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Microwave oven arcing

Discussion in 'Electronic Repair' started by MikeC, Dec 15, 2006.

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  1. MikeC

    MikeC Guest


    I'm not sure if this isn't a bit banale for this group, so please direct me
    elsewhere if I'm in the wrong place.

    My microwave oven has lasted quite a long time (it must be heading for its
    10th birthday). In the top of it, there is what appears to be a mica window
    through which the UHF arrives. Intermittently (but getting more frequently)
    it produces, on the magnetron side of this window, what looks, smells and
    sounds like arcing - like a welding set. The light comes through the mica
    window and lights up the inside of the oven quite brightly. It arcs for two
    or three seconds then stops, and may start again after 10 seconds. The
    problem has been there, then disappeared for three weeks, but it's back at
    the moment.

    I haven't taken it to bits yet, but can anybody tell me if they have had the
    same experience, and whether it is even worth dismantling. If it's the
    magnetron, how much do they cost? Ovens aren't very expensive, so would it
    be worth repairing? It's quite a good one (Matsui 170TC, if you know it),
    so it is worth more to me than a bottom-of-the-range replacement.

    Incidentally, I started my carreer as a microwave engineer (S-band), though
    I haven't been near that side of the business for a long time (in computer
    storage now), but I'm aware of the consequences of running it with the
    covers off, so please don't let that danger steer you to recommending a

    Many thanks,


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  2. Jim Land

    Jim Land Guest

    This is covered in Sam's FAQ:
  3. Dave

    Dave Guest

    Intermittently (but getting more frequently)
    The inside of the plastic cover gets dirty over time. When there's enough
    gunk built up, arcing can and does occur. Remove, clean, replace.
  4. tlbs

    tlbs Guest

    I've seen this before, a couple of times on my Panasonic over-the-stove
    model microwave oven, only not on the mica window. The arcing I've
    seen always happens at the HV anode of the magnetron, and it it usually
    fatal. Since the microwave cost more than 2 times the replacement cost
    (even in today's dollars), I have the magnetron replaced (total cost
    for part + labor around US$75) versus a new 1200Watt over-the-stove
    model starting at US$199. The problem is the buildup of grease and
    cooking aromatics in-and-around the magnetron. It usually arcs when
    the humidity gets high (that's the variable factor you might be looking
    for). If it happens to me again, I'll ask my repairman to put alot of
    HV silicone around the anode terminal.

    If your microwave is a cheap ($50 - $70) microwave, it's a throw-away.
    If it is a $150+ high power fancy model, consider replacing the

    good luck
    Tom P.
  5. ian field

    ian field Guest

    The OP said its mica, these tend to de-laminate so they're impossible to
    clean adequately to prevent future occurrence of arcing.

    Often the only solution is to remove the mica and throw it away - then hope
    the oven lasts long enough without any food splattering into the waveguide
    to save up for a new oven.
  6. MikeC

    MikeC Guest


    Thanks to all who responded. I read Sam's FAQ (and got pretty interested in
    his work on lasers, and will probably order Dr. Shawn's science CD), and
    took the "take the cover off and clean it" advice. In fact, I found (going
    by the burn marks) that it had been arcing from the top of the waveguide to
    the bottom. There is a U-section welded to the top of the oven to form the
    box-section waveguide, which is angled down into the oven at about 45
    degrees at its end. The end of this 45 degree section meets the far end of
    the aperture where the UHF goes into the oven (the aperture that is covered
    by the mica plate), and above the leading edge of the aperture, there is a
    round bump on the inside top of the waveguide about 1/2" high and with a
    diameter of an inch or so. It has been pressed in from the outside of the
    waveguide, and presumably, it disperses the microwaves to fill the inside of
    the oven. The highest point of the bump is directly above the leading edge
    of the aperture, and it appears that the arcing was between those two
    points. The mica cover was half-way burned through at the point where the
    arc was hitting the edge of the aperture hole.

    I've cleaned it up - though there didn't seem to be much to clean. I was
    expecting that the arcing would be through a film of muck - but it appears
    to have been across half an inch of open space.

    Regards to all,

  7. ian field

    ian field Guest

    If the waveguide window is some sort of plastic it shouldn't de-laminate so
    take it out and scrub it clean and inspect it for any carbonised points -
    these can be scraped off with a sharp blade.
  8. MikeC

    MikeC Guest

    I've just measured the distance, and in fact, it's a little over an inch.
    Would you expect to see an arc over this distance? The top of the bump had
    the paint burned off, as had the edge of the aperture hole, so it suggests
    an arc between those points. As I said originally, it made a light like a
    welding set, though, of course, the mica window was in place, so I didn't
    see the arc itself.

    Incidentally, it isn't mica. It's some sort of manufactured material, but
    it's grey like Mica.


  9. Franc Zabkar

    Franc Zabkar Guest

    Here in Australia we can buy sheets of the stuff:

    - Franc Zabkar
  10. Guest

    well it was initially, but arcing burnt it away.

    You'll need to replace this bit of sheet, if you run it without or with
    a hole, muck will get into the waveguide and it'll screw up, only this
    time you may have great difficulty trying to clean it out.

    Any sheet plastic that can handle microwaves and steam will do. Self
    extinguishing is a real plus.

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