[volt-nuts] Agilent 3458A Issues

[Randy Evans]

Illya,

Can you explain why you say " If ACAL DCV does not remove drift then A3 is
probably fine".  I don't really follow the argument.

Randy

On Thu, Nov 23, 2017 at 5:18 PM, Illya Tsemenko <illya at xdevs.com> wrote:

> Well, one thing you can know for sure , that reference is indeed broken.
> Drift over a day should be way below the noise floor (<0.1ppm). With 1.7x
> gain of ADC scale that drift rate gives you around 0.8ppm +/- 0.3ppm due to
> zener noise. So it is in line of 1.1ppm/day. If ACAL DCV does not remove
> drift then A3 is probably fine.
>
> Since reference is much easier to troubleshoot and fix I would go with
> replacing LTZ chip and testing if drift go away, if that have any help on
> your desire to keep meter.
>
> Also serial number range in SN doesn't mean much for you, as meter history
> is unknown and it still may have been serviced at some time.
>
> On November 24, 2017 8:40:00 AM GMT+08:00, Randy Evans <
> randyevans2688 at gmail.com> wrote:
>>
>> Illya,
>>
>> I ran the test you suggested over 4 days and got 7.19114068 VDC on the
>> start of the test and 7.19113736 VDC at the end of the fourth day (96 hours
>> later).  I calculate a total of 0.46 ppm drift, which seems excessive but
>> does not account for the 1.1 ppm/day I measured overall.  I suspect the A3
>> card is the primary source of drifting.  Since the unit is a late model
>> Agilent unit, that is well beyond the expected range of units described in
>> Service Note 18.  What do you think?
>>
>> Thanks,
>>
>> Randy Evans
>>
>> On Sat, Nov 18, 2017 at 4:47 AM, Illya Tsemenko <illya at xdevs.com> wrote:
>>
>>> 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
>>>>
>>>>
>>