[volt-nuts] RE "new" 3458A

[R.Phillips]

Randy
I would guess that your display is almost certainly 'on its way out'.  I had 
a new display fitted to my 3458A earlier this year and the difference is 
astounding. The display is of the vacuum-fluorescent type, and they will 
deteriorate over time, especially if your instrument was used in a Lab. for 
24/7 - and many had been. As I live in the UK, I am not able to give you a 
price as it was a 'package deal' - but I'm sure you get better deal in the 
US.
Roy


-----Original Message----- 
From: Randy Evans
Sent: Sunday, August 17, 2014 6:36 AM
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] RE "new" 3458A

Interesting note.  After the room cooled down from about 79F to 73F, and
another ACAL, the meter now reads +000.00035 mVDC, a more reasonable value,
although it does bounce around a couple of tenths of a uV.

Maybe that is OK?  If so, then the only issue would seem to be the  display
has some faint pixels, which a new display should fix.

Randy


On Sat, Aug 16, 2014 at 9:59 PM, Randy Evans <randyevans2688 at gmail.com>
wrote:

>
> The unit seems to be working so far except for one issue.  After doing an
> ACAL, and making sure the Auto Zero is ON,  I short the input leads with a
> copper wire shunt across the inputs and the reading is approximately
>  -000.0023 mVDC.  That seems rather high.  I would expect the unit to 
> short
> the input leads internally and force a zero reading during the ACAL.
> Anyone have any comments on this reading?
>
> Thanks,
>
> Randy
>
>
> On Sat, Aug 16, 2014 at 3:37 PM, Richard Moore <richiem5683 at gmail.com>
> wrote:
>
>> Hi Randy -- sounds like your unit is in cal, based on your measurements
>> of DCV and precision 10k resistor.
>>
>> Using autocal all is recommended before doing precision measurements, and
>> I do that if it's been more than a day or two since last use. The autocal
>> uses the internal Vref and an internal 10K resistor to do cal on 
>> everything
>> else, so that tells you what the basic cal procedure is. I just got my 
>> 3458
>> back from Loveland, and that's what they did for me -- warmed it up, then
>> ran autocal, then measured everything against a Fluke 5700, aided by an 
>> HP
>> 3325, and another 3458.
>>
>> It has been 5 years since I replaced the display board (no "exchange"
>> deal was available then AFAIK, so I don't know what's changed) and also 
>> the
>> NVRAM board, which was dead, with one with the Snap-cap RAM chips. I did
>> those replacements, then sent it home for cal, which was complete, since
>> all the RAM was new. Now after 5 years, the unit passed all incoming
>> performance tests and was sent back to me without a cal process of any
>> kind. This tells me that an old, well-aged Vref module is a good thing. 
>> The
>> 10VDC test had changed by a bit under 5ppm, or roughly 1ppm/year.
>>
>> They have a cal deal -- use code 1.090 -- press them for it -- and that
>> saved me 30% off the normal price. I think this deal lasts until
>> mid-September, so my recent "cal" ended up at just under $400 including
>> shipping. I'm not sure the deal is available on new or first-time cals; 
>> my
>> unit was in their data bank.
>>
>> But this is a long way of saying I don't think you need to send it for
>> cal -- just push Auto Cal and Enter and wait about 10 minutes and you
>> should be good to go.
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