frank.stellmach | 7 Oct 12:50 2015

users of Fluke 750A's ...

Hi  Stephen,

The problem you describe is related to the relatively low  impedance of such divider.This has always to
be taken into account, like the voltage  drop over connection leads.

This special effect (inside the instrument) is not explicitely described in  the 750A, 720A, 752A manuals,
but indirectly in the Fluke book, Calibration -  Philosophy in Practice, 1st edition, p. 52, description
of 7105 system, page 63,where the lead wire resistance error is  calculated. (This can be downloaded
via google  books.)

In your example, you find a 20uV error.
The 750A draws 1mA  from each input voltage applied, so that equals an internal lead resistance of  about 20
milli -Ohm, which is quite reasonable. another internal wiring would not change the game.In this sense, I
do not agree, that the 750A is wired in a wrong manner, this divider techniques generall has its accuracy
limitations.External lead resistances might  be 10 times as high, causing 10 times higher errors.

So this internal error depends on the input voltage,  or equivalently on the divider resistance in use.

A 20ppm error occurs  only at 1.1V input, 10V create 2ppm, and so forth.

This may be  compensated either by using Kelvin sensing of the voltage source, or in some  cases by the 721a,
lead compensator.
These methods are also described in detail in  the Fluke Calibration handbook.


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Stephen Grady | 6 Oct 03:07 2015

users of Fluke 750A's beware

Fellow Volt-Nuts,

I notice that in recent discussions regarding the replacement of the
batteries in Fluke 750A’s seems to indicate that they are still being used
to make precision voltage measurements.

Now I haven’t used one in about 30 years and when I did it was part of a
Fluke 7105A with a Fluke 335D, Fluke 720A, 721A, 750A and 845AB. The voltage
reference was a bank of four standard cells in an enclosure.

Now I am going to describe an issue I had with my Fluke 750A I cannot say
that every Fluke 750A was like this but I feel it is important enough to at
least make everyone who uses these to ensure that theirs is not the same.

The problem comes down to current flow in the divider in my Fluke 750A . In
my divider current flowed between the output low terminal and the input low
terminal and this developed a potential difference of approximately 20 µV
between the input and output low terminals and depending how the divider was
guarded or earthed could lead to an error of approximately 20 µV/V (ppm). 

For those who are reading this out of interest and not familiar with the
750A it is a single unit rack mounted box containing input terminals on the
left hand side and output terminals on the right hand side;  there are also
an input switch (10-1000V) and an output switch (0.1V-1000V); there are also
terminals to connect a standard cell, switches to select the standard cell
voltage, terminals to connect a null meter and a toggle switch to make the
divider – standard cell – null meter circuit. The idea of this divider is
you supply an input voltage to the input terminals and set the input switch
to 1000V and supply approximately 1000V to the input terminals, this
supplies 1000V to the 1000V tap of the divider and if a meter is connected
(Continue reading)

Pete Lancashire | 1 Oct 22:22 2015

Just A FYI _ One the big auction site a Datron 4902

No assoc with the seller etc

mgoldshteyn | 30 Sep 21:31 2015

any voltnuts in the Chicagoland area?

I am looking for fellow voltnuts in the Chicagoland area to share calibration, voltage standards, etc...

This hobby can get quite expensive when the costs don't get shared...

Please reply to the list or direct via e-mail...

David Garrido | 30 Sep 15:22 2015

540b Arrival...........

Hello All,

My new to me 540b arrived last evening.  I have never used one of these so I put it on the bench, opened it up and
inspected it looking for leaks primarily.  EVERYTHING is spotless.  Fluke factory cal stickers
everywhere.  AND………….after plugging her in over night, all of the batteries take a charge and
test in the middle of the BATT scale.


All of the batteries in the 20 cell pack are individually shrink wrapped in light blue and the two others on
top are AA size lithiums.  Two of them side-by-side.  Was this a factory mod?  All the work looks to be factory,
but who knows if the PO was just REALLY good at clean work.

Anyhoo………thanks for the bandwidth.


David Garrido | 28 Sep 21:59 2015

Fluke 5205a Repair

Hello All,

I know this is not specifically discussing precise voltage, but it is about producing it.  We should
consider starting a FLUKE calibration equipment repair forum.

Anyhoo………I have been scratching my head over this 5205a issue for a couple weeks now.  It presents
with a OVERLOAD light after the warm-up period of about 30 seconds. If I try to apply a voltage from the 5200a
I get a FAULT light and I have to reboot the power in order to clear the FAULT led, but the OVERLOAD led still
comes back on after the warm-up period.

Going through the TS procedures on page 4-24 in the manual tell me to check for an issue “…….in the
disabling circuit between the Warm-Up Delay circuit and U8-11 on the logic assy.”

The first place I looked was at both of the bases of Q104 for a difference of more than 125mV. Pin 2 reads -5.83v
and pin 5 reads -5.19v, so obviously it sends it into OVERLOAD mode.

Knowing this, I continued deeper into the circuit.  I am currently working in the Neg HV Sensor circuit on pg.
3-7 and it tells me “If the output (HV) becomes less negative than -2225a, a high output results at the
coupler U52, setting the instrument to standby.”  Pin 3 of U167 is +4.97v, Pin 2 is +6.25v, Pin 7 is +21.1v
and so is Pin 5 of U52.

Now, there are two adjustments in the cal procedure, 4-82 & 4-85, that adjust the POS and NEG Current Trip
Level, but I am not certain I want to disable the OVERLOAD circuit to make this adjustment without knowing
what may be wrong for fear of doing more harm than good.  But on the other hand, this may very well be exactly
what is causing the tripping in the first place.

I could use any help one would offer and the manual is located here: <>

(Continue reading)

John Ackermann N8UR | 27 Sep 16:19 2015

TASS Computer-Controlled Switch System

Every so often, I hijack the list to do some shameless self-promotion 
when I think it would be of interest to the 'nuts community.

I've spent the last several months developing a computer-controlled 
relay switch system.  The impetus was failure of the old HP 59307A GPIB 
switches in my PPS measurement system and discovery that replacement 
relays are unobtainium.  The only similar remote switches I could find 
cost >$1K, which seemed a bit rich.  So, I decided to roll my own.

We ended up with an 8 port switch board controlled by an Arduino.  I 
think it will have lots of T&F applications as well as usefulness in ham 
shacks.  Like virtually everything I do, it ended up as a TAPR kit.

There's an introductory video at

The relay board kit is $119 and we have a shield for the Arduino Mega 
2560 that can control 4 relay boards and costs $19.  The software is 
open source.The boards are now available from TAPR at

(BTW, I contribute my designs to TAPR and don't have any financial 
interest in sales.)

Special thanks to time-nuts Bob Camp and Tom Holmes for a lot of 
engineering assistance.

Here's the TAPR product announcement:

Now Available: The TASS Computer-Controlled Switch System
(Continue reading)

David Garrido | 25 Sep 14:53 2015

Fluke 750a batteries.............

Hello All,

I posted this over at the HP yahoo group as well, so pardon the redundancy if you belong to both or more.

My new to me Fluke 750a reference divider arrived last evening and I have been having one heck of a time
finding info on replacement battery options others have used on their 750a's.  Fluke spec'd (2) 6.7v
mercury cell batteries for the overload protection circuit and these are clearly no longer available. 
What is everyone using for power?  I am hoping to not use a separate DC supply for this app.

The only manual I can find is here: <>

I have been able to find Wein batteries that are 1.35v zinc/air and I could stack and shrink wrap 5 of those in
series to arrive at 6.75 very stable volts, but they are a minimum of $4.50 each cell.  I was hoping to find a
more affordable / elegant solution.

Will the circuit handle (2) 3.6v Li-Ion in series instead?

Any thoughts?


David Garrido | 24 Sep 18:38 2015

Fluke 720a self calibration repair

Hello All,

I am trying to track down what I think is a small issue in the calibration of my new to me 720a.  I am dreadfully
paranoid of jacking something up in the process and need a little hand holding.

I was going through the Self Calibration steps and every thing was going well up to the adjustment of the "A
1.0" decade switch position.  All of the variable resistors up to that point needed to be turned CW in order
to null the appropriate switch position.  The “A 1.0” adjustment pot does not null the switch position
and is not getting any closer than about 2.5uV.  I verified the meter balance between every other step.  I
have checked, in circuit, R302 and R303 and I am getting a VERY SOLID reading of 9.896 on all of those.  Of
course, checking R1006 and R1010 give me significantly different readings than the stipulated 8.45k.

I cannot seem to be able to find a CCW or CW limit to this resistor and I think I read somewhere that there are
supposed to be run from end to end several times if an issue arises.  Do these var resistors have limits?  I
need a little help moving forward.


Ken Goodhew | 17 Sep 01:45 2015

Re: volt-nuts Digest, Vol 73, Issue 9 fluke 540b

	I have a fluke 540b that I am selling cheap  AUD 120 , but postage from Australia would probably be a killer

-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@...] On Behalf Of volt-nuts-request@...
Sent: Thursday, September 17, 2015 2:00 AM
To: volt-nuts@...
Subject: volt-nuts Digest, Vol 73, Issue 9

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   2. Fluke 540B... (Chuck Harris)

Message: 2
Date: Tue, 15 Sep 2015 16:43:08 -0400
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To: Discussion of precise voltage measurement <volt-nuts@...>
(Continue reading)

Hank | 16 Sep 19:05 2015

732A CORRECTION..... Voltage Standard IC PARTS

REALLY sorry guys , when I opened the box , it turned out to be a complete
set of FLUKE 382A voltage standard spare parts kit.  NOT A FLUKE 732A !!

I had a number of Fluke voltage standards, turns out the 382a has a separate
oven controlled 6.2 volt standard ( not a PCB ) however it uses a pcb that
is the oven proportional temp. controller.

I also have an  Fluke 335A voltage standard card that is an operational
spare , this is fully self-contained operational PCB , about the same
voltage ( 6.1xxxxx)  but an older Rev. version.  The newest Fluke 335D uses
a quarter diameter size standard with a solid state heater built into the

SO...   the Fluke 382a Parts and PCBs and voltage standard are all going the
hamfest next month, but willing to sell individual parts/PCB.  The PCBs sell
for $15 at the Hamfest , voltage standard $35.

Willing to negotiate if someone wants more than one 382a part , all
assemblies are available , meter , Calvin Varley divider assembly , and
transformers ( but they are real heavy { double shielded } )

Sorry about the incorrect part number . not a 732a.