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microwave riddle?

Discussion in 'Electronic Repair' started by poster, Sep 20, 2008.

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  1. poster

    poster Guest

    Microwave burned the fast fuse and 'rectifier' 2x062 on this schematic
    http://geocities.com/anglomont/mw.jpg
    Replaced it with hvr 6x2 diode however once again it and fuse burned
    though "high voltage rectifier" diode hvr1x stayed ok
    (hvr 6x2 short, hvr 1x conducting 0.25 ma when tested with 4.5v)
    Magnetron ok, so what could be the reason for this-perhaps capacitor
    or some contact with oven interior along points A or B?
    thanks in advance
     
  2. Franc Zabkar

    Franc Zabkar Guest

    I can't find anything in the Microwave Repair FAQ that discusses the
    high voltage protector diode:

    http://www.repairfaq.org/sam/micfaq.htm

    I did find a datasheet for your HVR6x2 diode, though:
    http://www.dccomponents.com/products/Rectifiers/Diode/High_Voltage/HVR6x2.pdf

    This device consists of two back-to-back HV diodes, one drawn larger
    than the other, the smaller one with a 1500V reverse breakdown
    voltage, the larger with 6000V. AFAICS, if your HV capacitor were to
    go open, then the smaller diode could see a reverse voltage close to
    the peak secondary voltage of the power transformer (~2000V). This
    would exceed its rating and presumably it would fail.

    So IMO the capacitor is one possible suspect, although I would think
    that anything which allowed a good capacitor to charge in the reverse
    direction may also cause the same failure. Therefore it may be wise to
    replace the HV diode as well, just in case it is leaky. Also check
    that the magnetron's filament is not shorted to the case.

    - Franc Zabkar
     
  3. Franc Zabkar

    Franc Zabkar Guest

    No, I was thinking that the smaller diode connects to the transformer.
    That's how it's drawn in the schematic diagram.

    If the dual diode were reversed, then the smaller diode would always
    see the voltage of a fully charged capacitor, which would presumably
    exceed its breakdown voltage.
    If the capacitor is open, then, during the negative half cycle of the
    mains, you effectively have two reverse biased diodes (HVR and small).

    10kV 6kV 1.5kV
    Gnd o---|<|---|--|>|--|<|----o transformer secondary
    HVR | big small
    |
    magnetron
    |
    Gnd

    How this reverse voltage is shared between the two diodes would
    probably depend on the reverse leakage characteristics of each. I'm
    assuming that no reverse current flows through the magnetron (its
    filament is cold). If current does flow via the magnetron, then the
    small diode would see the full reverse voltage. Perhaps the diode
    survives until just after the filament begins to warm up?

    - Franc Zabkar
     
  4. poster

    poster Guest

    thanks again
    What is actually the curved line close to the transformer and
    directly connected to magnetron?
    What if upon disconnecting the both diodes fast fuse blows again
    when either microwave or grill cooking is started?
    What is it exactly connected between a8-a7?
     
  5. poster

    poster Guest

    also here-http://www.repairfaq.org/REPAIR/F_micfaq2.html#MICFAQ_017
    i found this

    Problem: Fuse blows when initiating cook cycle.
    Possible causes:

    1. Defective interlock switches or misaligned door.
    2. Shorted HV capacitor.
    3. Shorted HV diode.
    4. Shorted magnetron (probably won't blow main fuse but HV fuse if
    used).
    !5. Defective triac.
    6. Old age or power surges.
    7. Defective HV transformer.
    !8. Short in wiring due to vibration or poor manufacturing.
     
  6. Franc Zabkar

    Franc Zabkar Guest

    That's the transformer winding that supplies 3VAC to the magnetron's
    filament.
    According to Sam's Microwave FAQ ...

    "An open HV diode will result in AC instead of DC across the magnetron
    with a peak negative value (the only one that matters) about 1/2 of
    what it should be. The result will likely be little or no detectable
    heat but no other symptoms".

    If the fuse blows again, then I'd be looking at the power transformer
    or the magnetron.
    That's the transformer (and metal oxide varistor) that is part of the
    low voltage DC supply for the control board. One secondary winding
    supplies the +5V (?) logic, the other probably supplies the display
    filament.

    - Franc Zabkar
     
  7. poster

    poster Guest


    thanks for input
    if the transformer secondary was shorted inside it would not be able
    to supply voltage high enough to burn the rectifier diode when it was
    replaced so maybe wiring is a good suspect since magnetron seems ok-
    that could be tested with turning on grill operation only ?
     
  8. Franc Zabkar

    Franc Zabkar Guest

    I agree that the transformer is probably OK. All I can add is that
    whenever I've seen a failure in the protection diode, I've always
    replaced it and the rectifier diode as a pair, and I've tested the
    magnetron and cap. I can't see what else could cause the protector to
    fail.

    BTW, I don't understand how you can confidently state that the
    magnetron is good unless you've tried it in another oven.

    - Franc Zabkar
     
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