MWO Door Switch problems

Discussion in 'General Electronics Chat' started by Gdrumm, Feb 10, 2015.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I have several new (rejected but not repaired...yet) Microwave Ovens, all seemingly with door switch alignment issues.

    I just need confirmation of which switch should be doing what, and when.
    Here goes.

    Switch--------------Door Open--------Door Closed

    Top(NC)------------Continuity--------No continuity
    Middle(NO)--------No continity------Continuity
    Lower(NC) --------Continuity--------No continuity

    Does this look right?
    Thanks,
    Gary
     
    Last edited: Feb 14, 2015
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Is microwave door switch logic standardized across all devices? I'd think not.
    But clearly the switches are "switching"
     
  3. Reloadron

    Active Member

    Jan 15, 2015
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    This is a good read on the subject of testing microwave oven door switches. Not seeing your setup it's hard to tell. Generally the door switches are in series (depending on the microwave) and all switches are Open (No Continuity) when the door is open and closed (Continuity) when the door is closed. My microwave only uses two door switches and they are configured as I mentioned.

    Ron
     
  4. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Thanks for the feedback.
    Most of the ones I've worked on had three switches (at least what I can remember).
    But good points, and thanks again.

    Gary
     
  5. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Here is an update on the "several ovens I had for repair".

    On the first oven, the middle switch was partially melted. From YouTube I learned that the wire normally used to connect this switch is not rated for high enough amps.
    Question? When these switches fail like this, is it possible that someone "popped open the door before the timer shut off, and that increased the load on the switch"? I've seen several switch failures and that's my Sherlock Holmes thought...

    The second oven literally had a wire connector unplugged, that was supposed to hook up to the transformer (on the "power out" side). It was a cheap aluminum connector that may have jarred loose during transit.

    The third oven (also brand new, still with bubble wrapped parts) had a wrong door switch at the bottom. Instead of a NC, it had a NO.
    I fixed that problem and got rid of the error code, but I still could not get it to start cooking.
    I suspected the board, but I couldn't find a used one
    A new board costs $139.00 (it is a Commercial MWO).

    Thanks for all the input.
    Gary
     
    Last edited: Feb 14, 2015
  6. Reloadron

    Active Member

    Jan 15, 2015
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    A partially melted switch could have a few causes. In this case the most likely cause could have been, as you noticed, poor connection at the switch terminal because of cheap or loose connectors. The resistance of the connection increases and the switch can't handle the current. The hotter the connection gets the more the resistance increases till finally things simply burn up. While I doubt it is a good practice I open and close the door on mine many times when the thing is running and have yet to cook a switch. The thing has been going almost 20 years with me abusing it. :) The display is beginning to appear a little dim, that or my eyesight is getting worse. Anyway yes, a loose switch connection or underrated wire will cause heat and it becomes like a snowball effect.

    Ron
     
  7. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Ron,
    Good info, and I can see where loose connectors can cause these kinds of failures.
    Thanks,
    Gary
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    The usual scheme is two (double for fail-safe) break switches that remove power. A third switch closes to short the power and blow the fuse if *BOTH* the break switches fail.

    When I repaired microwave ovens for a living - any with dodgy door switches became parts donors for repairing the others.

    Setting up the switches isn't trivial - the clear fluid in the front of your eyes is the same kind of protein as egg white, and that's pretty much what it becomes when exposed to microwaves.

    Take my advice - watch the kerb collection zone (while you can still see!) for one with something else wrong with it, then strip the parts you need from the one with dodgy door switches.
     
  9. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Ian,
    Thank you, I did not know that.
    Would looking the other way while testing be protection enough, or would completely replacing the top cover be best?
    What is the kerb collection zone ? (I'll Google that).
    I've seen leak detection devices, but wrongly summized that the dangers (to anyone) was leaky microwaves that could cause heart problems for people with pacemakers, etc..

    This information will forever change my attitude toward these machines.
    Thanks again, I owe you one (better yet, I owe you two).
    Regards,
    Gary
     
  10. jamesd168

    New Member

    Nov 8, 2014
    21
    2
    is this a common issue for microwaves?

    we have gone through two microwaves already made by Panasonic, which made me thinking there might be something wrong with my kitchen electricity wiring.

    maybe I should go back to the microwave and check to see if it the door switch issue.

    what are the symptoms of the failed microwaves you are repairing?
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
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    The commonest fault with door switches (that anyone notices!) is random fuse blowing, sometimes contact bounce causes fuse blowing if its slammed shut.

    Most repairers just replace the fuse and carry on - but the "crowbar" safety kill switch will have sputtered contacts and shouldn't be depended on thereafter.

    The surge current might also have welded the contacts in one or both the break switches! If you feel any sensation of heat while near the operating microwave - switch it off immediately and go buy a leakage meter!

    A lot of people don't realise how easily food spatter & grease can render the door seal ineffective - a leakage detector will also tell you if you need to clean it.
     
  12. jamesd168

    New Member

    Nov 8, 2014
    21
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    wow this is serious stuff!

    are you saying that I might be frying my brain when heating up my food?

    our microwave happens to be at the head level, and I usually just stand there and wait because it cooks fast.

    maybe I should not stand nearby then
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    It depends on physical size of the various organs and the wavelength at 2.45GHz, that's mainly eyeballs and one of the intestines - there's at least 2 sizes of those.

    There might be some other balls you don't want to microwave either!!!
     
  14. jamesd168

    New Member

    Nov 8, 2014
    21
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    so other than balls, it's OK to be microwaved?
     
  15. Reloadron

    Active Member

    Jan 15, 2015
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    There is the ancient classic Gerbil in the Microwave. :) Click Play and then start clicking the power level buttons.

    Ron
     
  16. jamesd168

    New Member

    Nov 8, 2014
    21
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    oh my goodness, I just found myself to be very evil and cruel, and derived a lot of fun from the poor Gerbil's suffering.
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    Life is not a happy lot for the long suffering gerbil...................

     
  18. m.phillips

    New Member

    Apr 17, 2015
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    0
    Check the thermal cutoffs for both the oven and the magnetron. The thermal cutoffs are little disc shaped devices with a wire connecting the two of them. Remove a lead and set the VOM to RX1. Probe the terminals and look for a reading of zero. If not, replace. Check both thermal cutoffs. If these are OK, check the capacitor and diode as described above. The magnetron or transformer could also be bad, but they need to be serviced by a professional.

    Or it will require a professional in most cases. As always, have the make and model numbers handy when heading to the parts shop. If your microwave isn't the only appliance giving you headaches, this website has repair and information guides for you.
     
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