[volt-nuts] Voltage standards, Fluke 730A v.s. 731B

[Christopher Brown]

This is personal use, a fun way to fill the long winter time hours.

These days my work is non-travel, so have a house (ok, condo) with a
room set aside for the radio gear and a SGC235 fed inverted L.  Between
the suburban environ, compromise antenna and location, there are alot
more hours to fill than openings.  Spend most evenings listening and
building or repairing something.


This led to trying to put together the kind of workbench I used to drool
over as a kid on the other side of the room.



I have a tendency to take things a bit far (likely a common trait in
this list), if I am going to spend alot of time on something might as
well do it well and make it a challenge.


General flow is finding older but high quality gear, learning, etc.
Alot of it is "tests OK" or "appears to work" stuff, leading to alot of
time doing a complete teardown, cleaning and attempt at repair, service
and calibration.  The critical bits needed to bootstrap things have been
picking up stuff w/ currect NIST traceable cal where I can.


Currently have a basic TBolt setup feeding a 5087A dist amp I finished
cleanup on a few months back.

In current cal

HP 3456A Voltmeter
HP 8657B Signal Generator
HP 5335A TIC
HP 8116A Function Generator
Tek 2465B-DVM Scope

Also have a HP 8935/E6380A spec an, not in current cal, but all of the
periodics done except for the atten recal, and all functions check out
against systems in current cal.


Currently working on

2x 5370B TIC

second 5335A TIC w/ option 40

3586C differential voltmeter

second 3456A (think it has an issue with the A/D converter)

Fluke 750A Ref voltage divider (have not even started trying to verify
this one)

One of the side projects is putting together a better freq ref, dual
tbolts with independant linear supplies w/ good filtering and a properly
compartmentalized enclosure for both shielding and thermal control.


My main set of goals right now is to try and both line op work for the
winter and collect the gear needed to work on the other gear.


Have the 750A, shopping for a ref supply, voltage standard, null meter
or diff voltmeter, etc.

Looking at a Fluke 845AR, up in the air about the rest.

My general thoughts were.


I think I can test/cleanup/cal at least the null-meter function with a
counter, scope and a in cal 3456.

Get a voltage standard, 730A or 731B.  Attempt to clean it up, and put
it in good working order.

I could send off for cal, but I was thinking of the 3456As.  I have one
in cal, and one that is not and should be repaired by then.  I do not
have or have access to all that I need to cal a 3456.  Was considering
finding the best place to do a _full_ service on a 3456.

Figured I could use the freshly aligned 3456A to align the second one,
cal the voltage standard, verify and align the 750A, etc.


My thoughts on the 730A

4 is better than one when your looking at 30 - 50 year old gear, and
long intervals (if ever) between servicing and limited parts availability.

As I understood it, setting aside day to day the thermal variation,
using all 4 internal standards in parallel mode provided a large
improvment in long term stability v.s. a single ref.  My opportunities
to compare with valid external refs will be few and far between.

On the other hand, there are 4 standards inside, and a switching system
on the front end.  More complex, lots more to fail/degrade, maybe very,
very hard to clean/refresh.  Searching for info on the 730A I came up
almost empty.


Seem

On 8/19/12 2:40 AM, WB6BNQ wrote:
> Chris,
> 
> To the best of my recollection, the 730 was the designation of a "rack" that
> contained, essentially, four each 731As.  the 731A's were a decent voltage
> standard that, of course, has been superseded.  Having four references has value,
> if, and only if, you know the temperature coefficient of each of the four
> standards in the rack.  With proper technique, that information can be of great
> benefit compared to having just one standard.
> 
> Having said all of that, unless the temperature coefficients are known for each;
> and you are going to maintain a proper lab environment; and spend the time using
> the "rack" correctly, then there is no distinct advantage.  If the price is right
> and you can get it cheaper then the 731B, then by all means, IF THEY ARE WORKING,
> you would not be hurt by getting it.
> 
> Otherwise, if you are just looking for a working voltage standard then go for a
> 731B and have it calibrated by a responsible credited LAB.  You can possibly save
> calibration costs if you specify that you are only concerned with the 10 volt
> output.  The other output taps are only good if no current is drawn from them by
> using a proper null detector, specifically the Fluke 845.  The 10 volt output has
> a low output impedance providing the ability to handle some small amount of
> loading, such as the nominal 10 Meg Ohm input on good DMM's with very minor error
> due to the loading.
> 
> Bill....WB6BNQ
> 
> 
> Christopher Brown wrote:
> 
>> As I understand it, the 730A refs are a older design, and the one used
>> in the 731B is improved/better.  But, the 730 has 4 that can be run in
>> various combinations.
>>
>> Thoughts anyone?  Benefit of 4 refs in one chassis v.s. issues with the
>> switching combination controls? anything?
>>
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