Hello, is there anybody familiar with normal behavior and calibration of Keithley 2100 (or its brothers Picotest M3500A or Array M3500A, Atten ATM3511A, BNC 1201, Chroma 12061, DL-2060, DME 1600, Kusam-Meco M3500A, Time Electronic 5065, BNC 1201)? I have a DMM which claims Tektronix calibration, but it is ~80uV off on range 10V. I tried to calibrate it following calibration manual, and it appears to adjust readings and change calibration date. And it measures OK before next power cycle.
It is unclear if the calibration did not make it into EEPROM, or something else is wrong, but the readings now are different from before calibration (~40 uV), but still not the same as the measurements after calibration and below power cycle.
There is also strange behavior of readings at cold startup - they are appearing closer to used voltage standard, but then drift to incorrect values within next ~20 mins. The drift is slow, followed by a steep change of readings after ~150 readings, then slow again, then step etc... I am attaching the plot of drift vs time since cold power up
. It settles after 18-20 min, and at that time I am doing calibration. But on the next power cycle the behavior repeats...
The manual says that the DMM does autozero on every measurement, and there is no explanation to periodic step-wise moves.
Did anyone experience similar behavior or prooblem? Any suggestions what can be wrong? There is no schematics to look at, unfortunately.