Maytag oven control board temp sensing logic

[EldredB]

I have a Maytag gas oven that is running way too hot.  Model MGR5875QDS with control board 8507P13760 rev A / 74008629.
If I set the oven at 350, it initially heats to 350 (and yes, I've verified it is 350 degrees), the relay clicks, the burner shuts off.  Within about 30 seconds the relay clicks and the burner relights, runs for a minute or so, then clicks and the burner shuts off.
This sequence continues over and over, and the oven continues to get hotter and hotter and hotter.  I have verified the temp sensor is good and even replaced it with a new one for good measure...to no avail.

I'm very experienced with audio circuitry but I'm a little bit of a fish out of water with active oven controls.  I'd like to find out some information about the logic used to sense oven temperature and the related underlying technology used to accomplish this function.  At first glance, I see a couple capacitors early in the circuit board path feeding the oven temp sensor.  This makes me wonder if those caps are used to hold voltage for the oven temp sensing during the times when the burner/ignition circuit is not enabled?  If I have a faulty cap, could that explain why the temp sensing circuit is giving a false (under temp) signal back to the control circuit, thus restarting the heating prematurely?  Does anyone have any experience with this type of problem?  Any assistance would be greatly appreciated!!!

[tgoods]

You may be on to something with those capacitors.  It should be easy enough to test with a cap tester. Capacitors are cheap enough, you could just replace them and see what happens.  Another option is to send the board out to be repaired.

[EldredB]

My wife wants it fixed ASAP and I am supposed to be able to fix everything that breaks.  ....cars, electronics, plumbing, HVAC, you name it.  If it's broken, I'm fixing it.  I was just hoping to get some insight from industry experts before blindly throwing parts at a board.  Caps are cheap and generally easy to find locally, so I'm rooting for some kind of "that's common problem with those boards" or "yes, the cap(s) are there to provide power for the sensor circuit during the cooling part of the cycle".
Thanks for your moral support, though!   

[jimbo6679]

I suspect that manufacturers consider control boards to be a non field repairable item, and therefor may not publish component-level circuit descriptions.   There are companies who repair boards, so they get some info from somewhere!

[Bailey]

I was going to repair a board once. It had a burned resistor so i couldn't tell what value it was. Called the factory tech line for some help. I was told they did not know the value because the part was made by a third party vendor and was not considered a repairable part.As a technician I am more concerned about fixing it fast and right the first time so that usually means replacing the board. However like you if it were mine I don't mind tinkering with it awhile.

[fixyourboard]

AJ alerted me to this thread ... We often see temperature sensing failures. Various sensing methods are used depending on the control board. In the end they all do an analog to digital conversion. This includes several precision resistors and capacitors along with either an a/d chip or the microprocessor and sometimes an eeprom. We've seen failures due to any of these parts, often more than one. It's a sensitive circuit so all have to be pretty much right on for it to work properly. The capacitors can definitely cause trouble.

Note that, when you replace the capacitors, be sure to use a decent desoldering tool. A little too much heat will lift the trace off the pc board and then you end up with a new re-work headache! Also, if the part is thru-hole mounted, the leads may have been bent before soldering. In this case you may have to pull while heating to straighten the bend ... don't push back towards the board or again you risk detaching the pcb trace!

[EldredB]

Thanks for the input gents.  It turned out to be a faulty transformer.  At least I hope so. 
I'll know for sure after dinner tonight.