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Microwave problem.....again

Discussion in 'Electronic Repair' started by [email protected], Mar 3, 2006.

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  1. Guest

    Ok, so I got a new microwave diode for the microwave I posted about a
    few days ago (Samsung MW8490W). I should have checked the new one
    before installing it, but I was excited and tried it anyway. Turn
    microwave on....same exact thing. No smoke, popping noises, nothing
    odd. I take the diode out and hope it's at least still good. The
    original was shorted. Now this one has become open! I start checking
    other parts.

    There's one, very low resistance xformer winding that's connected
    across the magnetron terminals (cathode?). This makes sense. Then
    another winding that's higher (~70ohms) that's connected in series with
    the capacitor and diode. I tested the capacitor with a Sencore Zmeter
    and it says it's perfect. However, I can only check leakage to 600V.
    Still, no leakage detected. No shorts to the capacitor casing, either.
    The magetron is open between the terminals and the casing, but the
    resistance between the terminals is low as it should be. I start
    drawing out the connections to see how this thing works. This doesn't
    look like the voltage doublers I've seen, but whatever. I can't
    understand how it blew the new diode. Apparently something is
    happening only at high voltage. Any ideas?

    I'm ready to buy a new microwave anyway....one that mounts above the
    range and uses what I remember being called "inverter technology" or
    something like that. Instead of cycling at lower power levels, it just
    produces less power.
     
  2. Guest

    I just realized that my diode is probably not destroyed. If this thing
    has a forward voltage drop of ~10V, my meter won't show this. I'll
    test it later using another method. (I just assumed it has a drop of
    about 0.7V as most other diodes do!)

    This still doesn't explain why the darned microwave is still buzzing
    and vibrating loudly. This is the same symptom as when the original
    diode was shorted. (See post "Microwave buzzing a vibrating" from Feb
    26)
     
  3. What does it do if the HV diode is not installed? If the same thing,
    then either the magnetron is shorting at full voltage or there is a problem
    in the transformer that isn't severe enough to blow the main fuse but
    enough to result in no heating. That wouldn't explain why the HV diode
    blew originally though.

    Also, of course, double check you put it in the correct way around!

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  4. Cyrus®

    Cyrus® Guest

    It Seems you need new Magnetron for your Microwave,{ Assuming your inside of
    microwave is Clean including waveguide Mica window}Check this web site for
    more info,
    http://www.gallawa.com/microtech/mag_test.html

    With Best Wishes
    Cyrus®
     
  5. Ken G.

    Ken G. Guest

    Before you start any of this please discharge the capacitor .
    Yes make sure you put the diode in the right way .
    Try unplugging the magnatron wires and run the oven . If it sounds
    normal its probably a bad mag .

    Those inverter microwaves are a bit touch & go for lasting .. The first
    ones out were a disaster . The later ones are better . If you buy a
    ``mount in`` one make sure you want to be taking it out if it breaks .
    I have seen bad magnatrons in more newer microwaves than ever before .
    some of them are still brand new only used a few times not enough to
    even get it dirty . I have also been seeing samsung branded magnatrons
    in a few other popular brand ovens .. so much for quality
     
  6. Guest

    Well, it seems it's the magnetron. The diode is still good.
    In summary:

    Sounds normal with magnetron filament connector removed.
    Also sounds normal with magnetron filament connected, BUT everything
    else removed.
    Buzzes like crazy with everything connected.

    And, if I run it with the filament disconnected, then unplug the AC
    cord and go to discharge the cap, it actually sparked, for the first
    time. So it seems the HV is shorted through the mag. A new mag is
    $57+shipping....hardly worth it.
     
  7. Jeff, WB8NHV

    Jeff, WB8NHV Guest


    I have a Sharp Carousel microwave which I bought new in November
    1999. Magnetron and everything else are OK. I don't know what brand
    magnetron is in mine, but it must be a good make if it still works
    after more than six years.

    I agree with the advice regarding over-the-range microwaves. I've
    seen at least one of these (a friend of mine has one in his kitchen) ;
    I hate to think what a job it may be to remove it for repairs.

    I would think off-brand microwaves (and those ultra-cheap ones with
    spring-wound dial timers) which can be had for $39 or less in
    department stores, etc. would be a lot more prone to very early (read
    premature) magnetron failures. The micros with digital displays and
    touch-pad controls should last much longer assuming they are made by a
    reputable manufacturer, though these days it isn't uncommon to see
    appliances with well-known names on them that have actually been
    manufactured by some offshore company no one ever heard of.

    As to magnetrons only a few months old (or even newer [!]) going bad:
    This can happen if the user tries to operate the microwave empty; this
    will ruin any magnetron in almost no time flat. Also, the use of metal
    pans, utensils, even aluminum foil in a microwave will destroy the tube
    in very short order. Power surges on the AC line the microwave is
    plugged into can also wreak havoc on the internal circuitry (such as
    the microprocessor which controls the operation of the magnetron and
    everything else in all recent microwaves), but I wouldn't expect the
    magnetron to short or open from a line spike. This is not to say the
    latter cannot occur, but it is possible.

    Jeff, WB8NHV (email addy not shown to deter spammers)
    Fairport Harbor, Ohio USA
     
  8. Seafarer

    Seafarer Guest

    You have to carry out proceedure for discharging the h.v circuit then
    unclip the filement leads.Attach meter to leads then switch on,the
    reading will be between 3 to 4v a.c.
    Switch off,discharge Cappie,re-attach fill.leads then clip meter on to
    fill.terminals,switch on.Reading should be the same,if not,Mag is
    suspect.
     
  9. G.E.M.

    G.E.M. Guest

    Have been reading your microwave problem, and sounded a bit familiar...

    I just recently got 2 tossed out ovens, one had an arcing from the
    antenna...seems someone
    washed the mica RF window and carbon'd it up...which caused the antenna to
    start arcing over
    to the waveguide....(I googled about oven magnatrons and learned how they
    work)

    I used to work on Navy Ship Radar's so figured this can't be too
    hard...took off the mica cover...then took out the magnatron...cleaned it
    all up...but the tip had badly arced over...so put large shrink tubing on
    it
    to protect it from further arcing...(what the heck it was going in the
    landfill anyway)...
    Blew out lots of dead bugs while inside...measured the resistance's..all
    seemed ok..been
    working great for months now...no mica window..was going to try to find
    some plastic won't heat
    under RF, but seems ok without it, just I make sure no moisture gets up in
    the guide area...and
    keep it clean/dry.....

    2nd Oven sounds more like your problem...buzzing like heavy load on the
    transformer....so checked
    diode, cap...and resistances...seemed ok?? So figured better look further,
    took the mag out, and
    pried open the shielding lid and could see the RF filter coils inside...but
    strange the plastic where
    the filament leads come through....was loose...so pulled it back further
    and could see it had cracked
    and also on one lead was signs of a carbon arc path...funny didn't show up
    as high resistance??
    Maybe a High Pot meter would detect it?
    Anyway, cut the lead where the RF filter coil (leaving some copper on the
    steel lead) then used a hot
    iron to melt the potting epoxy so I could pull the male lead and all out
    with pliers...
    Then on inspection, seems there was a factory void in the potting epoxy or
    it just broke down from
    someone running the oven with nothing in it?
    Had to carefully drill out the charred epoxy and any remaining carbon, used
    acetone to flush...dry...then
    took the filament pin connector and put many layers of shrink tubing over
    it, all the way from the tip
    to the connector (could aways razor trim later).... Then used some GE clear
    silicone glue to get the
    inner part to glue and pushed the inner plastic back onto the pin through
    holes, then let sit dry overnight.

    Next day, soldered on the pin to the RF choke, and then mixed up some clear
    high temp epoxy, got
    some at ACE 2 mixes in one tube...used a long chinese stick and slowely
    pour drops into the connection
    side of the filament connectors (filled it up) and put some dabs over my
    solder connection too....let stand another day....cleaned it all up, been
    working great ever since!!!

    So I saved somemore things from the landfill... :))

    Figured about 1 hour honest labor....and some fun learning too...

    Now I wish I could re-program the presets, as it boils my coffee...and
    maybe get the timing so the
    coffee cup handle always returns to the front?? heh heh....
     
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