Update on 720A
I've added resistors in series with R1008 and R1044 to bring the resistance
for those parts of the A divider to a value that it would calibrate OK (two
of the resistor sets in the oil bath very low in value). I also had to
remove five turns (about 1.3 Ohms) from R1051 as the combined value of R313
and R314 was too high to allow me to calibrate S2. I also replaced R1041
as it was worn out (almost impossible to get a null).
I've successfully calibrated the C decade as per the maintenance part of the
manual.
Sadly the beast isn't giving me a nice linear voltage sequence when using
just the A decade.
Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to
zero except Decade A.
Decade A Output V
0.1 9.999,88
0.2 19.967,8
0.3 29.936,7
0.4 39.903,8
0.5 49.874,7
0.6 59.855,6
0.7 69.852,4
0.8 79.871,3
0.9 89.919,6
See also attached graph.
Any suggestions on where to look for trouble will be most welcome ...
All sections of the A decade attenuator now measure about 9.999,2 ohms.
It was suggested on eevblog that the next decade (or a subsequent one) was
loading it, but with all the other decades set to 0 I don't quite "get" how.
Thanks
Dave
David:
First suggestion would be to see what the output measures, with the
input still set to 100.0014 volts (same as before), when setting the dials
to:
0.99999X (equivalent to setting the "A" decade to "0.1")
1.99999X (equivalent to setting the "A" decade to "0.2")
2.99999X (equivalent to setting the "A" decade to "0.3")
3.99999X (equivalent to setting the "A" decade to "0.4")
and so on up to
8.99999X (equivalent to setting the "A" decade to "0.9")
What is the output when the dials are set to 9.99999X? It should be
100.0014 (or very, very close) just like the input voltage.
Also what do you mean by your statement "Sadly the beast isn't giving me
a nice linear voltage sequence when using
just the A decade." Do you mean that other settings when decade B, C, D and
so on are not "0", produce ok results?
Bill
----- Original Message -----
From: "David C. Partridge" david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Monday, August 07, 2017 1:49 AM
Subject: [volt-nuts] Update on 720A
I've added resistors in series with R1008 and R1044 to bring the
resistance
for those parts of the A divider to a value that it would calibrate OK
(two
of the resistor sets in the oil bath very low in value). I also had to
remove five turns (about 1.3 Ohms) from R1051 as the combined value of
R313
and R314 was too high to allow me to calibrate S2. I also replaced R1041
as it was worn out (almost impossible to get a null).
I've successfully calibrated the C decade as per the maintenance part of
the
manual.
Sadly the beast isn't giving me a nice linear voltage sequence when using
just the A decade.
Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to
zero except Decade A.
Decade A Output V
0.1 9.999,88
0.2 19.967,8
0.3 29.936,7
0.4 39.903,8
0.5 49.874,7
0.6 59.855,6
0.7 69.852,4
0.8 79.871,3
0.9 89.919,6
See also attached graph.
Any suggestions on where to look for trouble will be most welcome ...
All sections of the A decade attenuator now measure about 9.999,2 ohms.
It was suggested on eevblog that the next decade (or a subsequent one) was
loading it, but with all the other decades set to 0 I don't quite "get"
how.
Thanks
Dave
and follow the instructions there.
David:
Ignore my previous post. In thinking about this while out shopping this
morning I know the answer.
The output resistance of the 720A is about 66k ohms. Your voltmeter has
an input resistance of 10 megohms. You are simply loading the output of the
720A. The reason that the 0.1 ratio has about the expected voltage is
because the 10 Volt range your voltmeter probably has a very high input
resistance. You are on "autorange" and when the output is set for 0.2 ratio
the voltmeter is now on the 20 volt range which is now 10 megohms.
I get the same results as you do when measuring 0.9 ratio on my setup.
89.920 volts. Check your 720A by applying 10 volts and then do your ratio
checks on the first decade.
I just haven't used a 720A for a while and I didn't remember about the
output resistance. When you get to become a "well aged citizen" it takes a
while to recall things.
Bill
----- Original Message -----
From: "David C. Partridge" david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Monday, August 07, 2017 1:49 AM
Subject: [volt-nuts] Update on 720A
I've added resistors in series with R1008 and R1044 to bring the
resistance
for those parts of the A divider to a value that it would calibrate OK
(two
of the resistor sets in the oil bath very low in value). I also had to
remove five turns (about 1.3 Ohms) from R1051 as the combined value of
R313
and R314 was too high to allow me to calibrate S2. I also replaced R1041
as it was worn out (almost impossible to get a null).
I've successfully calibrated the C decade as per the maintenance part of
the
manual.
Sadly the beast isn't giving me a nice linear voltage sequence when using
just the A decade.
Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to
zero except Decade A.
Decade A Output V
0.1 9.999,88
0.2 19.967,8
0.3 29.936,7
0.4 39.903,8
0.5 49.874,7
0.6 59.855,6
0.7 69.852,4
0.8 79.871,3
0.9 89.919,6
See also attached graph.
Any suggestions on where to look for trouble will be most welcome ...
All sections of the A decade attenuator now measure about 9.999,2 ohms.
It was suggested on eevblog that the next decade (or a subsequent one) was
loading it, but with all the other decades set to 0 I don't quite "get"
how.
Thanks
Dave
and follow the instructions there.
Thanks
Dave
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@febo.com] On Behalf Of Bill Gold
Sent: 07 August 2017 19:22
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Update on 720A
David:
Ignore my previous post. In thinking about this while out shopping this morning I know the answer.
The output resistance of the 720A is about 66k ohms. Your voltmeter has an input resistance of 10 megohms. You are simply loading the output of the 720A. The reason that the 0.1 ratio has about the expected voltage is because the 10 Volt range your voltmeter probably has a very high input resistance. You are on "autorange" and when the output is set for 0.2 ratio the voltmeter is now on the 20 volt range which is now 10 megohms.
I get the same results as you do when measuring 0.9 ratio on my setup.
89.920 volts. Check your 720A by applying 10 volts and then do your ratio checks on the first decade.
I just haven't used a 720A for a while and I didn't remember about the output resistance. When you get to become a "well aged citizen" it takes a while to recall things.
Bill
----- Original Message -----
From: "David C. Partridge" david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Monday, August 07, 2017 1:49 AM
Subject: [volt-nuts] Update on 720A
I've added resistors in series with R1008 and R1044 to bring the
resistance
for those parts of the A divider to a value that it would calibrate OK
(two
of the resistor sets in the oil bath very low in value). I also had to
remove five turns (about 1.3 Ohms) from R1051 as the combined value of
R313
and R314 was too high to allow me to calibrate S2. I also replaced R1041
as it was worn out (almost impossible to get a null).
I've successfully calibrated the C decade as per the maintenance part of
the
manual.
Sadly the beast isn't giving me a nice linear voltage sequence when using
just the A decade.
Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to
zero except Decade A.
Decade A Output V
0.1 9.999,88
0.2 19.967,8
0.3 29.936,7
0.4 39.903,8
0.5 49.874,7
0.6 59.855,6
0.7 69.852,4
0.8 79.871,3
0.9 89.919,6
See also attached graph.
Any suggestions on where to look for trouble will be most welcome ...
All sections of the A decade attenuator now measure about 9.999,2 ohms.
It was suggested on eevblog that the next decade (or a subsequent one) was
loading it, but with all the other decades set to 0 I don't quite "get"
how.
Thanks
Dave
and follow the instructions there.
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
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Update: After changing the resistor I added in series with R1008 (A decade position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is slowly increasing in value back towards nominal) and re-calibrating again, the A decade is now within 1ppm of linear (using the 10V range and nominally 10V input).
I note that when I'd done this, with the A decade set to 0.9 (reading 9.000000V on the 3458A), the actual input voltage needed was 10.000040 or so. Is that to be expected?
Two of the positions on the B decade wouldn't quite adjust for a null when calibrating it. In this situation I think I have two options:
1) Add series resistors to compensate for slightly low value 9.898k resistors in the relevant two positions of the decade and recalibrate.
2) Adjust R203 to change the bridge balance slightly to the 10K ohm decades and recalibrate.
Your thoughts on which approach to take is much appreciated.
Thanks
Dave
Hi Dave,
Fundamentally, any input impedance from your 3458 is going
to throw your 720A out of wack. the 720A is not supposed to
have even picoamps of loading.... nothing, nada, zip.
I would suggest that before you damage your 720A further,
you read the manual for the 720A and follow the instructions
to the letter. An HP3458A is not a substitute for a null
detector and a voltage calibrator.
The calibration of a 720A requires that no current, none, zip,
be drawn from the 720A on any port. It can only be done by
using a voltage calibrator, and a null detector, as a
differential voltmeter.
Remember, and heed my words, no current may be drawn from the
720A.
-Chuck Harris
David C. Partridge wrote:
Update: After changing the resistor I added in series with R1008 (A decade
position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is slowly
increasing in value back towards nominal) and re-calibrating again, the A decade
is now within 1ppm of linear (using the 10V range and nominally 10V input).
I note that when I'd done this, with the A decade set to 0.9 (reading 9.000000V on
the 3458A), the actual input voltage needed was 10.000040 or so. Is that to be
expected?
Two of the positions on the B decade wouldn't quite adjust for a null when
calibrating it. In this situation I think I have two options:
-
Add series resistors to compensate for slightly low value 9.898k resistors in
the relevant two positions of the decade and recalibrate. -
Adjust R203 to change the bridge balance slightly to the 10K ohm decades and
recalibrate.
Your thoughts on which approach to take is much appreciated.
Thanks Dave
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instructions there.
Chuck,
I totally get your point that ideally you should use any KVD in null-balance mode. I do note however that you can load it up to 11mA without damage though I've not once gone anywhere near that (worst case insult has been about 0.1mA). However it does also say that to avoid loading errors you should load with an impedance >= 1TeraOhm (10**12).
Clearly a 3458A on the 10V and lower ranges is merely > 10GOhm and will show loading effects.
So, yes, I do recognise I was "doing it wrong"!
I don't believe I've inflicted any damage, and have managed to restore it to a useable state from a fairly sorry condition: When I got it, two oil bath resistors were way off value (202 and 100 ohms low respectively), and the S2 shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 calibration. There were were also sundry other problems like two open circuits in the final decade, a badly worn trim pot, and sundry wires broken at solder joints.
Ideally I'd like to replace the KVD "Bridge Balance" pot as that's a bit noisy but shudder to think what that might cost if I could even find one.
Cheers
Dave
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@febo.com] On Behalf Of Chuck Harris
Sent: 08 August 2017 13:34
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Update on 720A
Hi Dave,
Fundamentally, any input impedance from your 3458 is going to throw your 720A out of wack. the 720A is not supposed to have even picoamps of loading.... nothing, nada, zip.
I would suggest that before you damage your 720A further, you read the manual for the 720A and follow the instructions to the letter. An HP3458A is not a substitute for a null detector and a voltage calibrator.
The calibration of a 720A requires that no current, none, zip, be drawn from the 720A on any port. It can only be done by using a voltage calibrator, and a null detector, as a differential voltmeter.
Remember, and heed my words, no current may be drawn from the 720A.
-Chuck Harris
David C. Partridge wrote:
Update: After changing the resistor I added in series with R1008 (A
decade position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means
that R302 is slowly increasing in value back towards nominal) and
re-calibrating again, the A decade is now within 1ppm of linear (using the 10V range and nominally 10V input).
I note that when I'd done this, with the A decade set to 0.9 (reading
9.000000V on the 3458A), the actual input voltage needed was 10.000040
or so. Is that to be expected?
Two of the positions on the B decade wouldn't quite adjust for a null when
calibrating it. In this situation I think I have two options:
-
Add series resistors to compensate for slightly low value 9.898k
resistors in the relevant two positions of the decade and recalibrate. -
Adjust R203 to change the bridge balance slightly to the 10K ohm
decades and recalibrate.
Your thoughts on which approach to take is much appreciated.
Thanks Dave
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Hi Dave,
The 11ma loading is a threshold where over dissipation will
occur, letting the magic smoke out of the box. The 720A won't
be physically damaged, but will be far from usefully usable
long before that point. No load is the only acceptable loading
of a 720A.
The damage I am referring to is the damage done by you in your
attempt to diagnose and fix the 720A through faulty techniques.
A snip here, a solder joint there, and you can quickly, though
unintentionally, turn the KVD into a worthlessly unstable piece
of junk.
I know that the 720A is yours, and you can treat it anyway you
please, but if your intention is to bring it back to its design
specifications, you are going to have to follow the rules. No
shortcuts. The 720A is a bridge device. It works in that
magical region where no currents are sourced or sinked by the
720A.
Typically, trim pots don't wear out. They are accessed so
infrequently that they never get a chance to wear before something
else renders them unuseable... usually corrosion. It can probably
be saved by a little cleaning. The bridge balance pot may not
be so lucky, though. I don't think it needs to be anything very
special.
-Chuck Harris
David C. Partridge wrote:
Chuck,
I totally get your point that ideally you should use any KVD in null-balance mode.
I do note however that you can load it up to 11mA without damage though I've not
once gone anywhere near that (worst case insult has been about 0.1mA). However
it does also say that to avoid loading errors you should load with an impedance >=
1TeraOhm (10**12).
Clearly a 3458A on the 10V and lower ranges is merely > 10GOhm and will show
loading effects.
So, yes, I do recognise I was "doing it wrong"!
I don't believe I've inflicted any damage, and have managed to restore it to a
useable state from a fairly sorry condition: When I got it, two oil bath
resistors were way off value (202 and 100 ohms low respectively), and the S2
shunt resistors were about 1.3 ohms high which was far enough off to prevent S2
calibration. There were were also sundry other problems like two open circuits
in the final decade, a badly worn trim pot, and sundry wires broken at solder
joints.
Ideally I'd like to replace the KVD "Bridge Balance" pot as that's a bit noisy but
shudder to think what that might cost if I could even find one.
Cheers Dave -----Original Message----- From: volt-nuts
[mailto:volt-nuts-bounces@febo.com] On Behalf Of Chuck Harris Sent: 08 August 2017
13:34 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts]
Update on 720A
Hi Dave,
Fundamentally, any input impedance from your 3458 is going to throw your 720A out
of wack. the 720A is not supposed to have even picoamps of loading.... nothing,
nada, zip.
I would suggest that before you damage your 720A further, you read the manual for
the 720A and follow the instructions to the letter. An HP3458A is not a
substitute for a null detector and a voltage calibrator.
The calibration of a 720A requires that no current, none, zip, be drawn from the
720A on any port. It can only be done by using a voltage calibrator, and a null
detector, as a differential voltmeter.
Remember, and heed my words, no current may be drawn from the 720A.
-Chuck Harris
David C. Partridge wrote:
Update: After changing the resistor I added in series with R1008 (A decade
position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is
slowly increasing in value back towards nominal) and re-calibrating again, the A
decade is now within 1ppm of linear (using the 10V range and nominally 10V
input).
I note that when I'd done this, with the A decade set to 0.9 (reading 9.000000V
on the 3458A), the actual input voltage needed was 10.000040 or so. Is that to
be expected?
Two of the positions on the B decade wouldn't quite adjust for a null when
calibrating it. In this situation I think I have two options:
-
Add series resistors to compensate for slightly low value 9.898k resistors in
the relevant two positions of the decade and recalibrate. -
Adjust R203 to change the bridge balance slightly to the 10K ohm decades and
recalibrate.
Your thoughts on which approach to take is much appreciated.
Thanks Dave
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David wrote:
When I got it, two oil bath resistors were way off value (202 and 100 ohms low respectively), and the S2 shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 calibration. There were were also sundry other problems like two open circuits in the final decade, a badly worn trim pot, and sundry wires broken at solder joints.
The resistors in a 720A -- even the pad resistors -- do not drift that
much by themselves under normal use. Trim pots do not get used enough
to wear out under normal operation. If you have some that are worn, the
instrument has been abused. This is further indicated by the broken
wires. I have never seen a broken wire in a 720A.
Unfortunately, it seems clear that your 720A was traumatized in its
earlier life, and some of the divider resistors were damaged (in
addition to other probable damage, based on your reports). There really
isn't anything you can do but replace the bad resistors with equal or
better parts. Trimming them as you have done is just a temporary
band-aid. Resistors that have been traumatized will never allow the
instrument to meet its time and temperature drift specifications, and
you will be going back inside to replace (and re-replace) pad resistors
every time you want to use it.
Worse, the problem goes deeper than that. Even replacing the resistors
you know are bad will not restore the instrument to its accuracy and
stability specs. The techniques you are using to identify the bad
resistors will only identify ones that have been grossly damaged -- they
will not identify all of the resistors that have been damaged and are
preventing the instrument from meeting its specifications.
Further, the very act of replacing resistors in a 720A is likely to
cause additional damage -- to the resistors you are soldering, the
resistors attached to them, and the infrastructure of the instrument.
Add to this the fact that only a few qualified replacement resistors
will cost more than a properly-working used 720A, and you can see that
repairing a traumatized 720A -- if you can do it at all -- is not
cost-effective for an amateur.
The only 720As I've ever seen that worked correctly after being
traumatized had been repaired by Fluke at astronomical cost. I hate to
say it, but if you expect to use a 720A to calibrate meters of 6-1/2
digits and up, you will eventually learn that you need to replace the
one you have.
Best regards,
Charles
David:
I had the same problem as you when I got my used 720A years ago. Both
the "A" and "B" decade had a few resistors that had changed over the years.
I don't think as bad as you experienced but still they would not balance
within the range of the adjustment pots. So I "adjusted" those that needed
correcting with either parallel or series resistors as necessary. I used
the best tolerance resistors I could get at that time. I figured that the
"adjustment" resistors were such a small amount of the resistors I was
trying to adjust, that the TC wouldn't be much of a problem. However I did
have to go through these "adjustments" several times as I noticed that
correcting one would then affect others in the string. This is probably
what you are experiencing with needing to "readjust" others again. A few
passes in each string finally got the adjustments into a good range and they
have stayed there for years now. As far as I am concerned this is all "good
enough" for hobbyists or VoltNuts who don't want to spend $5,000 or more to
get FLUKE to refurbish your instrument, if FLUKE is even interested in doing
the job and has the parts. Notice that some resistors in the "A" and "B"
decades are selected at the factory anyhow. If you have the money, then be
a "purest". It might be cheaper just to buy a new 720A.
As you realize now the 720A input resistance is 100k. So you will get a
small voltage drop from the voltage source to the input of the 720A,
depending upon the size of the wire you are using. Of course the proper way
to insure that there is no drop is to use Kelvin connections from the
voltage source to the input of the 720A and then set the voltage source to
external Kelvin sensing. What I usually do is to set the ratio dials to
.999999X and then set the voltage supply to get exactly 10.0000000 on my
3458A. Then you have the best chance to get an idea of the accuracy of the
720A ratio at .8, .7, .6 and so on. Don't let the 3458A autorange when down
to the 0.1 ratio! Then to minimize the effects of the "B" decade, use
.899999X, .799999X down to .099999X. The only problem with this method is
that you don't get to check the .9 ratio. So what you can do is connect the
voltage source to the "1.1" input, set the ratio dials to 1.0000000 and then
set the voltage source to give you 10.0000000 volts on the DVM. Now you can
check the .9000000, .8000000 and so on ratios.
I am sure that I did the R203 adjustment as described in paragraph 4-25
of the manual. It has been years now.
All IMHO of course. You are now down into measurement "mud" so to
speak. This is probably why the 3458A development team used the JJA at
Loveland to check the overall linearity of the 3458A A/D. The 720A (or
equivalent) wasn't good enough at 0.1 ppm!
Bill
----- Original Message -----
From: "David C. Partridge" david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Tuesday, August 08, 2017 7:39 AM
Subject: Re: [volt-nuts] Update on 720A
Chuck,
I totally get your point that ideally you should use any KVD in
null-balance mode. I do note however that you can load it up to 11mA
without damage though I've not once gone anywhere near that (worst case
insult has been about 0.1mA). However it does also say that to avoid
loading errors you should load with an impedance >= 1TeraOhm (10**12).
Clearly a 3458A on the 10V and lower ranges is merely > 10GOhm and will
show loading effects.
So, yes, I do recognise I was "doing it wrong"!
I don't believe I've inflicted any damage, and have managed to restore it
to a useable state from a fairly sorry condition: When I got it, two oil
bath resistors were way off value (202 and 100 ohms low respectively), and
the S2 shunt resistors were about 1.3 ohms high which was far enough off to
prevent S2 calibration. There were were also sundry other problems like
two open circuits in the final decade, a badly worn trim pot, and sundry
wires broken at solder joints.
Ideally I'd like to replace the KVD "Bridge Balance" pot as that's a bit
noisy but shudder to think what that might cost if I could even find one.
Cheers
Dave
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces@febo.com] On Behalf Of Chuck
Harris
Sent: 08 August 2017 13:34
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Update on 720A
Hi Dave,
Fundamentally, any input impedance from your 3458 is going to throw your
720A out of wack. the 720A is not supposed to have even picoamps of
loading.... nothing, nada, zip.
I would suggest that before you damage your 720A further, you read the
manual for the 720A and follow the instructions to the letter. An HP3458A
is not a substitute for a null detector and a voltage calibrator.
The calibration of a 720A requires that no current, none, zip, be drawn
from the 720A on any port. It can only be done by using a voltage
calibrator, and a null detector, as a differential voltmeter.
Remember, and heed my words, no current may be drawn from the 720A.
-Chuck Harris
David C. Partridge wrote:
Update: After changing the resistor I added in series with R1008 (A
decade position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means
that R302 is slowly increasing in value back towards nominal) and
re-calibrating again, the A decade is now within 1ppm of linear (using
the 10V range and nominally 10V input).
I note that when I'd done this, with the A decade set to 0.9 (reading
9.000000V on the 3458A), the actual input voltage needed was 10.000040
or so. Is that to be expected?
Two of the positions on the B decade wouldn't quite adjust for a null
when
calibrating it. In this situation I think I have two options:
-
Add series resistors to compensate for slightly low value 9.898k
resistors in the relevant two positions of the decade and recalibrate. -
Adjust R203 to change the bridge balance slightly to the 10K ohm
decades and recalibrate.
Your thoughts on which approach to take is much appreciated.
Thanks Dave
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