You should also try twist the leads for the fan.
Alternately, if you can find a large ceramic or otherwise non-polarized capacitor, soldering it across the fan output on the main board might help. I did this mod and it seem to help but haven't had time to look at the PWM on an o-scope to see if it changes the PWM rise/fall time. The cap in theory should slow the rise/fall time. The faster the rise/fall time, the high the frequencies generated and the more likely they will interfere with the tc amp.
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Hey,
had the same proble half a year ago. As you said, the fan could be responsible for this. It´s using PWM for speed control and when not running at full speed a lot of pulses are being sent through its wire. Neither the fan cable nor the thermocouple cable is shielded and the thermocouple is very prone to interference. So the fan causes wrong temperature readings. Easiest way to avoid this is to either turn the fan completely on or off, but nothing in between. The better way is to separate these two cables from one another. That should help.
Cheers,
Philip
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