The later production Solartron 7071 and 7081 meters had an modification made to the RMS absoluter circuit to allow adjustment of the AC linearity. This is called out in the "Test Specification" document on page 6. Basically it involves disconnecting pin 3 of IC753 from ground and connecting it to two additional resistors and a 100K 25 turn pot as shown in the drawing. You can download the drawing from: <http://www.perdrix.co.uk/Solartron7081/7081_AC_Linearity_Adjustment_Modification.pdf> Currently there's no way to navigate to it except using this link, however I will update my Solartron 7081 web page at a later date. Cheers Dave