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Microwave not powering on???

Discussion in 'Troubleshooting and Repair' started by Irishpat, Aug 8, 2016.

  1. Irishpat

    Irishpat

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    Aug 8, 2016
    i have checked:
    all door/switch safety latches and continuity checks out;
    main fuse has continuity;
    1 of 2 thermal cutoff is open (but disconnecting it microwave still does not power on display, light, anything, if this is the problem should disconnecting it turn machine on?)
    Capacitor and transformer show proper resistance readings.
    Replaced main power board(there is another board in this sharp kb6524ps microwave, but not sure if that would give this symptom?)

    History machine stopped heating awhile ago, but all electronics operated. Replaced magnetron and everything was fine for a few weeks. Then while my wife was cooking rice for about 4-5 minutes there was a burning smell from the machine and then all power went off.

    This is a factory refurb unit but sharp says they have no technical people who can help me. I am rural and the few appliance repair people in the area either don't service sharp or are afraid to tackle this model as they are unfamiliar with it - so I have been doing the "warrantee" repairs out of my own pocket.

    Anybody out there have any help?
     
  2. shrtrnd

    shrtrnd

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    Jan 15, 2010
    That burning smell is not good.
    Sounds like you've been all-over the unit's insides. If you don't see what burned-up, your transformer is
    probably toasted. Transformers rarely burn open-circuit. When the windings overheat, the laquer on the
    wires burns off, and one or more of the windings touch, shorting the turns. You'll still read resistance on
    a meter, but the transformer has probably bitten the dust.
    You can keep replacing parts, but if this was me, I'd toss the unit and buy a new one, ...with a warranty.
     
    hevans1944 likes this.
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Gee, what a nice feature, a drawer that slides out! And costs only $900 more than a 1200 watt microwave with a door that opens sideways.:rolleyes: Country living at its finest!

    [​IMG]
     
    davenn likes this.
  4. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Check the diode
     
    hevans1944 likes this.
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Post a wiring diagram.
     
  6. Irishpat

    Irishpat

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    Aug 8, 2016
    Wiring diagram was JPEG, but looks like forum converted it to png file. Hope you can see it.

    Looked at the diode, but read conflicting reports about testing diode is xxx to you can't test diode because there are a bunch in series. Is there a reliable way to test the diode?
    Thanks
     

    Attached Files:

  7. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    High voltage diodes can't be read by most meters in diode mode, but you should be able to read a voltage drop across each diode. (Yes, all connected in series.)
    Disconnect anode side of the three diodes and apply an external +9 or +12 volt battery to it (obviously disconnected from household power) then land - (neg) side of battery to the neg lead of a volt meter. Then, probing with red lead, You should see about 2volts dropped across each diode or about 6 volts dropped across all three.
    Then swap the leads and they should block any current flow in the other direction.
     
  8. debe

    debe

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    Oct 15, 2011
    If its the HV diode just replace it they are cheep.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    There is, and @Tha fios agaibh described it. You have to apply enough forward voltage to overcome the 0.6 to 0.7 V forward voltage drop of each of the series-connected high-voltage diodes in the diode stack. And then you have to apply the voltage in reverse to make sure the diode stack is actually blocking current in the reverse-biased direction. But, as @debe mentioned, diodes are inexpensive so it wouldn't hurt to save some time and replace the diodes anyway. Being more isolated in a rural area, it probably would benefit having a spare diode stack on hand anyway.

    However, from your original description of the problem, it does sound like the original HV transformer and/or the replaced magnetron is defective. Below is a "simplified" schematic diagram copied from the Sharp manual, which is available online for download. Note that the magnetron acts as a diode in a half-wave voltage-doubler power supply. If the magnetron becomes internally shorted it will conduct heavily through the high-voltage capacitor and should cause the 20A line fuse to open. The so-called "monitor switch" will also blow this fuse if there is a mechanical fault that causes the monitor switch to remain closed when power is applied to the primary of the HV transformer via contacts on RY 2 to operate the magnetron. Unfortunately, because of nanny state regulations "for your safety" when a monitor fault occurs, the current surge created by this deliberate short circuit across the power line not only blows the fuse but also can damage any switch components in the path, including the monitor switch, the secondary interlock switch, contacts on relay RY 2 (primary interlock relay), contacts on RY 1, and the two thermal cutouts. Sharp recommends that the monitor switch and circuit-board mounted fuse be replaced if a monitor switch fault condition causes the fuse to blow.

    upload_2016-8-9_10-59-0.png

    The above image was copied from page 10 of the manual. This appears to be a standard microwave oven circuit with added bells and whistles for consumer operating convenience. The manual provided by Sharp is quite complete and well-written with testing and troubleshooting guides. The OP should download and read the manual.
     
  10. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Nice post Hop. That schematic now has me wondering how one would check the humidity sensor. Ohm value?
     
  11. Irishpat

    Irishpat

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    Aug 8, 2016
    Diode checks out with 3V drop. Opposite hookup blocks almost everything, a few millivolt said is all.

    @hevans1944 wouldn't the fuse and relays check bad with continuity testing if that was the case?

    But since clock and lamp drawer motor, etc. won't come on??? Could the 20A fuse be fine but something else on that board it is on blown that would stop the power? Just not that much upstream of all the different things that do not power on.
     
  12. Irishpat

    Irishpat

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    Aug 8, 2016
    Here is the humidity sensor info, but I could not make heads or tails of what they are talking about. image.png
     
  13. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Humidity circuit is fascinating. (Thank you)Basically it changes resistance as humidity changes.
    It did say it will display "Error" if the bridge circuit has a problem so that's probably not your problem.
    I'd start looking at power supplies at perspective test points. Like +5 and +15vdc.
    May want to look at the traces on the boards too, considering it blew out a thermal cut out.
     
  14. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Well, you said the fuse checked good, but does that mean you replaced a previously blown fuse?

    It is unlikely that the relay coils would be affected by a power-line short-circuit, but any contacts that were in the current path and closed at the time could be damaged, as well as circuit board traces, as @Tha fios agaibh mentioned, until the 20A fuse blows. You need to find the electrical fault, the source of the burning smell, before willy-nilly replacing every component in sight. It's just a guess, but I would guess the replacement magnetron shorted, overloading the HV transformer and the transformer insulation cooking is what you smelled. Just a guess. You should pull the magnetron and check for shorts between the filament (high-voltage connection) and the frame.

    To troubleshoot this, I would physically disconnect the primary of the HV transformer, and wrap tape around the ends of the two wires so they don't "accidentally" get connected to a power source. Then connect the microwave to the mains source, hopefully with a plug and receptacle connection, and start tracing the simplified circuit diagram with a multimeter on a 200 VAC (or higher range) to find out where the AC disappears. If you get AC all the way to the relay contacts where the HV transformer primary was connected, you should be able to operate the front panel controls and get some response, although the microwave magnetron won't be operating of course. But you should be able to determine whether the relays are actuating or not, the clock should work, the oven light should come on with the drawer open... things like that. It's possible that the switch-mode power supply that operates all the electronics is either not getting AC power or is defective, but you said you changed that power supply.

    A few years ago I had problems with my 1200 watt GE Profile Microwave with Inverter Technology. Sometimes, you had to jiggle the door up and down on its hinges to get it to start. Eventually this failed to restore normal operation, so I had to open it up for investigation. There was a nice service schematic in a little paper envelope inside, and of course the fuse was blown. It took me many days to finally come up with a permanent "fix" to the problem.

    All of my problems were related to the door interlock switches. The microwave itself performed perfectly. There are two door interlock micro-switches that MUST be closed and third so-called "monitor" switch that MUST be open before any attempt is made to start the microwave oven. I can see where the nanny state legislators wanted to prevent any possibility of the magnetron coming on if the door was not completely closed, much less left open, but it is ridiculous IMHO to do so by deliberately shorting the power line and blowing a fuse which then requires someone (usually NOT the end user) to replace both the fuse and the monitor switch, or in my case, GE says to replace all three switches. The switches all tested good. No contacts welded together. Switches open and close normally. So scratch GE's directions to replace everything.

    My problem turned out to be the cheap, die-stamped, thin metal frame the microswitches were mounted on. Closing the door causes two plastic, spring-loaded, claw-latches mounted on the door to operate an internal plastic mechanism that grabs the claw hooks, holds the door closed, and operates all three switches. But the door latches also exert a side-ward force on the sheet metal switch mount, and that was sufficient to cause one or more of the switches to not actuate properly from a purely mechanical point of view. I solved this by installing two stiff sheet-metal braces (I used the metal covers from unused expansion-board slots on PCs, cut to appropriate length and secured with sheet-metal screws) and these braces keep the interlock switches from moving when the door is closed. To ensure this problem would never happen again, I disconnected the "monitor" switch, but I am NOT suggesting that you do this!
     
    Irishpat and Tha fios agaibh like this.
  15. Irishpat

    Irishpat

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    Aug 8, 2016
    Thanks, will try your suggestion. If I understand this, by taking the hv transformer out of the circuit the capacitor and magnetron won't get juice. So after testing magnetron for a short I will be able to trace actual voltages to the component that is breaking the path.
    Interesting problem solving for your profile too!
     
  16. hevans1944

    hevans1944 Hop - AC8NS

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    It should go without saying again, but I will say it again: Be careful working around the high voltage circuits. Make sure you follow the instructions in the manual to discharge the high-voltage capacitor before working with the magnetron. The filament (high voltage) connection between the magnetron and the HV transformer MUST be disconnected to measure whether or not the magnetron is shorted. Never work on the high voltage section with mains power applied to the microwave oven.

    Microwave ovens have been on the market for a LOOOONG time and all the early bugs have been eliminated. Magnetrons are made and sold like popcorn, by the millions, today. They are very reliable unless abused by overheating, usually a cooling-fan problem. The filament is the main source of failure: either it opens or it shorts internally to the microwave cavity. The tube itself is a marvel of simplicity: just a hot filament to emit electrons into a vacuum, surrounded by multiple tuned cavities and an external strong permanent magnet. The magnet creates a magnetic field perpendicular to the electrical field that causes the emitted electrons to spiral around past slots in the cavities, inducing microwave oscillations in the cavities. It's pretty efficient too. The emitted electrons start at a very high negative potential and, were it not for the magnet, would be simply accelerated by the electrical field between cathode (filament) and anode (cavities) and deposit the energy gained from the electrical field as heat. But the strong magnetic field deflects the electrons in their journey from cathode to anode, forcing them to travel in spiral paths past the cavity slots, losing much of their kinetic energy to microwave energy induced by electron motion past the slots, before the electrons finally reach ground potential. I think it is amazing that I can hold a 1200 watt microwave emitter in the palm of my hand!
     
  17. Irishpat

    Irishpat

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    Aug 8, 2016
    So, this has become more unclear at least to me.
    Magnetron test each lead to case gives unlimited resistance and the two leads together give 0.25 ohm resistance. This seems ok.
    So current path into and out of the fuse path is fine, but drops to 25V into the first thermal cut off and down to 1V to the 2nd. ???
    Went ahead and tested power elsewhere and 0 to light whether door is open or not.
    Switches range from 1V to 3V with 3V getting to transformer leads.
    Not sure how it drops from 121V reading coming out of the fuse to 25V to thermal cut off - looks like direct path with connections and wiring looking good. Does this mean the wiring is fried even though it looks fine? Or am I missing some other more likely explanation?
     
  18. Irishpat

    Irishpat

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    Aug 8, 2016
    In case it makes a difference the hot wire running to the first thermal cutoff shows continuity and so does the wires between cut offs. Can find no cross continuity between wires or the case.
     
  19. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    You sure of that? Is your meter capable of reading high ohm readings? A meter that can read into the meg range will do, but a megger meter would be the best way to check insulation.
    The 25v drop across the thermal sounds like a problem. It may read continuity fine but not be capable of passing current to rest of circuit. You could try jumpering it out by putting a fuse holder across it with a 20 amp fuse in it. This is a safer technique than just a wire across it.
     
    Irishpat likes this.
  20. Irishpat

    Irishpat

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    Aug 8, 2016
    Up to 40Mohms.
    Those buggers can show correct continuity and not pass current, wouldn't have thought that. I'll just replace both of them then and see where it stands. They are cheap, it's more of a pain waiting to get them w/o working microwave, oh well.
    Thanks.
     
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