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Old 01-19-2023, 01:32 AM   #1
Use2bavolvo
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Default 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.
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Old 01-19-2023, 05:19 PM   #2
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Default 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
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Old 01-19-2023, 06:23 PM   #3
redwire
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Default 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.
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Old 01-19-2023, 06:36 PM   #4
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Default 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 at 06:42 PM..
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Old 01-19-2023, 06:51 PM   #5
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Default 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 at 06:59 PM..
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Old 01-19-2023, 07:10 PM   #6
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Default 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 32F/0C if the oven is OK with such a low temp.
2,200Ω should read around 613F/323C.
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Old 01-19-2023, 11:39 PM   #7
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Default 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 at 12:38 AM..
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Old 01-20-2023, 12:09 PM   #8
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Default 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.
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Old 01-20-2023, 10:44 PM   #9
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Default 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??
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Old 01-21-2023, 12:45 AM   #10
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Default 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.
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Old 01-21-2023, 11:52 AM   #11
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Default 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 700F/370C 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.
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Old 01-21-2023, 11:11 PM   #12
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Default 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
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Old 01-22-2023, 04:28 AM   #13
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Default 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.
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Old 01-22-2023, 04:27 PM   #14
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Default 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.
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Old 01-23-2023, 07:10 PM   #15
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Default 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.
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Old 01-24-2023, 11:57 PM   #16
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Default 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!!!
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Old 01-25-2023, 12:38 AM   #17
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Default 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.
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Old 01-25-2023, 12:18 PM   #18
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Default 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.
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Old 01-25-2023, 09:39 PM   #19
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Default 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
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