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Microwave buzzing a vibrating

Discussion in 'Electronic Repair' started by [email protected], Feb 26, 2006.

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  1. Guest

    Samsung MW8490W, about 6.5 years old. Tried microwaving something
    today, and the microwave just vibrates and buzzes. It's definately a
    60Hz-based buzz. No arcing or odd smells. Everything appears OK
    except the metal enclosure vibrates violently like there's a loose
    transformer in there.
    Oh, it won't heat water, and is drawing only about 350W from the outlet
    when running (rear label says 1.5KW).

    I can tell you everything you want to know about electronics (well,
    almost), but microwaves are somewhat of a mystery to me.

    Any ideas on what's wrong, or where to start?
     
  2. Shorted high voltage diode or magnetron.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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  3. Guest

    Any quick checks that can be made? Should the two terminals to the mag
    have some low resistance? (I'm not sure what they connect to)

    I'm also going to read up some more on how these things work.
     
  4. Stay out of that thing. And if you could buy a new magnetron, it would
    probably cost more than a new microwave.

    Perce
     
  5. Guest

    Stay out, why? I know there's a HV capacitor in there, and I know
    electronics plenty good enough to know what not to do.

    And yes, you're right. Magnetrons are expensive. However, I just
    tracked it down to the diode. 0.040 V drop in both directions. Yeah,
    it's shorted. Much cheaper than a new microwave.
     
  6. First you MUST read up on the SAFETY issues. Even though this one appears
    quite dead, you don't want to take chances.

    Complete troubleshooting quide at the Web site, below.

    With the unit unplugged, make sure the HV capacitor is discharged.

    The two terminals on the magnetron are the filament and should be low
    resistance, close to zero ohms. But they ahould read open to the chassis.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
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  7. mm

    mm Guest

    The most important thing is to either not mess with the cage around
    the microwave part, or to put it back EXACTLY the way you find it. At
    first the Amana girl refused to send me a schematic (back when I
    didn't realize how simple they are) because she thought I'd radiate
    myself or my eyes to death. Apparently the metal side of the oven,
    where the electronics are, is not enough to keep the waves inside.

    You might want to get a microwave tester (10 dollars at Radio Shack)
    to make sure there is no leakage. I calibrated mine by pulling the
    door open a little but less than it took to have the oven turn off.
    now that the ovens latch instead of just close, this might be harder
    to do. The RS tester was calibrated fine to begin with afaict.


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  8. mm

    mm Guest

    Don't know about that part but for Amana model #2, they wanted 700
    dollars iirc for the transformer. After I whined (and pointed out
    that no one would pay that price, they lowered it to the wholesale
    price of 400 dollars. I don't think anyone will ever pay that price
    either for an oven that had no special features, one power, and two
    timers. And weighed so much it was hard to carry. All that was
    good was that it was big and antique.

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  9. Guest

    There's no cage around the microwave parts. It's in the open once the
    outside shell has been removed. Nothing that I'm messing with here
    looks real critical....just high voltage stuff. I'm not remove the mag
    or anything.
     
  10. Right. Just try not to electrocute yourself in the process. If you don't
    remove the magnetron, there is no critical seal you'll be affecting.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    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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  11. mm

    mm Guest

    Interesting. I tguess this is part of how they lowered the weight
    of these things, like they lowered the weight of tv's in part by
    removing the cage around the high voltage parts. Of course if you
    don't touch the tv hv, there is no problem at all, but microwaves go
    zooming around the room. Maybe it was never the danger they thought
    it was, or they figured out how not to generate excess microwaves
    without and thus didn't need a cage.

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  12. I've never seen a cage around the magnetron in a microwave oven. It's
    all metal that's exposed outside the waveguide except for the filament
    feedthroughs. As long as holes are a small fraction of the wavelength,
    very little microwave energy will get through. And, the metal cover over
    the entire oven adds another layer of protection.

    TVs in the old days had high voltage rectifier tubes and flybacks with
    exposed terminals. Modern CRT-based equipment has the flyback entirely
    potted so there is no exposed HV.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    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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  13. PipeDown

    PipeDown Guest

    Usually when the magnatron goes out, the unit appears to run normally but
    the food don't get hot. If there is a short such that the HV circuits are
    making a lot of noise, then the unit is probably trash (for ex. the HV
    transformer is shot or there are multiple component failures). At the very
    least, you need basic troubleshooting knowledge of the circuit. Unless you
    are working on an expensive built in or over the range model, I would just
    replace it. As you probably know, small ovens cost way less than $100.

    The OP described a loud buzzing which could be a stuck motor. There are
    several in an oven. Notably the exhaust fan and the one that turns the
    blades that "mix" the microwaves around the oven cavity. A frozen motor
    will,often make a lot of noise. These moving parts have lower reliability
    than the HV circuits

    You may be able to operate the oven with the magnatron disconnected. In
    this manner, you can confirm the other functions (timer relay, fans,
    interlocks, no corona around HV circuits etc). Without an HV probe, you can
    often tell if HV is present by holding a fluorescent tube near (but not
    touching) the circuit while you ground the other end with your hand, it will
    light at the end near the HV. You will not know if it is the correct
    voltage.
     
  14. PipeDown

    PipeDown Guest

    Hope that's all it is. In my experiance, HV failures tend to cascade to
    several components.
     
  15. There aren't that many HV components: HV diode, HV cap, magnetron.

    So, conceivably, a bad magnetron took out the diode (though the exact
    mechanism isn't clear). The cap is still good or else the main fuse would
    have blown. If it was the HV transformer, that wouldn't damage the HV diode.
    So, you're left with the HV diode.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    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
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  16. mm

    mm Guest

    I have.
    Who says a cage can't have holes? AFAIK, almost all cages have
    holes.

    Above, I'm referring to a separate removable cage, with a woven metal
    gasket where the cage attaches to a piece of sheet-metal.
    I'm pretty sure I've seen flybacks with no normallly exposed HV that
    still had a cage, but I don't have anything that old anymore to check.


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  17. mm

    mm Guest

    I had one where all that was wrong was the wiring to the diodes. Some
    of the insulation was crumbling off. And I think the HV was sparking
    to an internal metal part. A heavy glob of GE silicone cement
    around the wire, and everything was fine for years.

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  18. Steve B

    Steve B Guest

    Buzzing is the last thing a microwave does before detonates.

    HTH

    Steve
     
  19. mm

    mm Guest

    Don't they have a red light timer inside that counts down to zero?
    All the bombs on tv have that.

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  20. And unmarked red and blue wires..... :)

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    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
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
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