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Re: Sharp microwave oven problem

Discussion in 'Electronic Repair' started by Electronics Tech., Nov 19, 2004.

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  1. This is probably a problem with your microwave oven door interlock switches.

    Here's a quote from repairfaq website:
    http://repairfaq.ece.drexel.edu/sam/micfaq.htm#mictstris

    "Even slamming the door really hard has been known to knock an interlock
    switch out of position, resulting in breaker tripping at the electrical
    service panel whenever the microwave oven door was closed. (Another reason
    to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So
    if there was some kind of "event" after which the microwave failed, check
    the interlock mechanism first - a switch may just need to be popped back
    into place."

    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.


    Testing and replacing of interlock switches
    With the oven unplugged, put an ohmmeter across the AC input just before the
    interlocks (but beyond the power relay or triac if it precedes these). Open
    and close the door slowly several times - there should be no significant
    change in resistance and it should be more than a few ohms. If it approaches
    zero while opening or closing the door, the interlock switches and door
    alignment should be checked. (You may need to disconnect one side of the
    transformer primary since its resistance is a fraction of an ohm. Refer to
    the schematic pasted inside the cover.)

    Replace with switches having a precisely identical fit and equal or better
    electrical specifications (terminal configuration, current rating). When
    removing the old switch make a note as to where each wire goes. Check the
    embossed marking on the old switch - don't depend on location as your
    replacement might just have a different arrangement. Make sure the new
    switch aligns correctly with the actuating mechanism and then check for
    correct electrical operation with an ohmmeter before applying power.

    Even slamming the door really hard has been known to knock an interlock
    switch out of position, resulting in breaker tripping at the electrical
    service panel whenever the microwave oven door was closed. (Another reason
    to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So
    if there was some kind of "event" after which the microwave failed, check
    the interlock mechanism first - a switch may just need to be popped back
    into place.


    Credit to author :

    Notes on the Troubleshooting and Repair of Microwave Ovens
    Version 3.44
    Copyright © 1994-2004
    Samuel M. Goldwasser
    --- All Rights Reserved ---
    http://repairfaq.ece.drexel.edu/sam/micfaq.htm#mictstris


    From Jim

    Remove 99 to reply.



    ----- Original Message -----
    From: Pat
    Newsgroups: sci.electronics.repair
    Sent: Tuesday, November 16, 2004 9:29 AM
    Subject: Sharp microwave oven problem


    My Sharp convection microwave (model R-9H66) trips the circuit breaker
    everytime it's turned on. It started doing this after its door was slammed
    shut by someone. It's obviously shorting somewhere when turned on, and I
    took the outer housing off to look at the wiring to see if there was
    anything obvious,but didn't notice anything.

    Does anyone have any idea what might be wrong or what to look for? It was a
    fairly expensive unit when I bought it 6 years ago, and I'd hate to just
    through it away, especially if it turns out to be something relatively easy
    to repair.

    Thanks for any help. I appreciate it.

    Pat
     
  2. Graham

    Graham Guest

    I was interested in the assertion that door interlock micro-switches are the
    most common failure in microwaves.

    I have just looked at our repair database and I have repaired about 280
    microwaves over the past 18 months makes include Sharp - Panasonic -
    Samsung - LG - old Toshibas & Sanyo.

    I would say the most common faults are
    1) Sharp CPUs
    2) HV diodes
    3) Magnetrons
    4) HV capacitors
    With microswitch trouble maybe joint fourth or maybe lower.

    The situation of the Sharp CPU perhaps needs a little explanation.
    Most of the ovens we repair are Sharp, with model numbers beginning with R-
    The CPU is integrated to an LCD display, they are connected to each other
    with a ribbon cable bonded to the glass and the PCB. After a while some /
    most LCD segments fail to display. I suspect the problem is with the bonded
    cable, but as there is no reliable means of repair (that I know) they all
    get replaced.

    It just occurred to me that here in the UK we use real electricity ie
    230-240V, therefore the current is only half of a N American oven of the
    same power. Typically the microwave side of a domestic oven here will be
    protected by an 8 Amp fuse. Does this explain the anomaly?



    Graham.


    %Profound_observation%
     
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