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GE Microwave ZMC1090

Discussion in 'Electronic Repair' started by Jim shedden, Jan 5, 2004.

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  1. Jim shedden

    Jim shedden Guest


    The above oven doesn't heat on microwave. Convection heating and all
    control functions seem O.K.

    Most notable symptom is that the primary transformer current is only 5
    amps AC when the magnetron is to be energized. I would expect over
    twice that much. It is modulated with a relay, whose contacts appear
    fine and will click on and off properly at 50% power setting.

    I discharged the High Voltage and did all the usual resistance
    measurements on the magnetron, transformer and cap. I did remove the
    diode and ran 10 volts in series with it and a 1.2K resistor. Got 3mA
    forward and 0mA reverse current.

    I have a high voltage probe on order to test the high voltage under
    operating conditions.

    Does anyone have a suggestions? The failure seems partial somehow and
    I don't know how to proceede without replacing parts that may be
    operating fine.

    Thanks in Advance!

    Jim Shedden
  2. I assume you checked the filament resistance of the magnetron, and the
    integrity of the connections. An open filament or bad connections would
    be the most likely cause, followed by a bad HV cap, though I don't know
    that they fail open very often.

    6 A is essentially the unloaded current into the HV transformer.

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  3. Andre

    Andre Guest

    That sounds a bit low- I would swap the diode and see what happens.
    (compare readings with the replacement first)
  4. Jim shedden

    Jim shedden Guest


    Thanks to Sam and Andre for suggestions. I was inspired to repeat my
    measurements, so I figured I would do a general thread update.

    FA to F terminal is 0.2 ohms (meter lead resistance) so assume
    filament OK. Neither terminal shorted to case.

    Rated 0.91uF measures 0.9uF. Infinite resistance either terminal to
    case. Between terminals measure exponential rise stopping at about 10
    Meg. Cap has internal bleeder. Effect is symmetrical and brief
    negative resistance implies charge storage when leads reversed. Dick
    Smith ESR Meter reads 2.1

    Used 15 volts in series with 1.2K and measured voltage this time.
    Forward drop across diode is 6.7, with diode reversed is 15.

    Filament winding resistance basically zero. HV winding is 58.4 ohms.
    Primary is about 0.2 ohms. No shorts between windings or chassis for
    filament and primary. HV is tied to chassis one end. With secondaries
    open and oven on primary is 120 volts at 4.3 amps. Secondary current
    with magnetron filament only connected is 11 amps.

    That's about the best I can do until my high voltage probe arrives
    tomorrow. I suspect a bad magnetron, or I'm missing something. Are
    there any doors that close off the microwave energy when the oven is
    used on convection heat? There is a damper that is currently open, but
    I thought that was to keep oven heat away from the electronics. I
    imagine in combined heat mode, the damper would have to open when the
    magnetron was energized to blow air across the magnetron fins and into
    the oven. In any case, it seems to be more of an air control than RF
    energy control, but I'm only learning...

  5. Jim shedden

    Jim shedden Guest

    That sounds a bit low- I would swap the diode and see what happens.
    Hi Andre,

    I picked up another diode and measured it. The forward voltage drop
    was higher than the one in the oven as you indicated that it might be.
    Unfortunately it opened up immediately when I placed it in the circuit
    and fired up the oven. I have another diode and a magnetron on order.
    Hope the high voltage winding is still OK.

    The diode is cracked in half. What a STENCH.

    Frustrating, but I guess part of the game....

  6. Bert

    Bert Guest

    Most likely cause is the 0.9 uf capacitor.
    Tranformer will be ok.
  7. Jim shedden

    Jim shedden Guest


    Looks like we back in service with a new magnetron! My guess is that
    something shorted in the old magnetron when the unit heated and dumped
    filament current through the diode, burning it open.

    Thanks to all for the help. (Bert, as a result of your suggestion I
    did power up the high voltage circuit with a new diode before
    replacing the magnetron. The diode held, so I figured the capacitor
    OK. Good to check because the new magnetron not exact and needed some
    work to fit in.)

  8. Dana Myers

    Dana Myers Guest

    Interesting. I recently replaced the magnetron in a
    GE JVM1642BB oven, only about 3.5 years old. In my case,
    the HVR-1X diode measured 6V forward drop and maximum
    reverse drop, the .86uF capacitor acted just like it
    should (ohmeter reversal test) and the transformer
    windings had appropriate DC resistance values.

    Removing the magnetron, I found that the antenna
    was partially burned/melted, though the filament
    was .2 ohms or so.

    Since the GE oven has a total of 10 year warranty
    on the tube, I called GE and finally found the right
    person to ask for a replacement tube. They sent one
    out, along with a new stirrer (slightly changed
    design) and a new HV diode. Kudos to GE for excellent
    customer service.

    I'd done a power measurement of the oven after
    buying it, since it seemed to cook things more quickly
    than expected. Sure enough, the original tube
    was putting out around 1000W. The FCC test report
    for this oven (really a branded Samsung oven with an
    OM75P(10) tube) indicated around 770W output. I thought,
    cool, I'm so lucky.

    The new tube is good for around 765-770W. I suspect
    now that the original tube was not quite right and
    ended up killing itself. The power tests were done
    using the calorimeter method, consistent with the IEC
    standard. It just takes water and a thermometer.

    My neighbor bought the same GE Profile oven the same
    month from the same store that I did. His oven died two
    weeks before mine did. My hunch is that Samsung made some
    odd OM75P(10) tubes. Is this your tube? Is it under
    warranty? Keep in the mind the GE ovens often have a
    total of 10 years warranty on the tube.

  9. Dana Myers

    Dana Myers Guest

    Let's employ Ohm's law. 3mA * 1.2kOhms = 3.6V drop. Diode
    drop is thus 10V - 3.6V = 6.4V. Sounds just about right to
    me for a typical HVR-1X type diode, which is apparently a
    string of ten Si diodes inside.

  10. Jim shedden

    Jim shedden Guest

    Hi Dana,

    My tube was an OM75S(10), I believe. No outward sign of failure on the
    tube. Oven is in service almost exactly 5 years, but I didn't try to
    get GE to give me a new tube. I fixed it myself, and I figured that
    they wouldn't enterain the thought of giving me one because it was not
    fixed by an authorized service person. If you have contact
    information, I will try. I'm the type of person that likes have spares
    around, especially if I'm entitled to them. I bought a replacement
    from MCM supply for between 40-50 dollars. The oven is almost exactly
    5 years from original installation in Dec. 1998. I think the line name
    is "GE Monogram", but it is all Samsung inside.

  11. Dana Myers

    Dana Myers Guest

    First, check to see if you have a warranty on the tube, read
    your manual carefully. For example, my warranty was 1 year on
    the entire oven, parts and labor, and then 9 more years on the
    magnetron, parts only. So the magnetron actually has a ten year
    warranty. They really have to honor that, they can't require you
    to pay their service person to replace it. They *can* require
    proof the tube is bad, which could be as much as selling you a
    tube and refunding the price when they receive the bad one in return.
    This was not my experience with them, though.
    I called the service parts number and explained what I wanted.
    It took a few tries but they eventually put me through to
    the *right* service parts number. They may ask you to return
    the broken tube, they did not in my case, but they did ask
    how I'd diagnosed it, after I explained it, they happily sent
    a replacement. I can't find the specific number I called
    at the moment.
    Well, double-check the warranty, and if the tube is covered,
    spend a little while on the phone.

  12. Jim shedden

    Jim shedden Guest

    First, check to see if you have a warranty on the tube, read
    Hi Dana,

    Thanks for pointing out the warranty and inspiring me to call GE. I
    have to say that their customer service is great. I received a new
    magnetron today.

  13. Dana Myers

    Dana Myers Guest

    Excellent! I too am quite pleased with GE's customer service.
    Despite the hassle of finding the right person to speak to, they
    did very quickly do the right thing, oddly enough it's made me
    willing to recommend them to friends even though I had an oven
    die prematurely :)

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