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Sharp Microwave powers down after 40 seconds, need assistance

Discussion in 'Electronic Repair' started by electroninja, Jan 7, 2005.

  1. electroninja

    electroninja Guest

    Sharp Model R1810 Carousel II microwave starts and cooks(heats) but
    shutsdown after about 40 seconds. Electronic control board display
    stays lit, countdown stops and displays remaining time--magnetron
    cooling fan works during the first 40 seconds but then shutsoff--just
    before shutdown, fan speed increases(doubling or tripling in speed)
    These items seemed to check out good with VOM--
    power fuse
    interlock switches (2)
    monitor switch
    capacitor
    diode
    thermal cutout switch(only found one)
    fan motor
    power tranformer

    Additional trouble shooting note--a power surge protector I had plugged
    the microwave into was discovered fried--suspect a near by lightning
    strike?

    Read an informative article by Sam Goldwasser-- "NOTES ON MICROWAVE
    OVEN FAILURE DIAGNOSIS AND REPAIR v1.01" but need more help. Thanks
     
  2. Does it do the same thing if set to defrost so that the magnetron only
    runs for a few seconds/30 seconds?

    Can it be restarted immediately?

    How is the behavior different than if the STOP button was presssed
    (maybe aside from the fan speed)?

    Isn't the fan simply on the AC line so how would its speed change?
    Or, is it a multispeed fan?

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  3. electroninja

    electroninja Guest

    Ref your questions Sam:
    Q. Does it do the same thing if set to defrost..
    A. Yes, will run longer in the cycle but eventually shutsdown
    Q. Can it be restarted immediately?
    A. Yes, I can hit the start button and the cycle replicates as many
    times as I restart-ie runs 40 seconds and shutsdown
    Q. How is the behavior different than if the STOP button was pressed
    A. No difference
    Q. Isn't the fan simply on the AC line so how would its speed change?
    A. Looks like an AC fan--speeds up only on first cycle--otherwise runs
    at top speed. Blade turns freely--no obstructions--low ohms on the
    coil--can sub another fan from a different brand if appropiate--your
    thoughts

    Sam--appreciate your help--thanks
     
  4. NSM

    NSM Guest

    | Ref your questions Sam:
    | Q. Does it do the same thing if set to defrost..
    | A. Yes, will run longer in the cycle but eventually shutsdown
    | Q. Can it be restarted immediately?
    | A. Yes, I can hit the start button and the cycle replicates as many
    | times as I restart-ie runs 40 seconds and shutsdown
    | Q. How is the behavior different than if the STOP button was pressed
    | A. No difference
    | Q. Isn't the fan simply on the AC line so how would its speed change?
    | A. Looks like an AC fan--speeds up only on first cycle--otherwise runs
    | at top speed. Blade turns freely--no obstructions--low ohms on the
    | coil--can sub another fan from a different brand if appropiate--your
    | thoughts

    Sounds a bit like an overheat situation. If the fan really is 120V and not
    speed controlled then I'd be looking for burnt wiring in the AC input
    system.

    However, I have a Panasonic doing much the same things. Pulled it apart and
    found the main MOV across the AC line was shooting flames out of the top!
    Then I noticed that the main power transistor (this is a inverter machine)
    has a big crack through the body. Only uWave I ever bought that failed on
    me.

    N
     
  5. electroninja

    electroninja Guest

     
  6. electroninja

    electroninja Guest

    -- No visual evidence of burnt wiring in the system
    -- Will monitor fan motor voltage vs speed thru shutdown cycle
    tomorrow--leaning toward possible fan replacement
    Thankyou for your input
     
  7. Not sure what you're saying here about the fan. If it runs at top (normal)
    speed, it's not the problem.

    The fact that you can restart immediately and get the same exact behavior
    suggests that it may not be a cooling problem but also check across the
    thermal protector if possible, or is there a thermistor to monitor
    magnetron temp? Could be a problem in that area. Is there a schematic
    with it?

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  8. electroninja

    electroninja Guest

    Ref your questions Sam:
    Q. Not sure what your saying here about the fan?
    A. Fan looks like an AC vs DC fan--one open air coil around
    core--armature alum--motor has appearance of bathroom vent fan. Fan
    runs on low speed only on first cycle but speeds up prior to shutdown.
    Todays test shows 115 volts across motor's coil at both low and high
    speed. Shaft rotates freely, but will try a drop or two of oil.

    Q. Is there a thermistor to monitor magnetron temp?
    Is there a schematic?
    A. Limited schematic (unit door closed/cook cycle off)
    One thermal switch on magnetron in series with power fuse--activation
    shutsdown all power (not our case.)

    - One oven thermistor shown as input to control unit circuit. No
    details given (including wire color coding). Had thought this might be
    convection oven temperature control.

    - Control unit connected to a "relay unit" --Triac, small transformer,
    relays etc. Schematic shows one relay that if activated would shutdown
    magnetron and fan while leaving power to rest of unit (our case
    possibly.)

    --Action plan:
    - Oil fan motor monitor affect
    - Locate thermistor and test--assume a two wire device whose resistance
    rises with temp. Uncertain what I should expect for ohm delta--comments
    appreciated
     
  9. There have been cases where for whatever reason, RFI from the magnetron,
    triac, who knows, gets into the controller and does strange things.
    Unfortunately, hard to track down those sorts of problems.

    But you said it happens on Defrost but takes longer. Defrost is 10 or 20
    percent duty cycle (I'm assuming this is your basic HV transformer/cap/diode
    design) so any heating would be minimal.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  10. electroninja

    electroninja Guest

    Oiled motor--unit runs now for 2 additional seconds before shutdown on
    first cycle--11 additional seconds on subsequent restarts

    Unit is HV, cap, diode system.

    Will attemp to track down thermistor--any input on how to test with VOM
    when found?
     
  11. electroninja

    electroninja Guest

    Additional info--after fan motor was oiled it runs at high speed
    continually now.
     
  12. So, at least that mystery is solved. From your other post, so something
    may be overheating but I don't know that there would be anythinb beside
    the thermal cutout, and that isn't activating.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  13. NSM

    NSM Guest

    | Additional info--after fan motor was oiled it runs at high speed
    | continually now.

    Makes sense. A small rise in voltage when the magnetron cuts out was
    affecting it. Now you have to look for what is cutting the thing out. Maybe
    the uProcessor got zapped?

    N
     
  14. electroninja

    electroninja Guest

    I tried shielding the control board with aluminum foil to isolate any
    stray RF--no change in unit response.

    A spare microwave I recently purchased for parts to fix the Sharp has
    two thermal cutout switches--the R1810 doesn't according to the
    schematic we discussed. Still looking for the thermistor. Contacted the
    company repair reps--no response yet.

    This unit fits as a built-in wall unit in our Foretravel RV--if not for
    that, it would be in the trash can.
     
  15. electroninja

    electroninja Guest

    Sam what's the downside of this idea--

    Substitute a dummy load for the magnetron (1500w low ohm external
    load-maybe a heating coil) and run unit to see if controller shutsdown
    the unit. Test could rule out overheating issue.
     
  16. NSM

    NSM Guest

    | Sam what's the downside of this idea--
    |
    | Substitute a dummy load for the magnetron (1500w low ohm external
    | load-maybe a heating coil) and run unit to see if controller shutsdown
    | the unit. Test could rule out overheating issue.

    You could use a small electric heater/fan heater as a load for a test.

    N
     
  17. Should work. Of course, what you really mean is substitute a dummy load
    for the high voltage transformer so that there is no power to any of the
    high voltage components.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
  18. electroninja

    electroninja Guest

    Like your idea better i.e. substitute for transformer loading--nice
    gentle caution

    Will try a 1500W toaster--power circuit fused for 15A--a little
    inductive load should balance well.
     
  19. electroninja

    electroninja Guest

    Test results with 1500W toaster--

    Load worked well--toaster heated normally

    Microwave unit functioned as in the past--it shutdown the transformer
    load (toaster in this case vice magnetron and cooling fan) after a
    period of time with the exception that the run cycle continued for an
    additional 10 sec over actual magnetron and fan loading tests.
    (Additional ten seconds may have resulted from no magnetron heating at
    all.)

    Suspect: 1. Faulty temperature sensor--perhaps opened thermistor or 2.
    bad relay/control unit

    Any thoughts Sam?
     
  20. Guest

    I often use my wife's electric iron as a high-power capabilty load.
    Just be sure to use the highest possible heat to keep the iron on
    continuously. It works better than a toaster becuse it will stay "on"
    much longer than a toaster stays down.

    H. R.(Bob) Hofmann
     
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