[time-nuts] Voltage standards

[WB6BNQ]

Will,

I beg to differ with you on the military statement.  The Navy's Primary LAB
received 100 of the 731's that specifically were not hand picked, as you
suggest.  Out of that 100 a number of them were used for a vast array of testing
including destructive testing.  A few units had problems right at the start, but
the majority held up very well.  The purpose being to define the value of the
item regarding the Navy's mission and support there of.  I know all this because
a close friend of mine was the LAB person who did the project.  His
responsibility, at the LAB was maintaining the Navy's primary voltage reference.

>From this large test base came a number of improvements my friend suggested to
Fluke that were incorporated in later production units.  I guess it would be way
more accurate to say Fluke used (wisely) the LAB as a "beta tester" of their
product which was brand new at the time.  Gaining the Navy's acceptance was a big
deal in the scheme of things in those days.

The quality of the Fluke equipment was outstanding and, no, the military did not
hand pick.  The same went for the HP equipment and Tektronix.  Simpson was
another cornerstone with their 260 VOM as the Navy had a ton of those.  So was
Triplett who had some very good VOM's in the day.

A very good instrument to grab when they show up is the HP 6920 Meter
calibrator.  Yes it is old but it was specifically built to do VOM's like the
simpson and individual meter movements as well.  It outputs DC and AC voltage (60
Hz) (zero to 1000) and current (zero to 5A) and is quite handy to have.

As for the "small transfer boards" you refer to, I completely understand what you
mean.  While they may serve a purpose for the masses, their advertising is a bit
misleading to put it kindly.


Bill....WB6BNQ



Will Matney wrote:

> Bill,
>
> Yes, Fluke did cherry pick on what they bought, or paid to have Motorola to
> do it. HP does the same when purchasing voltage references, but they both
> do burn them in themselves after they receive them. The duds are used in
> less tolerant meters, etc. That is what Fluke wrote about doing in an app
> note of theirs. Also, the military gets the pick of them all, and is the
> reason I try to buy military surplus that's in good shape. My Fluke 730A is
> ex-Navy.
>
> Actually, back in 2008, this subject was discussed here on Time Nuts, and
> was the reason why I made this post. At the time, nobody had crossed the
> Motorola part numbers, and just called them the part from Motorola. It took
> digging out an old Motorola manual from 1968 to find them. I am hoping it
> will help others if they have an older Fluke transfer standard they need to
> repair, as there are parts still available by a few dealers. However, to
> get any kind of accuracy, the parts will have to be burned in at least
> around, or over, a three month period.
>
> I also wanted to note that the small transfer boards being offered, by
> several, should be looked at by the way they are calibrated, if they are to
> have any sort of precision.
>
> Best,
>
> Will
>
> *********** REPLY SEPARATOR  ***********
>
> On 5/26/2011 at 4:18 PM WB6BNQ wrote:
>
> >Hello Will,
> >
> >I think you meant to post this on the "Volt-Nuts" list ?
> >
> >The reference parts used in the Fluke 731/732 are not "off" the shelf
> parts, they
> >are selected.  While they are referred to as transfer devices, if working
> >correctly, they exceeded their specs versus time on the 10 volt port.  I
> base
> >this on the history of over 50 units that were tracked by Navy's Primary
> Lab, as
> >these were used to feed the second level labs in the Navy's system.
> >
> >Bill....WB6BNQ
> >
> >
> >Will Matney wrote:
> >
> >> Hello all,
> >>
> >> I was reading some old posts about voltage standards from Dr. Kirby, and
> >> some others, and one that was mentioned was the older Fluke standards. I
> >> did some research on this, going back to the Fluke 730A transfer
> standard,
> >> which I have one of, and found out what the voltage reference and the
> >> differential amplifier was from an old Motorola parts book dating from
> >> 1968.
> >>
> >> The voltage reference in the Fluke 730A was most likely the Motorola
> part
> >> number: MCA1914N or the MCA1924N in a can package. Both are rated at 6.8
> >> Vdc, but there is a difference in the maximum voltage change between the
> >> two. The MCA1914N is the tightest, with a change of only 0.005 volts
> from 0
> >> to 75 deg C. The MCA1924N is made for a broader temperature, with a
> change
> >> of only 0.010 volts from -55 to 100 deg C. These are not an oven zener,
> but
> >> a voltage reference. Motorola first produced these using two diodes and
> a
> >> transistor sealed in a can, literally. They used a temperature
> compensated
> >> zener diode, consisting of a zener, and a regular diode, turned with
> their
> >> cathodes facing, and connected together. These were placed inside the
> can,
> >> along with a transistor across from them, then sealed. They used this
> same
> >> reference transistor on up through several transfer standards, including
> >> the 732A.
> >>
> >> The Fluke 730A did not use an op-amp after the reference, but used a
> dual
> >> transistor in a can, and created their own comparator amp. Motorola made
> >> several of these along with Fairchild. There are two possibilities here
> in
> >> the parts. I think they used a MD918A or MD918B, which are dual NPN
> silicon
> >> annular transistors, with a Vceo of 15 Vdc, and an Ic of 50 mA. The
> MD918
> >> series was to be used for "differential amplifier applications requiring
> a
> >> matched pair of transistors with a high degree of parameter uniformity
> >> under varying environmental conditions".
> >>
> >> The other possibility on the dual transistor was a 2N2914, which was a
> >> "dual npn silicon annular transistor, especially designed for low-level,
> >> low noise, differential amplifier applications, featuring very high
> Beta,
> >> guaranteed from 10 uAdc to 1 mAdc, and excellent noise characteristics".
> >>
> >> Next, the Fluke 731B used the same voltage reference, but it did not use
> >> the dual transistor amp. Fluke changed this to an op-amp, which turns
> out
> >> to be a LM308. This op-amp was then used in the newer models after the
> >> 731B. Also, this op amp used a few of the same resistor values, and
> >> circuitry, as the two transistor amp.
> >>
> >> Now, a word about "voltage transfer standards". They are not meant to be
> as
> >> stable as a true primary voltage standard, as they are meant to be used
> to
> >> "transfer" a voltage from a known source to other equipment being
> >> calibrated. The transfer standard is made to be recalibrated in one year
> >> increments or closer. Tight tolerances in the short term, in one year or
> >> less, are all that is needed. The Fluke 730A, 731B, and 732A and B are
> all
> >> transfer units. The small voltage transfer standard boards, now sold on
> >> ebay, are meant for calibration every six months, and are not meant to
> be a
> >> long term primary standard. They should be calibrated from another
> transfer
> >> standard, which has been calibrated from a primary voltage standard,
> such
> >> as a Josephson type, by Fluke, or the NIST. The ones calibrated by using
> a
> >> DMM, no matter who made it, are not calibrated correctly. To calibrate a
> >> board correctly requires the use of a voltage transfer standard, that
> was
> >> calibrated by a primary voltage standard, and a very sensitive null
> >> meter-detector such as a Fluke 845A, etc.
> >>
> >> Hope this helps the ones who had a few questions.
> >>
> >> Will
> >>
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