So when the enlarger light is on and the probe is under that light you are getting readings in the LED display. What are those readings? You haven't said but I take it that you are projecting a negative onto the easel where the probe is? I ask this because if there is no negative the easel is lit by a bright white light and the probe, I'd imagine, is unable to cope with such a bright light so the readings may be "crazy" I am unclear what you mean by no activity but if there is no negative projection then the light intensity, even if the probe is coping with it will be the same over all the easel so the reading in the LED display will not change. Is this what you mean by no activity?
I have a PDT2020 but the principles of how it operates are likely to be the same. If there is no negative, place one in the carrier then take readings on light and dark parts of the negative. There should be differences. Secondly leave the probe in one place on the negative projection and change the lens aperture. Again there should be differences.
pentaxuser