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 <randyevans2...@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 <richiem5...@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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