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Magnetron replacement,, (substitution)

Discussion in 'Electronic Repair' started by [email protected], Oct 13, 2013.

  1. Guest

    I have a large Kenmore over the stove type microwave oven that started growling last week. We pulled it apart and found that the magnetron had a shortbetween anode and filament. The diode and cap both check good. The cap is a 1.05uf 2100V unit. I have several new magnetrons in stock meant for comparable GE over the stove units. I don't know what value of cap is used with these though. In my travels I have seen caps as low as .90uf. If the GE magis the same physical size and mounts the same as the bad Kenmore does whatwould be the harm in using it if the present 1.05uf cap in the Kenmore were a different value from what the GE calls for? Thanks for any input. Lenny
     
  2. Baron

    Baron Guest

    dave Inscribed thus:
    I agree !

    If the magnetron is physically the same and the heater voltage is the
    same then just replace it. It should work ok.
    You can always do the cup of water test...

    HTH.
     
  3. Guest

    I hadn't considered the heater voltage. I guess I figured that they would all be identical. Isn't it generally around two volts? Is there an easy way, or site where I might compare mag. Specs? This tube is real bear to replace. The oven was literally built around the magnetron. I would hate to openthe filament on the new tube the fist time I fired it up. Lenny
     
  4. Guest

     
  5. Phil Allison

    Phil Allison Guest

    "dave"
    ** Completely irrelevant.

    ** Not in a microwave oven it ain't.



    .... Phil
     
  6. Jeff Layman

    Jeff Layman Guest

    Interesting. Took some time on Google to find out why. I had wondered
    why the capacitances were so carefully stated over a very limited range,
    and why the tolerance was so tight at +/- 3%.

    For anyone interested, the answer appears to be here in section 9.5
    (assuming it's correct):
    http://www.repairfaq.org/REPAIR/F_micfaq7.html

    By the way, if you want a laugh (and possible pair of future Darwin
    award winners), have a look at the fourth paragraph of section 8.21!
     
  7. Jeff Layman

    Jeff Layman Guest

    You might find some info here:
    http://s3.amazonaws.com/szmanuals/8b1cbf510190d2b758c9b5cb5e139167

    In the "typical" schematic on page 1, the heater voltage is shown as 3.15v.
     
  8. Phil Allison

    Phil Allison Guest

    "dave"

    ** Huh ??

    Has this guy never heard of tolerance bands ?

    Most film capacitors are rated at 10% tolerance but test much better.

    Film caps with 1% tolerance are available and they are darn accurate.


    ..... Phil
     
  9. Phil Allison

    Phil Allison Guest

    "Jeff Layman"

    ** That explanation is highly simplistic.

    A crucial thing with a microwave oven is that the current drawn from the HV
    transformer is the same on both half cycles of the AC supply - otherwise
    the iron core saturates and that would blow the AC fuse.

    On one half cycle, the transformer charges the film cap and on the next the
    magnetron draws current from the cap and the transformer in series. There is
    also series resonance between the cap and the transformer, due to its very
    high leakage inductance.

    Taken together, this explains why microwave ovens are made to suit the local
    AC supply frequency.

    BTW:

    If you examine the AC current draw of a microwave oven, it is a pretty good
    sine wave - remarkable considering what the load inside is like.


    .... Phil
     
  10. David Lesher

    David Lesher Guest

    Wouldn't it make more sense to use a switcher supply now-a-daze?
    The ovens I've seen had large ergo $$$ transformers.
     
  11. Phil Allison

    Phil Allison Guest

    "David Lesher"
    ** Nope.

    ** Made in China for about $1 or $2 each and normally way outlast the rest
    of the oven.

    Any 1kW SMPS that does the same job is gonna cost 20 times more and be the
    first thing to fail.



    .... Phil
     
  12. Guest

    Thanks for the great explanation of how the oven works Phil. I figured there was a resonance factor associated with this but wasn't completely sure. So then can I surmise that a different cap shifts resonance slightly and then does that affect maximum power transfer to the load, (Tube)? And then while we're at it, it occurred to me that the food must be the tube load, (output). I'm trying to understand this better so is at least part of that correct? Lenny
     
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