I found a problem on my DMM6500: The terminal switch (Front/Back) had a contact problem. With the inputs shorted ohms measurement was between -0.5 and 2-3 ohms, depending on how quickly I released the switch. Put some Contact Cleaner in the switch and now it always reads -0.5 ohms. This offset goes over the whole range, even at continuity measurement (as expected). I never used the switch before, and all readings were good until I used it the first time. Seems they calibrated it in the factory with the unwanted contact resistance. The unit is eight months old, so I suspect Keithley won't recalibrate it for free? Or is Keithley customer friendly?
Magnus