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Emerson MW8779W doesn't heat but turns on!

Discussion in 'Troubleshooting and Repair' started by Benny7440, Feb 28, 2018.

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  1. Benny7440

    Benny7440

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    Jan 14, 2016
    This microwave oven didn't come with any schematic. What I've found is this:
    ---> 0.8 ohms between primary of power xformer;
    ---> infinity resistance between primary & Gnd;
    ---> a cable between primary electrode & Magnetron shows infinity to Gnd;
    ---> another electrode from this xformer to the capacitor shows infinity to Gnd;
    ---> between the capacitor's electrode it shows a downward count starting at ~30 Mohms & reaching ~10 Mohms in about 30 secs (both polarities);
    ---> capacitor electrodes to Gnd shows infinity resistance;
    ---> diode electrodes = infinity (both polarities);
    ---> magnetron electrodes = 0.3 ohms;
    ---> magnetron's electrodes to Gnd ~ infinity.

    What I beleive is that there's a problem at the control panel that involves the "Powering Relay" but haven't conduct any test around this hipothesis preferring to wait for the schematic.

    Any ideas instead?
     
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Check the h.t. fuse.
     
  3. 73's de Edd

    73's de Edd

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    1,246
    Aug 21, 2015
    On the control panel PCB is it using a 42 pin uProcessor and is there a Daewoo board marking anywhere on the PCB ?
     
  4. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    Thanks for replying, kellys_eye & 73's de Edd!

    First, this unit has no High Voltage Fuse (see photo below).

    Have not taken out the board yet. I was thinking of checking the switches & their actuations synchrony next (still to be done).

    Also, due to my BAD experience with Whirlpool online services I hadn't considered Emerson's until now. I'm going to try with them to see if I can obtain the schematic and/or the user's manual, sometimes it's included with it...
     

    Attached Files:

  5. Benny7440

    Benny7440

    88
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    Jan 14, 2016
    There's no DAEWOO logo on either side of the control panel & the processor is a 28 pins chip: TMP47C443N1C81 Japan 0020H.

    Can you, 73's de Edd, explain the importance of those characteristics you mentioned above?
     
  6. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    I was just curious as to who the REAL maker of the unit was. Daewoo made some units for them.
    But your unit looks like an o o o older unit . . . as, when you look up some parts, they are . . .No Longer Available.
    That TMP u-Processor prefix used to and may still be Texas Instruments proprietary / domain.
    You can make some analysis of the functioning of the control board by confirming if the display works, the clock sets and runs and that the countdown timer function works. Then typically the MCW switching functions should work.
     
  7. Alec_t

    Alec_t

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    784
    Jul 7, 2015
    Seems rather low. I've just measured 2.3Ω primary winding resistance on the MOT tranny in my junk box.
     
    Benny7440 likes this.
  8. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    This's a rather unusual wiring for this application compared to at least 8 other microwave ovens I've seen. See attachement below.

    No visible label for the xformer! Do you think it's a good idea to take it out & check if there's one at the other side or below and then trying to find what measurement reading I should have at the primary?

    Edit: Have taken out the xformer & it has no label nor printed info on any side???
     

    Attached Files:

    Last edited: Mar 4, 2018
  9. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    How feasible is the possibility of using another xformer instead? Since I haven't found the specs for it but have another MOT (not phisically analogous) it ocurred to me that there might be a reasonable dynamic way to test the original unit as well as the proposed substitute. For ex., connecting the secondary to 115 VAC & measuring the output at the primary side, in both xformers.

    Has anyone around tried this already? What was the outcome of the test? Maybe putting a 60 Watts bulb in series with the secondary, measuring the voltage drop at the bulb & then at the secondary; subtracting the bulb's voltage from source might give the last quantity to extract the transformation factor... Waiting for your ideas!
     
  10. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    I've found another Emerson oven older than mine & took out its xformer (OBJY2) but it measures exactly the same primary resistance as the other one. It also looks very similar to the original one.

    The magnetron of this older unit measures ok, also. The model of this micro is MT3060 Suffix F, rated at an output power of 600 Watts but consumes 1050 Watts? Strange! Too much subsidiary power consumption...

    Tried to gain access to its specs in the internet but failed. Any ideas?
     
  11. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    I've tested the HV diode by measuring the voltage across it when connected in series with a 240 ohms resistor at a source voltage of around 17 DCV & it gave me around 6.6 V. Took out the HV Capacitor (0.90 microF @ 1800 VAC with internal discharging resistor), my digital VOM start going down from around 35 Mohms until it approaches 10 Mohms, when it's still coming down but very slowly, both ways.

    Since I don't have a capacitance meter is it possible to do other verifications to test the capacitor? I'm still waiting for recommendations on additional tests for the transformers!
     
  12. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    Today I've tested the HV transformer by connecting ~1.8 VAC at the secondary & obtaining ~20 VAC at the primary. At another forum someone has advised me to try with another magnetron. Think I'm going to try that next...
     
  13. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    Yesterday, I put another Magnetron (not with the same physical characteristics but screwable in place but twisted 90°) that, prior to any connection, measured ok but the fuse was blown immediately. The model of the substitute was different from the original and the size of it is bigger.

    What might have happenned? Have not taken yet any post-morten measurements... The tip of the original (almost sure) magnetron was fused with some holes on it. Took out the metal tip thinking of putting a good substitute & test it but refrained myself because of the measurement to ground it gave me when disconnected.

    Thanks for any ideas on this!
     
  14. Benny7440

    Benny7440

    88
    1
    Jan 14, 2016
    I've found another xformer very similar to the original and tried with it but the fuse was blown again. After inspecting around found that the HV-diode wasn't screwd in place but touching the chassis firmly while the 2 previous tests: could this lead to the blown fuse?

    I, after so many months of this unit been disassembled, have grown suspicious of the overall connections. Certainly, a connection failure may lead to this and other worse things like damaging other good components. Any idea of how to proceed from here?
     
  15. 73's de Edd

    73's de Edd

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    1,246
    Aug 21, 2015
    Sr Benny7440 . . . . .

    Well at least I can now give you some more techno info and commend you on your perseverence, as do your countrymen in keeping their classic '57 Chebbys . . . .'55 Fordecitos and '52 Olds-mo-bubbles . . .still a going.

    Since you blew the fuse, that confirms that you are getting solid AC line voltage power to the marked ORANGE and BLUE square terminal markups at the primary of the power transformer.

    Why not now totally pull the substitute maggie and unscrew and lift the cathode of the HV rectifier diode high enough so that it can't possibly arc to case ground.
    Then re- fuse and power up only for ONLY 3 seconds or so to confirm that you are now not blowing the fuse again.

    I need to now know if you happen to have access to use of a Triplett 630 series volt ohm meter.
    ( My current inventory . . . . . . 6x )
    That is the only thing that I would entrust for measuring a microwaves HV supply.
    It has a 6000VDC scale with a separate isolated plug in receptacle for your + probes lead wire.
    ( Someone else chime in . . . if you have a Simpson equivalent . . . and confirm that it also has the same mechanical isolation and HV range capability. )
    I have never considered trying to use a voltage divider and one of my GOOD DVM's. . . . . figuring for the very worst for my meter.
    One of my other procedures was to use a TV high voltage probe where you have to jury rig insulated (string) mechanical suspension of the central body of the probe unit, since its normal probe tip goes to microwave case ground and the probes ground lead is being the maggie filament Hi Voltage connection, since you are reading kilovolts of NEGATIVE voltage.

    Clue me in on your metering situation . . . . .
    and if you have powered down the microwave . . . . it's now safe to take your left hand out of your pocket.

    MICROWAVE TECHNO REFERENCING . . . .

    [​IMG]

    73's de Edd
    .....
     
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