In our lab, we have had sometimes strange behaving on 432A+478A-H76 mount. We measuring Vcomp and Vref voltages backside of the 432A using 3458A. Normaly when mount zeroing is fine etc, 3458A should show around 85mV when power meter DUT Ref-Out power is ON. Time after time even zeroing is perfect (less than +/- 200uV) and turn
Ref-Out ON voltage staying 82-83mV and it meas that Ref-Out is out of spec. Problem can disappear itself and all working perfectly long time. Same issue has happened with another 432A meter, mount, cables etc.
All used equipments are annualy re-calibrated. What can cause this strange behaving?