Jobo CPP2 display temperature deviates by 10C from actual bath/chemistry temp

Hello,

I can across this thread recently, and could use some help with my Jobo CPP2 temperature disparity issue.

The difference between the temperature readout and the batch temp (including temp inside chemistry bottles) is 10C plus or minus.

The difference between the temp set on the dials and bacth/chem temp is much smaller but non-trivial --- about 1C.

Temp was measured by a glass drugstore thermometer, and by food thermometer!

I cleaned the heating element and the the temp sensor... no difference.

I appreciate any pointers.

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35mm and 6x7
Color and B&W

DrZorba said:

The difference between the temperature readout and the batch temp (including temp inside chemistry bottles) is 10C plus or minus.

Click to expand...

OK, I'd start reading on page 16 (numbered page; p21 in the pdf): https://125px.com/docs/manuals/darkroom/jobo/jobo-cpp-2-service-manual.pdf
That's the section of the service manual that concerns temperature calibration.
Given the magnitude of the error you're seeing, it seems likely that there's (partial) hardware failure. There's a somewhat similar issue here: https://www.largeformatphotography....-CPP-2-Temperature-board-question-for-experts but sadly no follow-up was given.

DrZorba said:

I cleaned the heating element and the the temp sensor... no difference.

Click to expand...

Cleaning the sensor isn't likely to do much. It's possible the sensor itself has failed, that connections are corroded or that there's component failure in the signal conditioning circuit. So..it can be pretty much anything at this point, but the magnitude of the error suggests something serious and not a case of normal lifetime drift.

For reference, here's the schematic of the temperature signal conditioning circuitry;

New style unit (pdf page 73):

The dangling net bottom left connecting to C52 is analog ground.

Old style unit (pdf page 73):

IC45 (new) = IC48 (old) = CA3162E (ADC; https://www.experimentalistsanonymous.com/diy/Datasheets/CA3162.pdf)
IC46 (new) = IC49 (old) = CA3140E (opamp; https://www.futurlec.com/Linear/CA3140E.shtml)

Note the reference voltages you can check at the upper leg of the probe; this will tell you (with a reasonable degree of certainty) whether the temperature probe/NTC is OK.

DrZorba said:

I can across this thread recently

Click to expand...

But your question is more about failure of the CPP2's temp readout circuit as such, so I've split off your question from the linked thread where you initially posted it. Hope that's OK.