Question about Panasonic NN-T990SA Microwave Oven

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Blows fuses! Toaster oven underneath microwave accidently left on overnight. Not realizing this, food placed in microwave for reheat burned to a crisp. Found blown fuse and tried replacements which blow when attempting to cook at any setting. Suspected and checked thermal cutout which shows continuity across terminals. Understand this is a normally closed ceramic relay and should show continuity. Could it still be defective and cause my problem? No signs of burned or loose connections. Have I lost the inverter electronics?

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  • stanthe4man Mar 11, 2008

    Panasonic NN-T990SA 1300 Watts

  • stanthe4man Mar 11, 2008

    not acceptable

  • stanthe4man Mar 11, 2008

    "Expert" advice offered is absurd and less than competent! I did not ask for web search advice.

  • stanthe4man Mar 11, 2008

    Not acceptable "Expert" advice offered is absurd and less than competent! I did not ask for web search advice.

  • stanthe4man Mar 13, 2008

    One of the experts indirectly addressed my questions, one told me to do a web search and read general, non-specific information and the third told me to take it to a service shop. Could someone familiar with the Panasonic microwave ovens please address my qustions?

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To blow fuses need only 2 reasons: one is the PRIMARY LATCH SWITCH who can be stuck in shorted position due to prolonged use time, or trhe second (wich I belive more) is the HV Inverter. The last one can be fixed in component level repair as well. You need or the bridge diode or 2 transistors. First check your primary latch switch if open when you open the door and if it's ok contact me to give you the part numbers for the inverter components.

Posted on Mar 13, 2008

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Another possible cause of a blown fuse is a partially bad triac. Some ovens use a triac rather than a relay to control the main power to the high voltage transformer. One type of failure of a triac is for it to be totally shorted causing the oven to come on whenever the door is closed. Alternatively, the gate may be defective preventing the triac from ever turning on. A third, and most interesting possibility, is that one half of the triac is bad - shorted or open, or doesn't turn on or turn off reliably. Recall that a triac is in effect a pair of SCRs in parallel in opposite directions. If one side is defective, the main fuse will blow due to transformer core saturation since the triac will act as a rectifier and transformers really do not like DC.

Posted on Mar 13, 2008

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  • dewan nafees ahmed Mar 13, 2008

    The fuses used in microwave ovens are usually ceramic 1-1/4" x 1/4" 15 or
    20 A 250 V fast blow type. Replace with exactly the same type and rating.

  • dewan nafees ahmed Mar 13, 2008

    The following can cause the fuse to blow (in approximate order of likelihood):



    >>Defective interlock switches or misaligned door. At least one of the
    interlock switches is across the power line and will blow the fuse if not
    activated in the correct sequence. See the sections: "Fuse blows when
    closing or opening door" and "Testing and replacing of interlock switches".

    >>Shorted HV capacitor.
    >>Shorted HV diode
    >>Shorted magnetron (filament to anode
    >>Defective triac (shorted or partially shorted)
    >>Old age or power surge. Fuses sometimes blow for no apparent reason.
    >>Defective HV transformer (shorted windings
    >>Shorted wiring due to vibration or poor manufacturing quality


    Note that a shorted magnetron or shorted HV diode - which you would think
    should blow the fuse - probably will not do so because current will be limited
    by the impedance of the HV capacitor (assuming it is not shorted as well).
    However, there will likely be a loud hum from the HV transformer as it strains
    under the excess load. Such a sound in conjunction with no heat is a likely
    symptom of a shorted magnetron or HV diode. If your oven has a separate
    high voltage fuse - somewhat rare in domestic ovens - it may certainly blow
    due to a fault in any of the HV components.




    Fuses also die of old age. The types of fuses used in microwave ovens are
    subjected to a heavy load and you may find that all that is needed is to
    replace the fuse with one with equivalent ratings. (but check for shorts
    first). There could be an intermittent problem as well which will only show
    up at some random time in the future. A poorly timed power surge (as opposed
    to the well timed variety) could also weaken the fuse element resulting in
    eventual failure.



  • dewan nafees ahmed Mar 13, 2008

    Testing and replacing the triac

    A triac may fail in a variety of ways:




    • A shorted triac would result in the oven coming on as soon as the door is
      closed or the power being stuck on high no matter what the touchpad setting.



    • An open triac or one that didn't respond to the gate would result in no
      heat and possibly other things like the fan and turntable not working as well.



    • A triac that didn't turn off would result in the parts of the oven
      continuing to run even after the timer counted to zero.



    • A triac where one half was shorted would result in a blown fuse due to it
      acting as a rectifier pumping DC through the HV transformer.



    • A triac where one half doesn't properly turn off would result in the main
      fuse blowing when the cook cycle completed.


    Nearly all triac failures will be shorts. Thus, measuring across the
    MT1 and MT2 terminals of the triac (the power connections) should read
    as a high resistance with a multimeter. A few ohms means a bad triac.


    As noted above, triacs can fail in other - possibly peculiar ways - so
    substitution or bypassing may be necessary to rule out all possibilities.


    Replacement is very straightforward - just don't get the wires mixed up.


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You gotta love it..so, you are perfectly competent on this. The thermal shutoff was a good approach, and would have come up in our communication. What we need here is a good copy of the specific service manual for that component. I'll look for that..In meantime, its advisable that the heating engine for your unit, is clearly pulling too much juice. I suspect that the heating effect from the description, most likely compromised the control panel circuit board(not that hard to do) or the microwave engine is shot(not likely, due to it's vital design). I'll be back in a few..

Posted on Mar 13, 2008

  • Patrick Michaels
    Patrick Michaels Mar 13, 2008
  • Patrick Michaels
    Patrick Michaels Mar 13, 2008

    Sounds like what's on your plate. Can't imagine that the toaster oven would 'cook' the interlock contacts, but stranger things have happened, no?

    this is an excerpt from the link I sent you for reading.


    Interlock switches

    Various door interlock switches prevent inadvertent generation of microwaves
    unless the door is closed completely. At least one of these will be directly
    in series with the transformer primary so that a short in the relay or triac
    cannot accidentally turn on the microwaves with the door open. The interlocks
    must be activated in the correct sequence when the door is closed or opened.

    Interestingly, another interlock is set up to directly short the power line
    if it is activated in an incorrect sequence. The interlocks are designed
    so that if the door is correctly aligned, they will sequence correctly.
    Otherwise, a short will be put across the power line causing the fuse
    to blow forcing the oven to be serviced. This makes it more difficult for
    an ignorant consumer to just bypass the door interlocks should they fail or
    to run the oven with an open door as a room heater - and protects the
    manufacturer from lawsuits. (That interlock may be known as a "dummy switch"
    for obvious reasons and is often not even mentioned in the schematic/parts
    manifest.) Of course, should that switch ever actually be used, not only will
    the fuse blow, but the switch contacts will likely be damaged by the high
    initial current! This also means it probably wouldn't be a bad idea to
    replace the interlock switch which might have been affected if your oven
    fails with a blown fuse due to a door problem.


    Failed door interlocks account for the majority of microwave oven problems -
    perhaps as high as 75 percent. This is not surprising considering that two
    of the three switches carry the full oven current - any deterioration of the
    contacts results in increased resistance leading to their heating and further
    deterioration. And, opening the door to interrupt a cook cycle results in
    arcing at the contacts. Complete meltdowns are not unusual! If any defective
    door switches are found, it is probably a good idea to replace all of them as
    long as the oven is already apart.


    The typical door switches and their function:





    • Door Sensing: Input to the microcontroller to indicate the state of
      the door.



    • Interlock Monitor: Shorts out the AC line (and blows the main fuse)
      should the Primary Interlock not open due to incorrect sequencing of the
      door switches or a failed switch.



    • Primary Interlock: In series with the high voltage (magnetron)
      power supply so cuts power when the door is open.


    Note that if the Door Sensing switch should malfunction, peculiar behavior
    may occur (like the fan or turntable operating at the wrong time) but should
    never result in microwaves being generated with the door open.







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3. Why does a fuse sometimes blow when my air conditioner is on?
Air conditioners require a lot of current to operate. It is recommended that all of our units be placed on their own dedicated circuit.

If fuses are used, they should only be 15amp Time-delay fuses.

If the problem persists, move the unit to another location or call a qualified electrician to check for proper electrical supply. Any electrical work or modifications must be referred to a licensed electrician.
i Hope that this resolve the matter.
Sergio

Posted on Mar 13, 2008

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Check the high voltage capacitor for shorted condition. Check this site for instructions on how to do it.
As you browse through that site you will also find more info on how to test the interlock switches under the troubleshooting section.
If after all the measuring is completed and everything seems to be in working condition, It could be that a "fail safe" switch or it associated circuitry failed on your microwave and is blowing the fuse.
Good Luck.

Posted on Mar 11, 2008

  • Jaime Hernandez Mar 13, 2008

    If you need more specific info about your appliance, you can get the service manual from HERE.

    All the experts have given you the theorical info, now is up to you to put that into practical.

    Good Luck.

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Because this has lethal voltages involved, dont let anyone work on this device without experience and training.

please contact an authorized technician for this:

http://www.pasc.panasonic.com/WhereTo/Fi...

btw...to save money, contact an authorized panasonic techy rather than bringing this to a service center.

Posted on Mar 11, 2008

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Sir,

Try this link for the manual to repair it.

http://www.eserviceinfo.com/browse.php?id=36

www.yinyanghome.com/Products/Icebox/Microwave.pdf

Posted on Mar 11, 2008

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