To me it looks like:
R1 - factory adjustment (to adjust for actuall chamber temp)
R4, R6 - 22 deg set point fixed (the two fixed resistors set the voltage drop)
R2, R5 - the switch position selects these two for the voltage drop setting the chamber temp
Have to go to work very soon. Will look further into this. I suspect that by changing R5 we can find a simple solution ie a single digital pot. Just a matter of keeping the ratios correct.
Bill, do you know what the 5k pot you are using is set at? and the range that you normally adj?
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