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Microwave help (electronics and Power issue)

Discussion in 'Do It Yourself (Not Computer-Related)' started by kgt, Jan 21, 2007.

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  1. kgt

    kgt Thread Starter

    Joined:
    Mar 31, 2005
    Messages:
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    I have a Panasonic microwave NN-S950WA which belongs to my parents. It will start but stops in few seconds (10 to 15 secs.) Clock and functions are working fine. During a lightning strike it hit entire house that killed their microwave, TV, cordless phones, etc. I tried to trouble shoot microwave. The fuse is OK. I believe it is Low Voltage Transformer which is located on a circuit board that is behind touch pad board. Transformer is reading 200-300 ohms which is OK. But there is a varistor to protect circuit in high power surges. I checked this varistor and it seems like bad. I removed it and microwave still does the same. Do you think I have to replace the varistor for microwave to perform if everything else is OK? I learned that it can run without varistor as it's function is to only absorb high voltage spikes during abnormal cases. Please help and let me know if I need to check anything else.
     
  2. O111111O

    O111111O

    Joined:
    Aug 26, 2005
    Messages:
    894
    If it's starting/stopping like that, it could be the HV doubler diode/Magnetron.

    Unplug, discharge all HV capacitors, check, check HV doubler diode to Magnetron, check impedence across magnetron with it disconnected.
     
  3. johnpost

    johnpost

    Joined:
    Dec 17, 2005
    Messages:
    317
    you said that the low voltage transformer might be at fault and then
    you gave a resistance value and said it seemed OK. It can't both
    maybe be at fault and maybe OK. Transformers also have more
    than one resistance value.

    If the varistor's function is to protect against surges and
    you remove it then the circuit should still work. So everything
    is not OK except for the varistor being blown.

    A varistor is placed where it could divert a surge and maybe
    protect other circuitry. If a varistor is taken out that doesn't
    mean the other circuitry was protected, the other circuitry
    could have been destroyed along with the varistor.

    There are things in a digital microwave you couldn't
    test without microprocessor test equipment. There
    are clocks, counters and memory that could all have
    been destroyed. If it can't count past 15 seconds
    then it could easily be that.

    Be careful even people with years and decades of
    electronic experience have died or been severly
    injured repairing microwaves. If you aren't really
    skilled and know what you can mess with, it might
    be better to just replace the whole unit.
     
  4. kiwiguy

    kiwiguy

    Joined:
    Aug 17, 2003
    Messages:
    17,584
    Also from experience I will confirm that afetr any lightning surge, even repaired appliances seldom last long.

    Everything is stressed to the point that MTBF (mean time before failure) rates for all components are all past their use by date.

    As above the varistor (if shorted) would blow the fuse, that is what it is there for. If open circuit, it would have no effect, it simply would no longer be able to offer protection. So it will not be the cause of your problems.

    You will find that the microwave will make a satisfying "thud" when you chuck it in the rubbish skip.

    You could well be endangering your own and others lives if you persist in trying to repair it without the knowledge or experience, or the tools and test equipment. As well noted above.
     
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