Keithley 2001 - Floating DC

[dhillman]

I recently acquired an older 2001 and have noticed something odd with DC Volts with no connection.   The displayed voltage will float up to roughly +/-2 Volts.  When connected to a source or shorted, the meter reads correctly.    Is this an oddity of these?

Background-  all electrolytics have been replaced, and the board cleaned around them.   There was no sign of prior leakage thankfully.
Diagnostic tests pass repeatably.   

Possibly related, or due to old calibration is that the AC self calibrated will fail with +415 error.   I do not know why this is not caught in the diagnostic test. 

[David Hess]

On the low DC voltage ranges (200mV, 2V, and 20V), no high input impedance divider is used and there is no 10 megohm input shunt so the input leakage current charges whatever input capacitance is available until the leakage cancels out.  This occurs with any DC voltmeter when a high, >10 gigaohms in this case, input resistance is used and it is completely normal.

On some meters but apparently not the 2001, the input resistance on the low input voltage ranges can be selected between 10 megohms and "infinite".  This can be important for measurement consistency or if you are using an external divider which is expecting a 10 megohm input resistance.