Joe
I guess that there are many of us with a reference board from the HP3458A
who would be pleased to have a good design making use of this item. For
those of you who have been around a long time, I recently obtained a
Weston - Rotex DC Absolute Voltage/Current Standard which came from a
Standards Lab. Much to my surprise it still performs very well, and its
interesting to note that each standard output is delivered to a separate
pair of terminals, unlike the Fluke 731B, but more in keeping with the
latter models. So it would seem that the old-timers knew a thing or two
about 'high standards', but at a price of course. It required a mass of
components to produce the result - it would appear to be 1960's
design/manufacture. Are there many of these out there ? - I would be
interested to see its specification as I cannot find any reference to this
item.
Roy
-----Original Message-----
From: Fred Schneider
Sent: Monday, September 17, 2012 3:29 PM
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Fluke 731A Transfer Standard and Fluke
WireWoundResistors
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" <[email protected]> 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: [email protected] [mailto:[email protected]] 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 <[email protected]> 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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