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Magic chef oven - thermistor ref voltage issue

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    Magic chef oven - thermistor ref voltage issue

    I am having an issue with my wall oven temperature control is ~180f out of spec. The thermistor is measuring correctly but I noticed the ref voltage appears to be off. With the thermistor wires attached it's reading ~3.4vdc with the thermistor disconnected it's around I9.8vdc. my understanding is the temp is read off the resistance from the thermistor which leads me to believe the issue is with the ref voltage.

    Another observation is the temp setting and readout at lower temps is way off meaning the oven thinks room temp is 300f and bake relay doesn't engage unless temp setting is much higher..

    Anyone have any thoughts on what's going on? I plan on rreplacing the caps this thing is 30yo.

    I've also I'd a fan resistor that's not registering any ohms ATM the fan runs all the time so not sure if that's also causing the issue.
    Attached Files

    #2
    Re: Magic chef oven - thermistor ref voltage issue

    Are there any components on the other side of this board if so could you post the picture of the back of the board
    9 PC LCD Monitor
    6 LCD Flat Screen TV
    30 Desk Top Switching Power Supply
    10 Battery Charger Switching Power Supply for Power Tool
    6 18v Lithium Battery Power Boards for Tool Battery Packs
    1 XBox 360 Switching Power Supply and M Board
    25 Servo Drives 220/460 3 Phase
    6 De-soldering Station Switching Power Supply 1 Power Supply
    1 Dell Mother Board
    15 Computer Power Supply
    1 HP Printer Supply & Control Board * lighting finished it *


    These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

    1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
    2 Hem Saw Computer Stack Board

    All of these had CAPs POOF
    All of the mosfet that are taken out by bad caps

    Comment


      #3
      Re: Magic chef oven - thermistor ref voltage issue

      What's the oven's model number? If the controller thinks the oven is hot, I'd expect the fan to be running. Have to take ohmmeter readings of the sensor or the control board has an issue.

      Comment


        #4
        Re: Magic chef oven - thermistor ref voltage issue

        Model C9875VRV
        Control Board part number 7601P225-60

        The top board has 3 separate single layer boards this top board does not have any other components just traces.

        Middle Section is where the IC is

        Bottom Section is for the controls and display

        The thermistor is reading correctly at room temp and when hot.

        The R4 resistor is a bit of an oddity I read it as Violet, Yellow, Yellow, Red, Red which is a odd ohm 744k (there is no resistance across this) Can I just replace with a 750k 1% resistor here?
        Last edited by Use2bavolvo; 01-19-2023, 06:42 PM.

        Comment


          #5
          Re: Magic chef oven - thermistor ref voltage issue

          R4 value looks like 2.2Ω 5ppm/K, have you checked it's resistance?
          Last edited by R_J; 01-19-2023, 06:59 PM.

          Comment


            #6
            Re: Magic chef oven - thermistor ref voltage issue

            Oven temperature sensor WPW10181986 commonly used in Whirlpool, Maytag, KitchenAid, Jenn-Air, Amana, Magic Chef, Admiral, Norge, Roper, Sears/Kenmore and others.
            It's an RTD (not a thermistor) aka Pt1000 so resistance goes UP with temperature. Resistance table is like this pic.

            Check the sensor resistance with a multimeter, as well check isolation resistance to the sheath/chassis to see if it has a ground fault. Check the connector is not corroded/melted as well because only a few ohms change causes large swings.

            You can also jumper in a resistor in place of the sensor, to see what the control board displays.
            1,000Ω should read around 32°F/0°C if the oven is OK with such a low temp.
            2,200Ω should read around 613°F/323°C.
            Attached Files

            Comment


              #7
              Re: Magic chef oven - thermistor ref voltage issue

              The resistor is between the relay and fan pin. I did some more research it looks like a fuse resistor does that seem right?
              Last edited by Use2bavolvo; 01-20-2023, 12:38 AM.

              Comment


                #8
                Re: Magic chef oven - thermistor ref voltage issue

                Yes it could be a fuse resistor, What resistance value does it measure?
                Also post a clear picture of the back of that board so we can see what it connects to.

                Comment


                  #9
                  Re: Magic chef oven - thermistor ref voltage issue

                  R4 is open no resistance

                  Back of board traces

                  Second board pic

                  Also transformer between
                  pin 1-4 142ohm
                  pin 8-5 175k ohm

                  Diode 4 and 5 both registered .526 but when tested other way around went up to 2.0 then 0 not sure if that's from the transformer??
                  Attached Files

                  Comment


                    #10
                    Re: Magic chef oven - thermistor ref voltage issue

                    After reading about capacitors when degrade creates resistance on the circuit I'm gonna replace the power caps to ensure this is generating a stable voltage. 30y its due.

                    Comment


                      #11
                      Re: Magic chef oven - thermistor ref voltage issue

                      R4 fusible resistor is for the DLB safety relay K3, I'd say it blows if there ever is a DLB problem. Who notices? Must be a repair indicator.
                      Has the oven ever had a failed heating element (ground fault)? That can take out the temp sensor and circuitry.

                      K3 has a fractured soldering joint on the spade BTW. Other touch ups look like not enough heat/time for soldering technique on the big through hole parts. Jack up the iron temp at least 700°F/370°C and hold for a second or two longer.

                      Given the age 30 years and rarity of this board, I'd replace the electrolytic capacitors certainly on the PSU/Relay board, and only 3 on the MCU board.

                      The temperature sensor circuit is another thing to troubleshoot.

                      Comment


                        #12
                        Re: Magic chef oven - thermistor ref voltage issue

                        Looks like we may have found the issue..these caps in macro look buldged out. Digikey didn't have this form factor they are height of 6mm 4.7uf 35v
                        Attached Files

                        Comment


                          #13
                          Re: Magic chef oven - thermistor ref voltage issue

                          Such small caps don't bulge, what you are seeing is just the heatshrink sleeving.
                          That said they do dry out and after 30 years they are probably bad if they ran at all hot, so good idea to swap them out.
                          "The one who says it cannot be done should never interrupt the one who is doing it."

                          Comment


                            #14
                            Re: Magic chef oven - thermistor ref voltage issue

                            Parts ordered was able to get the 4 relays as well. Wil update after I get em soldered in. I wasn't able to get current trim on the caps so just went with uf and v plus length which were requirements for the in between board caps.

                            Comment


                              #15
                              Re: Magic chef oven - thermistor ref voltage issue

                              With the longer replacement caps, you can just lean them sideways on the board. That should allow you to fit standard 5x11 mm caps. As a bonus, since they are bigger, they should probably last longer too.

                              Comment


                                #16
                                Re: Magic chef oven - thermistor ref voltage issue

                                Update of everyone.
                                I replaced 6 caps
                                3 on power board and 3 on logic board

                                Happy to report oven is fixed

                                Recap - thermostat was correctly measuring 1060 ohms at room temp

                                Oven temp would register 300f at room temp
                                Relays would come on when temp set to 515 and will get to 350 but wouldn't stay there.

                                Measured VDC at thermostat connection to the board ~3.8 or lower vdc. With thermostat disconnected 9.8v

                                The thinking was the ref voltage was low so temp was being read wrong by ic

                                One of the PSU caps was toast dumno if the others were bad but replacing them fixed the issue.

                                Oven temp at room is now normal and relays bake and broil are all turning on when they should.

                                Thx folks learned a lot about bad caps and sad to see the manufacturer killing off smaller spec replacements.

                                Invaluable learning experience and super helpful group!!!

                                Comment


                                  #17
                                  Re: Magic chef oven - thermistor ref voltage issue

                                  Nice work, great to hear you got it working!
                                  Also, thanks for coming to report back. I think this will definitely help others who may experience this issue, since now we know the caps did fix it.

                                  Comment


                                    #18
                                    Re: Magic chef oven - thermistor ref voltage issue

                                    Can you post the cap size and values you replaced, to help others doing this repair.

                                    It's great you got it fixed, the oven control boards are either super expensive (trending at $450) or discontinued. All this e-waste when people are broke.
                                    I thought it was the caps or the op-amp chip LM324 is the only thing left.

                                    Comment


                                      #19
                                      Re: Magic chef oven - thermistor ref voltage issue

                                      Power board 2x 1000uf 35v
                                      Power board 1 47uf 25v
                                      Ic board 1 47uf 25v
                                      Ic board 2 4.7uf 35v

                                      Attached component bought from mouser. I bought the 4 relays but didn't install them. These become unobtainium so figured best get spares.

                                      I agree with your comment about the disposable appliance. These things are dead simple devices which should be repairable but most die within 5 years.

                                      My appliances are all analog so they have survived a long time dryer is circa 1993. Only thing I've gotten for that is a blower fan blades plastic and a overheat switch which died and some drum rollers
                                      Attached Files

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