Mounting a Microwave Oven Waveguide

Discussion in 'General Electronics Chat' started by Gdrumm, Jan 26, 2015.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    It is a Sharp R-318AV Microwave Oven (no parts are available from dealers, or from Sharp)
    Someone tossed it out.

    It had a bad door switch, (which I replaced), it was missing the turntable plate (and I do have one), and the Waveguide is missing (I have some Mica Waveguide material, so I have made one).

    So, the problem is that the original waveguide was apparently made of molded Plastic, and had lugs that inserted into small holes inside the cabinet. My Mica waveguide can only be fitted so close, but now it will need an additional form of attachment.

    I'm pretty sure that I can't use metal screws, but can I drill a hole in the cabinet and use a Plastic push anchor like those used to mount a car door panel?

    I'm hoping someone knows something more about waveguides and how they can be mounted than I do.
    Do they get excessively hot?

    Any suggestions and all help appreiated.

    Gary
     
  2. Brevor

    Active Member

    Apr 9, 2011
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    Waveguides are made of metal, they can't be made of mica or plastic. Maybe you are refering to the "window" where the microwaves enter the cooking chamber.
     
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  3. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    There's a brand new one available on ebay for $80.

    That's probably cheaper than the time you are spending, and safer, too.
     
  4. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    My mistake, yes, it's called a Waveguide Cover.
    I've seen many older ones made of Mica, and newer units have the plastic ones.
    In retrospect, I know that cooking trays must be Microwave Safe, and that regular plastic will not hold up.

    Does anyone have specific knowledge of the "Plastic Waveguide Covers"?.
    I have Mica sheets, and I have made numerous "old school" waveguide covers from that, but I've never made a "Plastic" one, because I don't really know what the properties of the Plastic Covers are.
    I'm guessing they are simply "Microwave Safe" materials.

    If you have any insights on that, plase let me know.
    I'll do some Internet searches as well to see if I can learn more about it.

    I'm a scrounger, with time to burn, so I'm simply trying to fix broken stuff, without wasting a lot of money.

    Thanks for the input.
    Gary
     
  5. #12

    Expert

    Nov 30, 2010
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    If you have mica, stick with it. Those windows pass a lot of energy. Wrong materials tend to burst into flames.
     
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  6. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Okay,
    Thanks
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    The door safety interlocks are safety critical, setting them up isn't trivial even if you know what you're doing!

    My advice to you; is to strip it for spares to use on a salvage unit that doesn't have door problems.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    I've been known to not bother replacing damaged dielectric windows - the downside is food spatter can end up in the waveguide.

    The only time that was ever a problem, was when I tried the exploding egg trick - a couple of large chunks of hard-disintegrated egg landed in the waveguide and caused an overload.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Mica is harder to keep clean, if it starts to de-laminate it can trap food spatter which will burn and arc.

    The plastic dielectric windows are generally less hassle as long as you remember to give it a wipe after each time you use the oven.

    When I used to service microwaves - it always seemed like less people bothered to clean the mica windows than the plastic ones.
     
  10. #12

    Expert

    Nov 30, 2010
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    Only if you HAVE some of the right plastic.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    I think people would notice fairly quickly if the manufacturers used the wrong plastic!

    The right ones last the life of the oven, if you just give them a wipe after using the oven.

    As I've said times before - the only time I ever had a problem just discarding an arced window, was when I tried the exploding egg trick.
     
  12. #12

    Expert

    Nov 30, 2010
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    Gdrumm is not a manufacturer, and he has already said he doesn't know exactly which plastic to use.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    For repairs, its best not to use any - just don't try the exploding egg trick.
     
  14. sheldons

    Well-Known Member

    Oct 26, 2011
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    Dont know what the problem is, here in the Uk mica sheet is available from quite a few places and you simply cut it to size and it can be screwed in place or held in place in the cavity over your waveguide to act as a waveguide cover.....do not use anything else as it is a fire risk .....
     
  15. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Ah, that's the question is a nutshell.........
    Can I use metal screws to secure it inside the cabinet?

    I have the mica, and I have already cut it to size.

    So I hope the answer is yes!
    If so, my problem is solved.

    Thanks,
    Gary
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
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    There's one of those scientific type questions doing the rounds years ago; "why can't I microwave an ant?".

    The answer of course is that its physical size isn't an efficient sub multiple of the microwaves wavelength.

    If you left a protruding screw about 1/4 wavelength, it would make an efficient antenna and probably get red hot, standing up from the sheet metal compartment by the thickness of mica sheet - it shouldn't get any more than slightly warm.

    Another alternative is plastic rivets, they have a sort of hollow claw that you push a plastic pin through the rivet head and it spreads the claw to grip the metal. Some microwave manufacturers use these as original equipment - I don't know whether they use a special dielectric plastic, but so little of the rivet is exposed it probably doesn't matter.
     
  17. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Excellent explaination, and thank you.

    I do have some plastic push in type rivets that have a head with a low profile.

    I'll try that first, and if that doesn't work for some reason, I can use some small button head screws.

    Either way, I have enough information to get started.

    I apprecaite all the fedback.

    Gary
     
  18. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I used my plastic push rivet with the low profile head, and it fit like a dream.
    I mounted it dead center, and it looked likegreat, like it was designed that way.
    I put a coffee cup inside about 1/2 way full of water, and turned it on for three minutes.

    It ran great for about 30 seconds,then it began popping and smoking.
    The rivet melted into a glob before I could shut it off.
    There was actually a small fire (part of the rivet) that I was able to blow out like a candle on a cake.

    So now I'm on my way to plan B, or maybe a plan C.

    I'm going to scrounge thru my boxes of spare MWO parts and see what I might have.
    Perhaps I have an old Waveguide cover, with mounting hardware.

    I'll post back.
     
  19. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Plan C worked.
    I did have another Waveguide cover, and it is about twice as thick as the one I made.
    It fit over the hole in the cabinet perfectly.
    I drilled three small holes on the outer edges of the wave guide and cover, and secured the new cover with three screws with low profie heads.
    I blew it all out with compressed air.

    I set my cup with water for 30 seconds and it produced a warm cup of water, and the screws were not warm at all.
    Encouraged by that, I set it for three minutes, and the water in the cup got boiling hot, but the screws were just barley warm, about the same temp as the Waveguide cover itself.

    During the preperation for Plan C, I felt around inside the waveguide, and found that the hole for my original plastic rivet was within an inch of the nozzle of the magnatron, so no wonder the rivet melted.

    With that in mind, the new cover is mounted as far as possible away from the nozzle. and I think it's going to work fine.

    Thanks again for all the input.

    Gary
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    More recently; the plastic dielectric windows have integral moulded clips to fix them in.

    Most screws on a microwave are shallow pan-head with taptite threads. They're what always used to be used for mica windows.

    If you see a microwave you can strip for spares in a kerbside collection zone - grab it.
     
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