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Sharp Microwave powers down after 40 seconds, need assistance

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

  1. James Sweet

    James Sweet Guest

    First thing I'd check is the fan motor itself, some hair or gunked up grease
    in the bearings could be making it run slow which overheats the magnetron.
     
  2. Clear the mag first so::Go to the Mag filament wires{ be sure the HV secondary
    wire is removed} then test for between 2.9vac and 3.9vac depending on model
    when powered up.Replace Filament wires and re-coneect clipping test leads to mag
    terminals and check again at switch on for the same a.c. voltage.A lower voltage
    can be the result of the mag lowering the filament transformer voltage.Move on
    from there if correct but get the MAG out of the loop first.



    "Texas Hold'em Poker"

    "The bigger you bet,the less you win when you lose!"
     
  3. Can't overemphasize this. There may be up to 5,000 V at very lethal current
    availability present on the magnetron filament wires while operating with
    the high voltage capacitor holding a dangerous charge after shutdown.

    ANY testing in the microwave oven high voltage circuits is extremely dangerous.
    Very knowledgeable and experienced professionals have been killed doing this.
    It's too easy to be careless and you don't get a second shot.

    If you insist, while unplugged, make sure the HV cap is discharged, then
    hook up meter, stand clear and turn it on. Take reading without touching
    meter including its test leads, power down and remove plug from wall socket,
    discharge HV cap. Etc.

    --- 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.
     
  4. electroninja

    electroninja Guest

    Located thermistor--results follow:
    - Thermistor resistance reading was 275K ohms
    - Substituted variable resistor in circuit for thermistor and ran
    system operational testing for multiple values from 16K-2.2Meg ohms

    --Result: Microwave ran for approximately the same length of time
    before shutdown (40-60 sec) except for extreme resistance tests ie
    extreme highs and lows then shutdown was 10-38 seconds.

    Conclusions: Suspect control board problem--haven't located one from
    suppliers yet--have have only found a control board varistor
    replacement part for this unit. Assume a varistor is a variable
    resistor--part photo from supplier looks like a small capacitor.

    Comments appreciated regarding possible function of varistor in
    circuit--are they possibly used in timing control circuits?
     
  5. NSM

    NSM Guest

    | Located thermistor--results follow:
    | - Thermistor resistance reading was 275K ohms
    | - Substituted variable resistor in circuit for thermistor and ran
    | system operational testing for multiple values from 16K-2.2Meg ohms
    |
    | --Result: Microwave ran for approximately the same length of time
    | before shutdown (40-60 sec) except for extreme resistance tests ie
    | extreme highs and lows then shutdown was 10-38 seconds.
    |
    | Conclusions: Suspect control board problem--haven't located one from
    | suppliers yet--have have only found a control board varistor
    | replacement part for this unit. Assume a varistor is a variable
    | resistor--part photo from supplier looks like a small capacitor.
    |
    | Comments appreciated regarding possible function of varistor in
    | circuit--are they possibly used in timing control circuits?

    No. They may be voltage variable (varistor) or temperature variable
    (thermistor, PTC or NTC). Very unlikely that they are used for timing. Is
    this a sensor model?

    N
     
  6. electroninja

    electroninja Guest

    This is a Sharp R1810 convection oven and microwave--it has a sensor
    probe if that is what you mean by "sensor model." Trying to figure out
    what on the control unit might be causing the magnetron to shutdown
    prior to end of cooking cycle.
     
  7. NSM

    NSM Guest

    | This is a Sharp R1810 convection oven and microwave--it has a sensor
    | probe if that is what you mean by "sensor model." Trying to figure out
    | what on the control unit might be causing the magnetron to shutdown
    | prior to end of cooking cycle.
    |

    Common fault?

    http://www.euras.com/repair_tips_1/Sharp/R1810/R1810_Sharp.htm

    Try the 30 day Free Trial?

    Is the probe a temperature probe that you poke in the meat? If so, maybe it
    is 'sensing' end of cycle?

    N
     
  8. electroninja

    electroninja Guest

    The probe is disconnected--are you saying perhaps the probe socket has
    shorted. Thanks for the www.... site--will check it out.
     
  9. NSM

    NSM Guest

    | The probe is disconnected--are you saying perhaps the probe socket has
    | shorted. Thanks for the www.... site--will check it out.

    I'd check that circuit area for sure. That's the sort of thing that would
    cause those symptoms.

    N
     
  10. electroninja

    electroninja Guest

    Tested the probe socket by inserting two small pins into the wires
    running to the socket. (Getting to the socket would require disassembly
    of a great deal of panels) Ohm meter shows and open circuit condition
    meaning the socket checks out good. Any other suggestions I might try?
    Thanks
     
  11. NSM

    NSM Guest

    | Tested the probe socket by inserting two small pins into the wires
    | running to the socket. (Getting to the socket would require disassembly
    | of a great deal of panels) Ohm meter shows and open circuit condition
    | meaning the socket checks out good. Any other suggestions I might try?

    Connect an ohmmeter to the probe and then put it in hot water. What are the
    ohm readings before and after?

    Is the oven rated to work OK with the probe disconnected? Is there a manual
    or automatic switch to detect that?

    N
     
  12. electroninja

    electroninja Guest

    Have never used the probe since I've owned it and not sure where it is.
    The probe socket wiring routes directly to the control board. Any
    suggestions for trouble shooting the board--part suppliers don't seem
    to carry control boards for the Sharp R1800.
     
  13. If it's a convection oven, there is a thermistor for controlling the
    temperature but only of the convection heater. On the ones I've seen, it
    is screwed in above the oven chamber. This does not affect microwave
    behavior.

    --- 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.
     
  14. Did you run the test disconnecting the HV transformer? Don't even bother
    with a load at first, just see what happens.

    --- 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.
     
  15. electroninja

    electroninja Guest

    Lets review where I'm at--testing done with a digital VOM when required

    --HV loads check out good (capacitor, diode, magetron, HV transformer)
    --Fan runs at high speed during cook cycle
    --Switches good (monitor, thermal cutout, interlock)
    --Power fuse good
    --Thermistor good
    --Temperature probe socket good


    Symtoms review:
    --Microwave system cooks/heats for about 40 seconds then shutsdown HV
    load circuitry only (fan, magnetron, timer coundown)--everything else
    appears normal (lights on, control panel displays timer countdown
    seconds remaining in cycle)
    --Cook cycle can be restarted immediately by pushing start
    button--system will cook for another 40-50 seconds and then shutdown
    again
    --Microwave was tested with dummy HV load (magnetron, cap, and diode
    disconnected) with same results--ie system ran for 40 seconds then
    shutdown HV load circuitry indicating no overheating problem due to
    magnetron.

    Possible cause: Conrol or relay board.

    Proposed action: (1) Replace board(s)--problem-none available
    (2) Repair current board(s)

    Suggestions anyone?
     
  16. NSM

    NSM Guest

    | Have never used the probe since I've owned it and not sure where it is.
    | The probe socket wiring routes directly to the control board. Any
    | suggestions for trouble shooting the board--part suppliers don't seem
    | to carry control boards for the Sharp R1800.

    Like so many things now, they probably figure the cost of the board plus
    labor to diagnose and install it plus the cost of any warranty afterwards
    makes for a non-economic repair.

    N
     
  17. I assume by no HV, you mean HV transformer disconnected. If not, do that -
    just disconnect the primary entirely with no dummy load.

    But it sure sounds like the controller at this point. If a relay was
    dropping out, pushing START again wouldn't do anything becaseu the
    controller would think it's still running.

    --- 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

    -- An earlier test subtituted 1500W toaster at transformer
    primary--unit ran normally (toaster elements heated) but then shutdown
    at the same point in the cycle as it had been doing with normal
    transformer, magnetron, capacitor load.

    --Disconnected transformers primary for test you suggested. Units
    shutdown pattern was unchanged--ie shutdown about 40 seconds into the
    cook cycle.

    --The schemetic shows one relay located on the relay board that
    controls power to the primary side of the transformer. I can hear it
    tripping, interrupting the cook cycle--prior to it tripping though
    there are one or two noises in that general area that may be other
    relays activating.

    --Both control and relay circuit boards appear normal from the top--no
    discolored parts, no odors, loose wires etc.
     
  19. electroninja

    electroninja Guest

    Sam

    Took another look at the schematic--it shows a TRIAC in parallel with
    relay boards power transformer. If the TRIAC were to fail that could
    possibly lower the voltage to the relays on the relay board causing
    them to open.

    I'm unsure whether the control board is telling the relay board to
    shutdown the magnetron--or if the relay controlling the power to the
    magnetron is tripping off line (for example because of TRIAC failure)
    causing the control board to think a manual stop has been issued from
    the front panel.

    Any thoughts?
     
  20. James Sweet

    James Sweet Guest

    Every time I've seen a triac fail, it's shorted out, causing whatever device
    it controls to be stuck on.
     
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