[volt-nuts] Agilent 3458A Issues

[Illya Tsemenko]

Since you have 732A, testing should be easy enough. Calibrate faulty meter for zero and DCV 10V to 732A, record CAL? 2,1 value. This is your LTZ output. Then leave it running for few days to drift away and calibrate again to same 732A. Check CAL? 2,1 again. Calculate the difference and if it matches output drift (that 1.1ppm/day you mention) - you can be 80% sure that A9 is a problem. Other 19% go to A1 and A3 circuits, as 7V is not used directly in the meter, and there are still gain parts to get +12 and -12VREF on A3 and 10Vish bipolar levels on A1. If your CAL? 2,1 stays same (within 0.3ppm) then A9 is fine.

On November 18, 2017 12:59:53 PM GMT+08:00, Randy Evans <randyevans2688 at gmail.com> wrote:
>I just received an Agilent 3458A that has a problem with noise and a
>drifting voltage measurements.  I am using two Fluke 732As to compare
>absolute voltage measurements over time against the Agilent and an HP
>3458A.  The HP unit has a new A3 ADC card and seems to be very stable
>and
>low noise, so is being used for comparison.  I have been doing
>simultaneous
>absolute voltage measurements and DC Cal Constant measurements several
>times a day and then calculating the drift rates of the two units using
>the
>HP Service Note 18 procedure.
>
>
>
>The results indicate the Cal Constant drift rate of both units is very
>similar and within spec per Service Note 18.  However, the absolute
>value
>measurements show the Agilent unit changing 1.1 ppm over a day whereas
>the
>HP unit is within a tenth of a ppm over a day.  In my way of thinking
>the
>Cal Constant procedure assumes the voltage reference board in the 3458A
>is
>stable, hence the absolute value reading should remain essentially
>constant
>after each ACAL DCV, which is the case with the HP unit.  Since the
>Agilent
>unit shows a steady drift in the absolute reading, this would indicate
>to
>me that the voltage reference board is likely the cause of the problem,
>and
>is also likely the cause of the noisy readings.  If so, this is a
>“relatively” easy fix (I have several 3458A voltage reference boards,
>one
>of which has been continuously powered up for several years).
>
>
>
>The issue is that I have to make a decision to keep or return the
>Agilent.
>It has a cal seal on it and if I open the unit up to change the voltage
>reference board, I own it and can’t return it.  I would appreciate an
>opinion from the members of the group as to what they think the odds
>are
>that the voltage reference board is the source of the problems with the
>Agilent 3458A.
>
>
>
>Thanks,
>
>
>
>Randy Evans