[volt-nuts] New HP3458A

Sat Aug 16 10:21:08 EDT 2014

-0.0025mV does seem a little excessive, I think that may be out of mfr.
specifications if I'm remembering right.  Charles is right you don't want
to use banana plugs, at least for low dc volts stuff.  The service manual
has a good picture of how you should use a section of clean copper wire to
short the terminals.  Also make sure you've done an ACAL recently and that
the ambient temperature hasn't changed by more than 1 deg. C since doing
it.  The air filter is also a good call, I've seen the internal
temperatures change by 2-3 degrees just by having the air filter dirty.
 Use the TEMP? command to get the internal temperature.  Use the CAL? 58,
CAL? 59 and CAL? 60 commands to get the internal temperatures when the last
calibration was performed.  There shouldn't be more than a 5 deg. C
difference or else there are adders you have to start throwing on the
accuracy spec's.

Finally after all of that the input offset voltage is still high, running a
CAL 0 command in both the front terminal and rear terminal positions with
the input properly shorted will re-'zero' the instrument.  Make sure the
cal security is turned off to be able to run the CAL 0 command.  All that
is well detailed in the service manual too.

On Sat, Aug 16, 2014 at 8:48 AM, J. L. Trantham <jltran at att.net> wrote:

> Randy,
>
> Congratulations on your 'new' 3458A.
>
> The 'CALRAM' back-up was just recently covered.  There are two options:
>
> 1.  Remove the NVRAM, read its contents with a programmer, program a new
> NVRAM and install the new NVRAM.  I chose that option, installing sockets
> for the 'CALRAM' and the two other NVRAM's that are used for other,
> non-calibration, purposes.  Be sure to let the NVRAM cool all the way down
> before trying to read it.  Learned by experience with that one.
>
> 2.  Read the 'CALRAM' via HPIB and the MREAD command (many steps involved
> but can be done as a program with your laptop/desktop), use that data to
> program a new NVRAM (I suspect there is a way to 'program' the NVRAM by
> HPIB
> as well but I don't recall ever reading anything about it on the list),
> remove the old NVRAM and install the new NVRAM.
>
> When I read the contents of the 'CALRAM' with a programmer and with the
> MREAD command, they matched exactly.  However, when you do an ACAL, the
> CALRAM contents change.
>
> Does your unit have the single EPROM or the 6 EPROM version of the
> firmware?
> Also what version of firmware do you have?  The latest is REV 9.  REV 8 is
> 'out there' on the net.  If you purchase an exchange display, and you have
> the single EPROM version of the A5 Outguard Controller Assembly, the REV 9
> EPROM is $24.10 per the Keysight website.  Might be worth adding to the
> order.   The 6 EPROM version is $220.
>
> Also, what is the serial number?  You can get a good idea of age by the
> serial number.  Also, the various covers inside often will have dates of
> manufacture on them and the chips, etc., will have date codes, including
> the
> NVRAM's.
>
> Also, you can add option 001 by plugging some memory chips, specific
> details
> I don't recall right now.
>
> As far as the reading when shorted, I recall the Calibration Manual calls
> for a piece of copper wire to short the input terminals.  It looks like a
> flying 'U' when properly fashioned.  The Calibration Manual gives the
> details, IIRC.
>
> Good luck.
>
> Joe
>
> -----Original Message-----
> From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
> Behalf Of Randy Evans
> Sent: Friday, August 15, 2014 8:40 PM
> To: Discussion of precise voltage measurement
> Subject: Re: [volt-nuts] New HP3458A
>
> I would like to thank everyone that replied to my query on what to look for
> in my "new" HP-3458.  I did find that the AUTO ZERO button does work fine,
> thanks to Bill's comments.  I had looked in the manual, just not far
> enough.
>
> The display is still perfectly readable but i would like it to be
> "perfect".
> I am particularly interested in the exchange display for $272 (better than
> $700 for a new one, as Todd suggested).  Does someone have a contact number
> at Keysight that I could call (my experience is that one can spend a lot of
> time calling around until the right person is finally found).
>
> I certainly need to figure out how to copy and replace  the NVRAM - I lost
> the calibration on my Datron 1082 by not realizing the memory backup
> battery
> died and now I need to calibrate it myself (the HP3458A should make that
> doable, I hope).  Any suggestions would be greatly appreciated.  I believe
> it has been covered before on Volt-Nuts and I need to go through the past
> discussions to find it.
>
> One question I have for the group is what should the display typically show
> with the input shorted?  I see a reading of about -.0025 mV.  That seems
> rather high.  I tried several different banana cables (gold plated, tin
> plated) used to short the input terminals  to see if thermocouple effects
> might be responsible but there was no change in the reading.
>
> Still learning.
>
> Thanks,
>
> Randy
>
>
>
> On Fri, Aug 15, 2014 at 8:06 AM, Jason Watson <watson.jason at gmail.com>
> wrote:
>
> > I've also seen excessive Guard to Low leakage when varistor RV501 has
> > gone bad (it's located on the front/rear switch pcb and it's possible
> > to replace it while leaving the circuit board in place if you are
> careful).
> >  HP/Agilent/Keysight Part number is 0837-0196, cross referenced to a
> > Harris Corp. V430MA3A.
> >
> >
> > On Fri, Aug 15, 2014 at 10:46 AM, Stephen Grady
> > <grady.steve at gmail.com>
> > wrote:
> >
> > > Randy,
> > >
> > > I have come across a few 3458A's that had leakage between Guard and
> > > Low when te guard is in external guard position. This was due to a
> > > leaky external guard switch and/or leaky front rear switch. This can
> > > be quickly
> > determined
> > > by measuring resistance between guard and low with guard external.
> > > This normally in not an issue except when you are using 3458A to
> > > measure
> > voltage
> > > with low above earth potential say in a bridge the guard low leakage
> > > will be loading other arm of the bridge.
> > >
> > > Kind Regards,
> > >
> > > Steve Grady
> > > Sydney, Australia
> > >
> > > -----Original Message-----
> > > From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com]
> > > On Behalf Of Randy Evans
> > > Sent: Friday, 15 August 2014 1:22 PM
> > > To: volt-nuts at febo.com
> > > Subject: [volt-nuts] New HP3458A
> > >
> > > I picked up a used HP3458A today, which I needed for some precision
> > > DC measurements i need to make. It passes all the self tests and the
> > > Auto
> > Cal
> > > but is there any thing else I can check ( I have a 14 day RR).  It
> > > reads
> > a
> > > 10V standard I made within a few tens of ppm, but it's not a 732A
> > > but
> > that
> > > is at least comforting.  It also reads an ESI 10Kohm standard
> > > resistor
> > dead
> > > on.
> > >
> > > The only problem is that the display has some faint pixels in some
> > > locations, with three in the second row for every digit location dead.
> > >  Likely a pixel driver I would think.
> > >
> > >  I am not too familiar with it yet but I noticed when I push the
> > > auto
> > zero
> > > button, the display has a blinking square until I hit a measurement
> > > mode button; e.g., DCV, ACV, OHM, etc.  Is this normal?
> > >
> > > Regards,
> > >
> > > Randy Evans
> > > _______________________________________________
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