[danner]

Hi,

You're not measuring the resistance between the "ends" of the pot are you?   It should be between the "wiper" (ground) and the end that is connected to the Op Amp input.  The value should be about 3.33 kohms to give a gain (amplification multiplier) of 4.  Increasing the resistance gives lower gain, which you possibly need because at 1500 w/m2 the SP-110 should give 300 mV output. 

I'm measuring between pin 2 and the ground pin on one of the screw terminals with the connector removed from the TX board and with the SP-110 removed from the Solar/UV board.



I was considering swapping out the TX1 board and nano, or moving the Solar/UV to my other transmitter, TX0. Either way that should allow me to see if the behavior follows or goes away.

But all this should make no difference to the "zero" (night time) value.  Zero times 3.75 (or any value) should still be just zero!  So in the "dark" you need to discover what isn't zero, which with a good multimeter shouldn't be too difficult:  

Measure the voltages on pins 1, 2 and 3 of the Op Amp relative to a good ground (probably the PCB track, wiper of the pot, or ground connection of the SP-110) to a resolution of better than 1 mV.  Those three voltages (which could be measured on the two ends of R1 and the PV input) should give an indication whether it's the SP-110, the Op-Amp (input offset error), the Nano ADC (zero reference error) or something else that is "wrong".

Thank you for that information. It is very helpful and much appreciated! I can do that.