[volt-nuts] Fluke 731A Transfer Standard and Fluke Wire WoundResistors

[J. L. Trantham]

Pete,

As far as the connection goes, the current resistor wire appears to be
'soldered' to what appear to be gold plated steel wires (very stiff yet
bendable).  If the temperature is relatively stable, it should not be too
bad a problem as far as the repair goes.  However, it seems to drift all
over the place.

I changed the Op Amp to no benefit.

I am leaning to switching to a different voltage reference.  I have a
'spare' reference board from an HP 3458A and it would make a good project to
install this, change the wiring to the Op Amp to adjust the 'other side' of
the Op Amp input to adjust to 10.000000 VDC and still be able to use the
output divider in the 731A to get the rest of the voltages offered by the
731A.

Joe

-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
Behalf Of Pete Lancashire
Sent: Sunday, September 16, 2012 4:03 PM
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Fluke 731A Transfer Standard and Fluke Wire
WoundResistors


There is a good chance the wire is Manganin or an allow that is similar.

If what you use to connect to the resistance wire is not the same you run
into things like EMF (think thermocouple) being created at  the repair.

-pete

On Sun, Sep 16, 2012 at 1:36 PM, J. L. Trantham <jltran at att.net> wrote:
> I have been following the discussion about the LM399 heater, noting 
> mention of the Fluke Wire Wound resistors used often in Fluke 
> precision equipment.
>
>
>
> I have a 731A that I have been resuscitating and have found that the 
> R8A R8B wire wound resistor is failing.  Initially, the source was 
> very unstable then went to about 8 VDC.  This led me to an open R8B.  
> I was able to ‘reconnect’ the broken ends, loosing about half a turn 
> of wire.  Then, again, unstable and went to about 13 VDC.  This led me 
> to an open R8A. Again, able to reconnect, and, again, loosing about ½ 
> turn.
>
>
>
> It appears that the wire is corroded with corrosion leading to 
> failure. This brings up several questions.
>
>
>
> 1.      What are the benefits of these wire wound resistors?  Tempco?
> Ability to construct precise resistance?
> 2.      Is there a source of the resistance wire used in these resistors
so
> that the resistor can be reconstructed?  I think the resistance is in 
> the 30 to 40 ohms per foot range.  Something like 36 to 38 gauge 
> Nichrome 60 would work but it has to be insulated.
> 3.      Would it be better to look for a collection of commercial
resistors
> to ‘replace’ R8A, R8B?  If so, what?  The manual states that IC2, the 
> voltage reference, R6, R7, R9, and R10, are ‘factory selected’ and all 
> appear to be OK.
> 4.      Would it be better to completely abandon the voltage reference
> section of the 731A in favor of a ‘new’ (perhaps LM399, LTZ1000A, 
> etc.,
> based) reference to generate 10 VDC and still use the output divider of
the
> 731A to generate the remaining voltages?
>
>
>
> The 731A manual and schematic are here:
>
>
>
> http://www.ko4bb.com/Manuals/Fluke/Fluke_731A_Transfer_Standard_Instru
> ction_
> Manual.pdf
>
>
>
> R8A is the ‘high’ side of a voltage divider with R8B, R9, R10 and R11 
> (a 10 ohm, 20 turn pot) forming the ‘low’ side, dividing 10 VDC to 
> feed the voltage reference, IC2, a DH80417B.  The op amp, IC1, used to 
> generate the 10 VDC is an LM301AH.
>
>
>
> R8A measures about 5903 ohms, R8B measures about 11558 ohms, R9 
> measures about 78.02 ohms, and R10 measures about 399.8 ohms.
>
>
>
> Thanks in advance for any and all information and suggestions.
>
>
>
> Joe
>
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