I have many geolinks in operation. Some seem to need frequent recalibration and others stay calibrated. Also after recalibration some of the geolinks show moving values for heading. The value can vary from 6-7 degrees and does not stay constant when viewing them from the laptop. I have seen that the proximity of the laptop to the geolink will change values if the laptop is too close. (must be some kind of magnetism from laptop).
My question is are some of the geolinks "defective"?

Hello,

Magnetic compass is very sensitive part of the GeoLink so it can sense anything that is affecting nearby magnetic field.
The laptop has for example built-in speakers that are changing the field significantly.

Recalibration shouldn't be normally needed if you are not changing the field and the angle should also not change on its own.

In a real flight condition you might not notice difference in range of 10°.

Since some parts of the model are changing magnetic field too you can see these changes because of this. I recommend just replace GeoLink at the same model to see if the values are changing there or not.
It should show same behavior with the same setup. But when a different ESC/motor, BEC, servos are used you can get a different readings.

The easiest way for comparison is to open GeoLink case (yet I do not recommend it for the most pilots) and just replace the PCB. This should not take more than minute to do this comparison.

So short answer to your question is: I don't think they are deffective but you can make sure with the comparison.

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Spirit System developer