replace the Mag, it will work for a while, but it will probably
    fail again due to being over-run.

    So it's a replacement Magnetron and a replacement inverter, Panasonic don't
    encourage you to repair this item, bur if anyone has, I would be interested
    to hear.

    I recommend people stay away from these models, and go for a conventional HV
    transformer type.


  3. Doug

    Doug Guest

    Maybe magnatron overheating opening thermal fuse. Is the fan working?
    Maybe mag bad getting too hot. Does the display stay lit when it quits?
  4. Yeah,I got stitched up with this model [AN772] HV blew a transistor in
    the inverter board and PAN wanted 160 pounds to repair it! Seems that
    the whole inverter board needs a replacement[if you exchange your
    All this for one transistor s/c.As a qualified[if retired]Microwave
    engineer Pan finally agreed to supply an exchange board for 90 pounds
    plus VAT.
    I refused as it is likely to fail again,and bought a Sharp Combi for
    120 pounds,does the same job with conventional HV .
    I have since found out that PAN have had trouble with this HV system
    and they were warned this would happen. They thought it would be more
    appealing if the microwave was lighter.
    Sharp,Sanyo etc would not use this method of HV supply.
    PAN. took no notice and left the repair bill to we customers,I'll
    never buy Panasonic again .
  5. Graham

    Graham Guest

    Totally agree with you about the Panasonic's. Mind you, the Sharp R-XXX
    models have their own issues don't they?
    I am of course referring to the CPU (digital display) which seems to fail
    sooner or later in nearly 100% if them, and at GBP39.02 (for most of them)
    one is left wondering if Sharp deliberately built in some unreliability.
    (call me a cynic)


  6. Ken G.

    Ken G. Guest

    I have worked on a bunch of these and have seen several problems . The
    first ones i saw had mostly one of those long square hi voltage diodes
    shorted , then i stared seeing the big 4 lead diode all shorted . I have
    seen a few with one capacitor split open and shorted transistor . These
    were all the first models large black ones and med sized white ones
    using the same board .

    Now i see the big ones with a mirror like door , very nice looking oven
    .. many of these got samsung mags put in them and i saw these bad often
    causing the thing to shut off in 6 seconds or so .
    These also had a defect in the plastic door latch where the spring would
    fall off because a plastic part in the door would break off ( to small )
    easy fix
  7. hoef

    hoef Guest

    I have got a nn a773 combi oven which blows the fuse when starting the
    microwave cooking cycle. It blows the fuse in a raction of a second but
    I dont find any signs on the inverter board at all. The fuse only blows
    when the inverter board is powered (with the magnetron disconnected) It
    almost looks like a safety switch makes the fuse blow. (I think it cut
    out due to the magnetron fan not running)The fuse on this model will
    also blow if the door switches are misaligned.
    Why does no one wants me to touch the inverter board, it has straight
    forward components on it?
  8. Bob Shuman

    Bob Shuman Guest

    Check the door safety interlock microswitches. There are usually two of
    these. One is normally open and closes when the door is closed, but the
    other is normally closed and opens when the door is closed. It is this
    second switch that creates a straight short across the magnetron and must
    open or else you will blow the fuse. It could be welded contacts, or simply
    a misalignment of the door causing it to stay closed. You should be able to
    verify this with an ohmmeter or by temporarily disabling the safety
    interlock. If you get it working, remember to replace the switch since it
    is there for good reason.

    Good luck.

  9. mrhoef

    mrhoef Guest

    Door switches are working fine and if they wouldn't they would still blow
    the fuse even if the inverter board is not connected, because the short is
    created straight from the relay that powers up the inverter board.
  10. Graham

    Graham Guest

    When I last had this same fault, the big power transistor on the inverter
    PCB was collector-gate short-circuit.

    (This device is an IGBT so I don't mean collector - base)

    I did consider repairing the board but I couldn't find a supplier for the
    transistor, and I didn't discover whether the board had suffered any other


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