[volt-nuts] HP-3458A questions

[Andreas Jahn]

Hello,

A bottom picture of the board is currently in a auction (so no need to 
deinstall the board from a life system):
http://www.ebay.com/itm/Agilent-03458-66509-DCV-Reference-PCA-for-3458-PART-/371306381959?pt=LH_DefaultDomain_0&hash=item567395c287

With best regards

Andreas

Am 03.06.2015 um 01:00 schrieb J. L. Trantham:
> Frank,
>
> Thanks for the picture of your board.
>
> I have been interested in the 'plastic cap' on the LTZ1000A and you provided me with information about this in the past.
>
> Do you have a picture of the bottom of the board?  I would love to see how the bottom half of the 'cap' looks.
>
> Thanks.
>
> Joe
>
> -----Original Message-----
> From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Frank Stellmach
> Sent: Tuesday, June 02, 2015 4:50 PM
> To: volt-nuts at febo.com
> Subject: [volt-nuts] HP-3458A questions
>
> Hi Randy,
>
> in advance, this change only improves the stability of the instrument, when the 3458A is powered.
> The LTZ1000 will not age (nearly), when the instrument is switched off.
> Therefore, it makes sense mainly, if your instrument is running for an extended period of time.
>
> (Reference: Spreadbury: "Ultra Zener..")
>
> If you change the oven temperature, you will loose the 10V calibration, as the LTZ1000A will change its reference voltage.
>
>
> Although my instrument is strictly switched off, when not doing
> measurements, I reduced the oven temp. by connecting a 100k precision
> wirewound (3ppm/K) in parallel to the 15k BMF resistor. That gives 13k
> in total, yielding about 65°C like in the LT datasheet.
> An alternative placement is already foreseen on the PCB; in the CLIP,
> it's component designator is X411.
> I'll try to link in a photo of my change.
>
> lymex on the bbs35.hot forum obviously was more agressive, and paralled
> 75k of Vitrohm Thin Film, giving 12.5k for about 55°C.
>
> I would recommend my solution above the one of lymex, although his one
> gives theoretically more stability.
> Also, the lower stability of TF reistors may affect the stability (T, t)
> of the 15k BMF.
>
> With 100k/65°C, the oven runs about 30°C lower.
> Therefore, the ambient temperature has to be limited to max. 35°C, the
> fan has ALWAYS to be kept clean, giving a maximum temperature rise of
> 15°C, and the instrument may not be put in a rack (which would give
> additional 10..20°C temperature rise). Under these conditions, (35°C
> ambient + 15°C 3458A inner heating + 10°C self heating of LTZ1000A + 5°C
> regulation margin), the oven will always be stable.
>
> These 75k / 55°C would be possible, only if you limit the ambient
> temperature further, to <= 25°C, and precisely monitor the 3458A inner
> temperature (TEMP?).
>
> Although my instrument is kept at 20..23°C throughout the year, i
> wouldn't feel comfortable with 55°C.
> The only possibility would be the exchange of the reference to the non-A
> version, as the self heating effect may vanish!
>
> Frank
>
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