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Inverter microwave ovens - reliable?

Discussion in 'Electronic Repair' started by Franc Zabkar, Mar 3, 2009.

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  1. Franc Zabkar

    Franc Zabkar Guest

    My brother's Panasonic model NN-ST756W inverter microwave oven failed
    after one year, just out of warranty. I found a short circuited HV
    diode, D702, UX-C2B. The associated 8200pF 3kV capacitor appears OK,
    as do the inverter transistors.

    The inverter PCB (240V version) is similar to the one in these service

    Here are several exploded views and parts lists: ASSY.jpg

    I'm wondering if inverter microwaves are any less reliable than
    traditional transformer/capacitor types. They seem to be overly
    complicated for what they do. I understand that they cook more evenly
    in low power modes as a consequence of not having to pulse on and off
    like conventional microwaves, but I'd rather not pay a reliability
    penalty for this feature. BTW, once the microwave is sitting on my
    bench top, the extra weight of a traditional mains transformer is of
    no consequence.

    The Microwave Oven Repair FAQ appears to bear me out:

    - Franc Zabkar
  2. GregS

    GregS Guest

    I didn't know much about it. I bought one maybe 3 years ago. Small lightweight,
    and the most powerfull one I have ever had. I use it everyday. The only problem, the light
    does not come on when you open the door. My brother uses one in a resturant,
    and they have failed after much use. That one the commercial job, costs a lot
    more. Like I said, it small, powerfull, and cheap. My Sears GE microwave is
    big, not nearly as powerfull, also has turbo oven, but I use the Panasonic
    to heat fast. By other brother has a Samsung which has failed. Replaced
    diode and found the transformer is bad. After two months still waiting for transformer.

  3. hr(bob)

    hr(bob) Guest

    Maybe the reason the light does not come on is because it is burned
  4. Jim Yanik

    Jim Yanik Guest

    the savings is in not having to build an expensive,big transformer.
    all that copper is expensive,too.
    transformers,being far less complicated,are more reliable.
    Inverters have a lot more components,particularly electrolytic caps,that
    increase the odds of failure.
    Many electrolytics are only rated for 3000 hrs or so of operation.
    and it seems that surface-mount electrolytic caps are even less long-
  5. Mike WB2MEP

    Mike WB2MEP Guest


    I don't think they're as reliable as conventional transformer/diode/
    power supplies. "Inverter" is just a switching power supply that
    up the voltage, and consumer-grade SMPS in a variety of products have
    rather high failure rate.

    There were several threads on SER a few years ago when Panasonic
    first came out with the Inverter oves, about failures of the Inverter
    circuit. At that time, Panasonic wouldn't sell replacement parts for
    the Inverter power supply, or even provide a part number for the
    switching trasistors that were failing. You had to replace the whole
    power supply, which cost almost as much as a new oven.

    The service manual for the NN-C2000 you linked to does have a parts
    list for the Inverter board, so maybe they are considered repairable

    3 or 4 years ago, we needed a new microwave for the break room at
    Somebody donated an old (1984) Panasonic with a transformer power
    and shortly thereafter a fancy new stainless-steel Panasonic Inverter
    microwave was purchased. The two were used side-by-side for just over
    year, then one day I came in and saw the Inverter oven set outside for
    trash pickup.

    So, I would definitely avoid the Inverter microwaves. Other
    may be using switching power supplies, just not using the "Inverter"
    Last time I checked, most of the microwaves were still using
    power supplies, but once the cost of a switching supply becomes less
    a conventional supply, I would expect most of them to change over to

    When you go into the store, just lift up the right side of each
    on display, and buy the heaviest one.

  6. Aside from the reliability issues which have been addressed in other
    replies, it's possible that the HV diode died for reasons unrelated
    to the inverter.

    What does the oven do when you go to "cook". If nothing at all, then
    there are almost certainly other bad parts, a fuse at the very least.

    sam | Sci.Electronics.Repair FAQ:
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  7. Franc Zabkar

    Franc Zabkar Guest

    I'm not sure where to look except for the magnetron.

    Page 20 of the following service manual has a detailed circuit diagram
    of the inverter:

    Page 42 of this manual has a block diagram of the controller chip:
    I didn't test it in case something else broke, but my brother tells me
    that everything appears to work except for the actual heating. That
    is, the control panel works, the turntable turns, and the fan spins. A
    shorted HV diode (there are two in a voltage doubler arrangement)
    would mean that one of the 8200pF caps is now connected directly
    across the secondary winding. I expect this would overload the
    transformer and presumably the inverter would sense this fault
    condition via the current transformer in the primary circuit. I'm told
    that the oven shuts down after a time. Anyway I've ordered the diode
    and I'll report back once I've tried it.

    - Franc Zabkar
  8. GregS

    GregS Guest

  9. jakdedert

    jakdedert Guest

    GregS wrote:
    That seems kind of obvious: door switch...probably gunked up with food

  10. GregS

    GregS Guest

    It never worked. I don't see any reason to not believe thats the way its
    designed. I need to hear from those who have these.
    But, one day I'll open er up and change it.

    I remember way back on an old Norelco microwave, the controller
    failed, so I got out the old mechanical timer just like the one
    on my first Heathkit microwave. I can't stress how this simple and easy
    to use feature is missing on todays ovens. My father loved it.
    I think on better models they used aother timer to cycle
    the defrost. Those older models had at least 3 fail safe
    microswitches on them, but the switch took the full current.
    I'm sure in a fail safe mode, at least one microswitch in
    ALL microwaves take the full current.

  11. Franc Zabkar

    Franc Zabkar Guest

    The transistors are now offered as "A691E4V10GP Transistor Kit Series

    Here are the datasheets for the GT30J322 (75W) and GT60M303 (170W)

    The oven has a rated output of 1100W. If the SMPS has an efficiency of
    90%, then I expect that the transistors would be dissipating around
    100W between them, although the mass of the heatsink appears
    relatively small for such a high heat load.

    - Franc Zabkar
  12. David Lesher

    David Lesher Guest

    My sister had one like that; light on during cook -- no light with the
    door open. Dumb design.

    I added a relay to solve the issue and was the big hero...
  13. Franc Zabkar

    Franc Zabkar Guest

    I replaced the HV diode and all appears well so far.

    - Franc Zabkar
  14. Hi!
    I'm not sure I follow what you are saying. Are you saying that the
    microwave, when its controller went bad, failed in an "on" position?
    If I understand you correctly, I think it would be a far better idea
    to have the oven fail in an "off" position.
    Older microwaves sometimes had multiple timers. My grandfather has a
    Litton microwave oven with two timers. One is the main timer, which is
    an analog "knob" on the front. The other "timer" is engaged and used
    in conjunction with the main timer when DEFROST mode is turned on. It
    consists of a slow turning motor that drives an irregular cam. When
    the cam rises up, it turns on a microswitch that enables the high
    voltage and magnetron.

    Today, the controller board in a "typical" oven has two
    that runs the fan, turntable and light and another that can be turned
    on and off to cycle the magnetron.

    I'm not sure how long that technique was used, as I have a slightly
    newer Litton microwave oven that is largely the same internally but
    has a "rolling digits" timer and a solid-state control board for the
    defrost and low power modes.
    Newer microwaves also have multiple microswitches. (At least I have
    yet to see one that does not, and that's on ovens manufactured as
    recently as a year ago.) However, in normal operation, the microswitch
    that interrupts the oven's operation when the door opens is actually a
    low voltage signal to the controller board.

  15. Still use our Sharp "Carousel II" from around 1987 and it's never even
    needed a new fuse. :)

    sam | Sci.Electronics.Repair FAQ:
    Repair | Main Table of Contents:
    +Lasers | Sam's Laser FAQ:
    | Mirror Sites:

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
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  16. Jim Yanik

    Jim Yanik Guest

    (Samuel M. Goldwasser) wrote in

    My Carousel from early 1980s blew the HV capacitor after about 20
    yrs,replaced it and the HV diode for $25 several years ago,it's still
    The MWs of today don't have as tall a cavity as my Sharp's.
  17. The MWs of today don't have as tall a cavity as my Sharp's.

    But I suspect it's not as wide.
  18. Jim Yanik

    Jim Yanik Guest

    15.5" wide,10.25" high,and 16.5" deep,just measured it.

    most MWs today are sandwich warmers....

    I like to put a 2qt.pitcher in the MW and heat water in it for tea.
  19. My dad bought it a few years ago for their motor home, and never
    The only thing "wrong" with a 700W unit is that it won't heat things as
    quickly. Which -- except for bacon and a few other things -- is all that
    microwaves are good for -- heating and re-heating.
  20. The only thing "wrong" with a 700W unit is that it won't heat things as
    I kinda, maybe, see the logic in that.

    One should not be cooking meat in a microwave. (Other than bacon, I'm not
    sure what sorts of mean /can/ be successfully cooked.

    If you're heating up already-cooked items -- such as chicken nuggets -- the
    only way you /might/ have a problem would be if you stuffed the oven with
    several dozen, and the food stayed at an incubatory temperature long enough
    for the bacteria to grow.

    Even this is highly unlikely, as we're talking about pre-cooked food.

    No obvious reason, other than that you raised the issue. (Nothing personal.)

    You'll probably be upset to learn that, when I eat frozen dinners at work, I
    let them completely defrost before shoving them in the microwave. It's
    quicker, saves a bit of energy, and I'm not dead -- yet.
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