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

Fred Schneider pa4tim at gmail.com
Mon Sep 17 14:29:15 UTC 2012


My 731A also drifts all over the place.  The LM399 I am making was first ment to become a replacement for use in the 731A, and like you, use the diverder.  But not any more, I have a LT1027B reference that is on 24/7 for about a year and nice behaving. The 1027 behaves better as my 731A and delivers 10V . Weak point is the resitors i used to adjust it to 10V. ( I was young and.... ;-) ) I plan to change that using better ones and a better construction and after that, use that to feed the divider of the 731A. Also replaced the opamp of the 731A for a better one but that improved only a little.
Then use a chopper opamp with LM10 as buffer on the output of the 731A so its output impedance will be lower and it will be able to be used direct without a null detector, or before a KV devider without collapsing.

Fred PA4TIM

Op 17 sep. 2012 om 14:38 heeft "J. L. Trantham" <jltran at att.net> het volgende geschreven:

> 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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