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Does my microwave diode look good to you?

Discussion in 'Electronic Repair' started by Danny D., Aug 26, 2013.

  1. Danny D.

    Danny D. Guest

    I just tested my microwave diode where, unfortunately, it seems to be good:
    http://farm8.staticflickr.com/7358/9594179589_09dac4708b_o.fig

    Forward biased with a 9 VDC battery, I measure 7 VDC across the diode;
    reverse biased, I read 9 VDC across the diode (given a 200 ohm
    current-limiting resistance in series with the battery).

    I was hoping the diode was bad, because otherwise, I don't know what's
    wrong with the microwave. It does everything but heat up the food.

    It makes noise, the lights light up fine, the controls all work,
    and it beeps when done, etc. - but it just won't heat up anything.

    Did I test the diode properly?
     
  2. Danny D.

    Danny D. Guest

  3. micky

    micky Guest

    Since apparently there is no current in that direction, how about
    taking out the resistor and seeing what it says then?
    Normally I'd say you should measure the voltages while it's in the
    circuit. That's a little different here since you'd have to have the
    cover off and that would expose you to a lot of microwaves. OTOH,
    it's not working so maybe there are no microwaves. I hate to rely on
    maybe. Do you have a microwave detector. I got one at Radio Shack
    about 30 years ago and it works well. I was able to test it because
    at that time I had Amana Model 2, and it didn't have door latches on
    it. So I could open the door a little before the safety switches
    turned the machine off, and I could see the detector reading rise from
    zero to at least half way across the scale (but I only allowed that
    for a second.) After that, I could check door leakage. RS doesn't
    still sell them iirc but you might find one used, or you might find a
    new one sold through another channel.

    Here's exactly the same one I have, no bids yet, 18 hours left from my
    post time. If it doesnt' sell he'll probably relist it so click
    anyhow.
    http://www.ebay.com/itm/Micronta-Microwave-Leakage-Detector-/290963447557

    and a new one for only 15 including shipping
    http://www.ebay.com/itm/General-MLD100-Microwave-Leak-Detectors-/200952444879

    Here's one for more, $35 http://www.lessemf.com/mw-oven.html

    And http://www.ebay.com/bhp/microwave-leak-detector
    I'm self-taught for the most part, but I would have just measured the
    resistance directly with an ohmmeter, using the meter's internal
    battery, in both directions. If one direction is many times higher
    than the other, it's good. Even if I did it your way, I would do
    it my way too, to see if the results confirmed each other.

    For more info, ask on sci.electronics.repair


    ***I used to have an Amana Radarrange model 2. I never saw a model
    1, but this one looked just like the image of a microwave everyone
    used for years (except compeititors).

    I got it used, fixed the door spring and had it for about 10 years.
    When it stopped working I called Amana and asked, figuring she might
    know which part failed most frequently. She suspected the
    magnetron. The next time I called, someone suspected the diode. I
    opened the high voltage cage**** and saw that there were cracks on the
    wires to the diode (one 20 or 30 times bigger than yours, counting
    heat radiation fins). I used silicione sealant by GE (available in
    black at auto parts stores, if color matters to the repairer) and put
    on large blobs of it, thicker than the thick wires. And it worked for
    another 10 years.

    ****The woman at Amana was very much afraid I wouldn't put the woven
    metal gasket back the way it had been and that it would leak. She
    also wouldn't send me a schematic. I had to promise her up and down
    that I had 20 years experience with electronics, and I would put the
    gasket just where it was, and she finally sent me a schematic, for
    free. I had to take apart another one^^ and I think they are
    designed differently now, but I would be very careful reassembling so
    as to not leak microwaves.

    The next time it failed it did nothing, so it was the transformer. I
    think Amana wanted 380 dollars I said, "That's the price from 1970.
    They are worth less now (since you can buy a whole microwave for under
    100.) After writing a letter -- I said, Save a few for your museum
    and the inventor's grandchildren and sell the rest at a price at which
    you will actually sell them -- and being referred to a place near
    Harrisburg, they lowered the price to the wholesale price, 250 or so.
    Much as I hated to part with model 2, I scrapped it.


    ^^This latest one has a bad relay, and I'll probably never get around
    to fixining it.
     
  4. What do you mean by a "microwave diode"?

    To me, it's a point-contact diode used in microwave measurements. My
    experience has been that sample-to-sample variation is high.
     
  5. "microwave diode" can mean many things to me since I've experience in
    electronics. The context in which it's being used in this thread is a
    discussion of the high voltage diode used in the power supply for the
    magnetron in a homeowner's microwave oven. ^_^

    TDD
     
  6. Danny D.

    Danny D. Guest

    I meant the high-voltage diode in the microwave oven.
     
  7. Danny D.

    Danny D. Guest

    The resistor is to prevent the diode from burning
    up with too much current, I believe.

    If I take out the resistor, I probably blow up the diode.

    That does not sound like good advice, so I didn't read the
    rest of the suggestions for fear of being led astray.
     
  8. Phil Allison

    Phil Allison Guest

    "Arfa Daily"

    ** The OP is just begging for a Darwin Award.

    So who are you to spoil his one moment of fame ??

    http://en.wikipedia.org/wiki/Darwin_Awards

    How amazing to find that the famous Award originates with USENET !!!!!!!



    ..... Phil
     
  9. JW

    JW Guest

    That's true of most DMMs, but there are exceptions. My Keithley 2000 will
    bias a diode up to 10VDC. Useful for checking smaller zeners as well.
     
  10. Sort of. A 9 volt battery isn't really a good test for a high voltage
    recifier. The forward drop will exceed that of that battery, at least a
    rated current of 0.3 to 0.5A, depending on the diode and oven.

    The results of your test though, show the diode isn't open and isn't
    shorted, so that's good. I'd not bother to replace the diode at this point
    of the oven was in front of me.

    I'd test the heater coil of the magnetron. It should measure close to dead
    short ohms if it's good. If it's open, well, that explains why there's no
    heat. You'd also want to make sure the transformer is even being turned
    on, you can usually tell this from just listening to the oven or watching
    lights flicker when the oven turns on.

    Be sure you discarge the cap before you mess around in a microwave oven.

    As mentioned in this thread by others, a normal volt meter cannot be
    safely used to test the HV section of a microwave. You really don't even
    need to for the most part either.
     
  11. micky

    micky Guest

    Since apparently there is no current in that direction, how about
    taking out the resistor and seeing what it says then?
    Normally I'd say you should measure the voltages while it's in the
    circuit. That's a little different here since you'd have to have the
    cover off and that would expose you to a lot of microwaves. OTOH,
    it's not working so maybe there are no microwaves. I hate to rely on
    maybe. Do you have a microwave detector. I got one at Radio Shack
    about 30 years ago and it works well. I was able to test it because
    at that time I had Amana Model 2, and it didn't have door latches on
    it. So I could open the door a little before the safety switches
    turned the machine off, and I could see the detector reading rise from
    zero to at least half way across the scale (but I only allowed that
    for a second.) After that, I could check door leakage. RS doesn't
    still sell them iirc but you might find one used, or you might find a
    new one sold through another channel.

    Here's exactly the same one I have, no bids yet, 18 hours left from my
    post time. If it doesnt' sell he'll probably relist it so click
    anyhow.
    http://www.ebay.com/itm/Micronta-Microwave-Leakage-Detector-/290963447557

    and a new one for only 15 including shipping
    http://www.ebay.com/itm/General-MLD100-Microwave-Leak-Detectors-/200952444879

    Here's one for more, $35 http://www.lessemf.com/mw-oven.html

    And http://www.ebay.com/bhp/microwave-leak-detector
    I'm self-taught for the most part, but I would have just measured the
    resistance directly with an ohmmeter, using the meter's internal
    battery, in both directions. If one direction is many times higher
    than the other, it's good. Even if I did it your way, I would do
    it my way too, to see if the results confirmed each other.

    For more info, ask on sci.electronics.repair


    ***I used to have an Amana Radarrange model 2. I never saw a model
    1, but this one looked just like the image of a microwave everyone
    used for years (except compeititors).

    I got it used, fixed the door spring and had it for about 10 years.
    When it stopped working I called Amana and asked, figuring she might
    know which part failed most frequently. She suspected the
    magnetron. The next time I called, someone suspected the diode. I
    opened the high voltage cage**** and saw that there were cracks on the
    wires to the diode (one 20 or 30 times bigger than yours, counting
    heat radiation fins). I used silicione sealant by GE (available in
    black at auto parts stores, if color matters to the repairer) and put
    on large blobs of it, thicker than the thick wires. And it worked for
    another 10 years.

    ****The woman at Amana was very much afraid I wouldn't put the woven
    metal gasket back the way it had been and that it would leak. She
    also wouldn't send me a schematic. I had to promise her up and down
    that I had 20 years experience with electronics, and I would put the
    gasket just where it was, and she finally sent me a schematic, for
    free. I had to take apart another one^^ and I think they are
    designed differently now, but I would be very careful reassembling so
    as to not leak microwaves.

    The next time it failed it did nothing, so it was the transformer. I
    think Amana wanted 380 dollars I said, "That's the price from 1970.
    They are worth less now (since you can buy a whole microwave for under
    100.) After writing a letter -- I said, Save a few for your museum
    and the inventor's grandchildren and sell the rest at a price at which
    you will actually sell them -- and being referred to a place near
    Harrisburg, they lowered the price to the wholesale price, 250 or so.
    Much as I hated to part with model 2, I scrapped it.


    ^^This latest one has a bad relay, and I'll probably never get around
    to fixining it.
     
  12. micky

    micky Guest

    I think so too.
    Now why would you think that? Did you fail to notice that if there
    is 9 volts across the diode, there is no current running through the
    diode. How can you imagine that removing a 200 ohm resistorr will
    increase the current much? And with a 9-volt battery no less.
    Since ;your answer does not sound like you think well, I won't read
    the rest of your posts, for fear I'll read foolishness but be too weak
    to avoid believing it. .
     
  13. Danny D.

    Danny D. Guest

    Thanks for that confirmation. Bummer. But now at least I know it's
    most likely the magnetron.
     
  14. Danny D.

    Danny D. Guest

    Except there isn't 9 volts across the diode.
    There was 7 volts across the diode.
     
  15. micky

    micky Guest

    That was forward biased. My reply was clearly about reverse biased.
    If you hadn't snipped so much and didn't want so much for me to be
    wrong, you'd have noticed.

    Quote
    End Quote.

    I added the blank line between your first and second line so there'd
    be no doubt which direction I was talking about.
     
  16. Danny D.

    Danny D. Guest

    "Normal" diodes would fry under those circumstances;
    but I don't know what would happen with high-voltage
    diodes. That's why I have the resistor there.
     
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