The tuning pots are logarithmic, with values of 2K2 and 220K.

They are wired in series : when used for very low currents, the 220K does the coarse value and it can be refined by the 2K2 pot.

But not the reverse...

A "range" switch should be added to connect the pots in series or parallel. The parallel mode would use the 220K for fine-tuning the coarse high current pot, above 1mA.

I'll do this when I can, but the box is already pretty crowded and I don't want to add dust...