Testing a Microwave board

Discussion in 'The Projects Forum' started by Gdrumm, May 29, 2013.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I'm trying to repair another microwave oven, and so far, all of the components are testing good. I'm down to the board level, and I don't see any visual signs of damage, etc. What would be an easy test to determine if the problem is with the board?

    It powers up okay, but no hum, and no heat.

    I've checked out YouTube, and didn't learn anything new there.

    I unpluged the two wires going into the transformer, and hooked up my DMM leads, and I'm getting nothing.

    Any thoughts, things to check further would appreciated.

    Door switches all checked good, Thermal switches checked good.

    Thanks,
    Gary
     
  2. alim

    Senior Member

    Dec 27, 2005
    113
    1
    From what you have written I would suggest you again check the door switches carefully, particularly the uppermost one, this is in series with the HV transformer. Try checking it in its place and with the door closed. If you check it manually it may show good.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,974
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    I trust you are aware that that the microwave transformer has an output voltage high enough to readily kill a human. :eek:

    I don't understand your measurement when you disconnected the transformer. :confused: Are you saying there's no voltage going to the transformer? In that case you need to trace the power back until you find where it's not getting through.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I don't know exactly what you mean but that sounds bad.

    FWIW, my microwave failed due to the magnetron. Replacing that was nearly as much as a new oven, soooo it's in the basement graveyard.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,766
    928
    The magnetron is the usual suspect in old units. If nothing smells 'burnt' electrically and all components look OK then chances are good that is the problem. Safety interlock should be checked as already mentioned.

    The transformer can be stripped of its secondary and rewired with some large 4 or 6 gauge wire to serve in a homemade spot welder. :)
     
  6. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Thanks for the replys.
    I'll check the door switches again.

    Thanks for the warning, I always discharge the HV capacitor (though I've only had one that actually made a sound when I did it, and that was several years ago) or, I let it set unplugged for a few days before attempting to work on it (them).

    Thanks again,
    Gary
     
  7. sheldons

    Active Member

    Oct 26, 2011
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    if you disconnect the mains supply to your hv tx in the microwave-set your meter to measure 240volts ac-you close the door of your microwave and enter a cook time and press start you should have 240 v ac present.
    If the display is counting down when you press start and stops counting down when you open the microwave door your control panel is at least half way working and part of your door interlock is working.
    does the internal lamp light and fan spin and turntable motor work while the cook time is counting down after you press start-if not you need to pull the board and look for blown tracks on the underside and replace your oven lamp.
    If the lamp lights,the turntable motor and fan works but no supply to the hv tx you need to look at the door interlock micro switches-they should be closed with the door shut and they work with the control board in such a way that your oven cannot be used in cook mode with the door open.
    Failure of the primary or secondary interlock will cause no supply to your hv tx-there is a third interlock called the Monitor Interlock which monitors the status of the other interlocks such that closing either primary or secondary interlock will instantly blow fuses with the microwave oven door open so check these switches with your microwave disconnected from the mains supply.
    Chances are when you find the micro switch responsible a replacement will get things going again as long as theres no other fault with the hv section of the microwave....
     
  8. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I pulled the top door switch, and opened it up, and the contacts inside were pretty bad.
    I replaced it, and the Microwave works great now.
    Many thanks.

    I fried my continuity tester in the process; like a dumy, I was checking the main fuse for continuity, when it was under power. See my seperate post/ Now, I can't locate a 1.5v fuse type bulb.

    Thanks again,
    Gary
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,086
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    Well done. Sure feels good to win one. :D
     
  10. sheldons

    Active Member

    Oct 26, 2011
    616
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    Heres a schematic of the mw tester i use for checking fuse ,thermal cut out,micro switch continuity.Also it is used for testing components on the hv side of the microwave-you can test the hv diode ,capacitor,A.S,M rectifier(if theres one fitted) and checks the filament on the magnetron isnt sc to the casing of the mag and that the filament has continuity....
     
  11. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    684
    36
    Here in the US, homeowner type microwaves are powered by 120v AC.
    I'll study the schematic to see if I can duplicate it.

    Thanks again everyone.
    Gary
     
  12. alim

    Senior Member

    Dec 27, 2005
    113
    1
    My experience is thus far, the top switch always cause the symptom you stated, glad you got it going.
     
  13. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    Much better looking avatar, sheldons. I was worrying that you might have a skin disease.:D
     
  14. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    i still look the same but thought id change it 4 a while due to whats happened over here....
     
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