Restoration : Fluke 720A KVD/Ratio standard

[TiN]

Intro

Something strange happened today. I bought F720A's resistor decades half a year ago, and they were sitting in the lonely box quiet some time.
Today they transformed themselves into complete Fluke 720A Kelvin-Varley Divider box, just like in those sci-fi movies!! 



After transformation:



Exterior



Nothing on the rear, as this box does not have a single semiconductor component in it

Old school Fluke logo, yellowed switches and dirty connectors:



And to make it more exciting and fun learning experience, this poor KVD is...



I don't want to disappoint Dr.Frank with more unverified calibrations, so I had to have KVD for transfers and nV-comparisons between various standards, such as LTZ modules and sources. I had preamp and card for K2002 for quiet a while, thanks to chuckb, but had no divider to calibrate that setup to 2mV,200uV and 20uV levels properly. Hope now with 720A I'll be able to do so eventually. The calibration reference for 720A will be of course 3458A.

Hope we can fix this toy and see what she's got for us.

[Vgkid]

I will be looking forward to this.

[TiN]

I already hear shouting - tear it apart, what you waiting for...

Alright... remove some screws and we're in like Flynn...



CAUTIONs everywhere. Keep children and TiN's away....



Bottom view with glorious oil-filled resistor chamber for most critical decade A.

All components seem to be in place, nothing stolen or missing. Maybe except screw or two...

Internal switch for calibration:



Little board with beautiful Fluke-made teal resistors to do something:



What's with that solder spec near screw?



If I'm reading that correct, datecode on on decade A switch is 6th week 1979, and 37th week 1978 on 720A-3001 board resistors. Older than me!

Function switch:



Take a look on decades more closely. A,B,C:



And F (D and G are same)



I had shown separate decade photos in my previous article, including individual resistor photos.

Bottom PCB have long array of low-resistance trimmer wirewounds, variable 1-turn potentiometers and multiturn trimpots accessible from front panel to implement calibration functionality.



Also connection to input and output binding post caught my eye, seem to be dodgy soldering. Seems like failed repair attempt by previous owner?



Let's see more oil-filled chamber:



Hermetic pass-thru terminals, color coded wires, filling port. Isn't it beautiful?



And some more wiring:





Little 5K KVD-pot and view on front panel trimpots location.

[TiN]

Input resistance measurements shows >1Gohm on any A-decade setting, but sometimes shows 108KOhm if I wiggle it here and there.
But even then 10V applied on input and setting ratio to 0.9xxxxxx gives not 9V but some arbitrary value. No good.

I checked output terminals resistance with HPAK 34970A..



Not sure if values are what supposed to be, yet, need dig that manual.

[Jay_Diddy_B]

TiN,

I am not sure what you are measuring. But, I am quite happy to make some measurements on my 720A for you.

Here is a link to mine: https://www.eevblog.com/forum/testgear/list-your-test-equipment-score-here!/msg946456/#msg946456

Regards,

Jay_Diddy_B

[TiN]

Just output resistance on output HI/LO terminals, each dial (rest of dials set to zero).

Thanks for link, I remember there was thread about 720, but could not find it quick enough.

What is your resistance on input 1.0/LO terminals of 720, with dials set at 1.0000000, in OPER mode?

[Jay_Diddy_B]

Hi TiN and the Group,

My Fluke 720A isn't in perfect condition there are one or two dirty contacts in a couple of places.

From the 1.1 input terminal to Lo the resistance is 110.00 kOhms with dials set to 1.00000000

From the 1.0 input to the Lo the resistance is 99.999 52 k Ohms, with the dials set 0.999 999 9

The 1.0 input with the dials set to 1.000 000 0 is not a valid operating condition but mine measures about 107.5k Ohms.


The input resistance in the manual is

100.00 K +/- 0.005% at 1.0 Input

110.00 k +/- 0.005% at the 1.1 Input

So mine is correct.

I checked the linearity very quickly. I applied 10V from a Fluke 720A to the 1.0 input and Lo. I measured the output with a Datron 1281.

I went through each dial, setting the other dials to zero, and the Datron displayed the correct voltage.

I did have an offset of 6uV (0.6ppm of the 10V input) This is probably thermal EMFs in the leads. I just used random leads.

 The combination of the Fluke 720A and Datron 1281 was near perfect.

I have not adjusted my F 720A at all. There are some built in bridge circuits to adjust the divider.

The output resistance that I measured on the first three decades are very similar to yours.


I would check your output voltage with your (famous) HP3458A using a 10V reference on the input.

Regards,

Jay_Diddy_B

[Macbeth]

I'm thinking this is nearly as easy a fix as the 3245A

Waggle some leads and get open circuit or ~ 100 - 110 kohm? 

[quarks]

Looking forward for more, thanks for sharing

[Dr. Frank]

I don't want to disappoint Dr.Frank with more unverified calibrations, so I had to have KVD for transfers and nV-comparisons between various standards, such as LTZ modules and sources. ...

Illya,

I really apologize, being a square, sometimes. I know, I might have been too critical..

Anyhow, congrats to your nice catch of this 720A, seems to be repairable..


Looking at your pictures, somebody soldered on the switch of  decade B, all these paper numberings do not belong into the instrument.
I assume, this decade has been exchanged.

Your output measurement clearly indicates, that the chain of Decade A resistors is unbroken, all resistors seems to be fine, as a few ppm variation only is visible, (@ 2W measurement!), so it's still very linear.
Decade A switch seems to be ok in first instance, as well as Function Switch.


The whole first decade ( 12 x 10k) is normally assembled in a fixed manner between the "1.1 INPUT " and "Low Input", so you must always measure constant 110k, disregarding any setting of Decade A switch and the Function switch. You must always measure constant 100k between "1 Input"  and "Low Input".

Therefore, if you measure open, or unstable, then there are two possibilities only (in order of probability):

1) R 1047, that is sort of contact resistance compensation, is defect, or not assembled correctly
2) Somebody unsoldered connection from "1.1 Input", "1.0 Input" and "/ or "Low Input" and did not re-assemble that correctly.
3) the cables, involved in 2), may be broken.

So check at first, if "Low Input" has a low impedance connection to pin 1 of oil bath assembly, or to "Low Output".
These junctions (and maybe the cables) are suspect, so check these, and measure connection from there to resistor chain of Decade A:



Check at first the fat black and red cables from the input jacks, if they are broken.
 
I think, if you find that bug, the divider will work principally, directly after repair.
Proper precision operation will be another point.. a lot of debris is visible, which might require careful cleaning.

Frank

[TiN]

I really apologize, being a square, sometimes. I know, I might have been too critical..

Nothing to apologize about, it's always a good thing to get some fresh cold vision on things. We all easy to get overexited of our own work or buying 8.5-digit meter, thinking "oh, NOW i can measure stuff", and proceed with horrid mistakes, only measuring errors to 8.5-digit precision. That's what forums are for, design reviews, eh?   

all these paper numberings do not belong into the instrument.

That was my thinking as well. I did not see much 720A photos on web, but those I saw never had any paper labels anywhere.

I'll have to draw a partial schematics, which wires go where, to make clear vision how all interconnected. After that isolation of the issue would be matter of minutes.

Not sure if that will be done these days, as there is something bigger going on with my homelab (no, not getting second rusty 3458A , so this project might suffer little lack of attention next few weeks.
I don't want to rush into fixing this, as 720A not kind of instrument to accept ungraceful handling and user mistakes.

[Dr. Frank]

I really apologize, being a square, sometimes. I know, I might have been too critical..

Nothing to apologize about, it's always a good thing to get some fresh cold vision on things. We all easy to get overexited of our own work or buying 8.5-digit meter, thinking "oh, NOW i can measure stuff", and proceed with horrid mistakes, only measuring errors to 8.5-digit precision. That's what forums are for, design reviews, eh?   

Thank you for your kind words 

I'll have to draw a partial schematics, which wires go where, to make clear vision how all interconnected. After that isolation of the issue would be matter of minutes.

Not sure if that will be done these days, as there is something bigger going on with my homelab (no, not getting second rusty 3458A , so this project might suffer little lack of attention next few weeks.
I don't want to rush into fixing this, as 720A not kind of instrument to accept ungraceful handling and user mistakes.

That's a pity.. I assume this will really be an easy repair.
The labelled switch is not involved, that may very well do fine.

If I understand your description of the input resistance behaviour correctly, the error definitely has to be a broken solder junction, or this black cable from "Input Low" is broken. That's one simple Ohm measurement, perhaps.

Frank

[TiN]

Wires to decade switch A, R1047 and wires are OK.
But resistance check between oil tank pins 22 and 23 is open.
Resistance across 23 and 24 is 9.896K. 20-19 and so on are 7.2xxK.

Also from 23 to Input 1.0 is open.

[Jay_Diddy_B]

Hi group,

TiN wrote:


Wires to decade switch A, R1047 and wires are OK.
But resistance check between oil tank pins 22 and 23 is open.
Resistance across 23 and 24 is 9.896K. 20-19 and so on are 7.2xxK.

Also from 23 to Input 1.0 is open.



For those following along at home, here is a partial schematic:



The full schematic is included in the 720A manual.

Link: http://us.flukecal.com/literature/product-manuals/720a-instruction-manual

Hopefully it is a wire somewhere.

Regards,

Jay_Diddy_B

[lowimpedance]

Looking at the oil tank filling port 'plug' , the cap screw seems out of place as a plug that fluke would have used  not sure without peeking inside another one !.

[Dr. Frank]

Me and my big mouth.. this section is (of course) not included in your output resistance measurement.

So the error is definitely (again?) in the trimming section of R302, (that is the 9.896k resistor between pin 21 and pin 22).

Either R1008 is open (~5 Ohm), or one of the 3 cables, between pin 23 and R1005/R1007, or R1006/1007 towards R1008, or between R1008 and pin 22.
 
Maybe the 4 fold junction is open, where the cable from "1.0 Input", from pin 23, to R1005/R1007 and to S1B meet.
Frank

[Jay_Diddy_B]

Looking at the oil tank filling port 'plug' , the cap screw seems out of place as a plug that fluke would have used  not sure without peeking inside another one !.

TiN has showed us his oil plug, here is a picture of mine:




I think they look pretty similar.

Here are some more pictures from my unit, serial number 164.

Top View



Cal switch and decades A,B and C






The Oil Tank with matching serial number



The adjustment wire R1047




Regards,
Jay_Diddy_B

[TiN]

Can someone take good pic of dec.A switch from bottom side (where oil resistor visible).
I checked wire from pin 22 to trim network - its ok, R1008 is ok too around 5.2 ohm, but pin 23  does not come to trim network, but just to switch S1B position.
Also Input 1.0 does not go same pin too. Seems wrong wiring on switch S1. As I clearly can tell it was soldered badly.

[ManateeMafia]

Here you go.

[Dr. Frank]

Gotcha, two cables missing, at the brown "1.0 Input" cable.
One of them is pin 23, black and white colored.

Frank

[Jay_Diddy_B]

Hi,

Here are my pictures of the same thing 






(Where else can you get pictures of switch) 

Jay_Diddy_B

[Jay_Diddy_B]

Hi group,

There are actually two black and white wires attached to the brown wire (1.0 input) and the switch:



Jay_Diddy_B

[ManateeMafia]

I recently picked up a second "720A" from an auction recently. I grabbed it for parts so wasn't expecting much.

I wanted to make some comparison measurements for TiN but something is definitely wrong here.... FYI, all resistors have 82 datecodes. No mica ww in this beast. Anybody recognize the sig on the backside of the pcb's?

The pics I uploaded are from 720A #1.

[TiN]

(Where else can you get pictures of switch)

....inside of 20000$USD resistor....

You guys the Best!

Here's what I see here:





Hmm, what is connected that point on to Decade B ?

[TiN]

...

That is no 720A....    I'd expect no 0.1ppm out of it, whatever it is 

[Dr. Frank]

...

That is no 720A....    I'd expect no 0.1ppm out of it, whatever it is 

Heads up, TiN!

You'll restore this instrument, as precisely, as you've restored this wrecked 3458A.



Seems to be your fate--

And you will be able to check its 0.1ppm linearity by your 3458A (which is superior)

Frank

[zlymex]

Very interesting thread with a lot of hi-res photos.
I have 4 720A and 2 of them had broken dec.A switch because the damage of the plastic joint inside, I have to fixed them by disassemble and glue.

[TiN]

Well, I put black wires on thick brown wire position, and now 1.0 input is 70.000K, 1.1 is 80.000K, on any A position. Closer, but not expected 100K.
Something else is off still.

[TiN]

Aha, I think I know what it is.
Taking second look on my pics and yours - all thin wire pairs from oil tank resistor network are shifted one position counterclockwise!
E.g. black thin wire pair should go to position 1, but goes to INPUT LO/R1047 shunt thing instead!

Lemme fix it!

[Jay_Diddy_B]

Hi,

Captain Obvious says ' I think your unit has other wiring problems, It looks like one or two of the switches were changed'.

ManateeMafia

If the resistors didn't have '82 date codes, it looks like the kind of simplification that IET Labs might have made.

Does anybody have pictures of the inside of a KVD-700?

Link: http://www.ietlabs.com/voltage-divider/kvd-700.html

Regards,

Jay_Diddy_B

[ManateeMafia]

That was one of my initial thoughts but didn't see anything other than an oil can like the 720A in their current model. It may have been an initial attempt but decade A probably isn't good enough.

[TiN]

Don't quote me, but I was under impression that KVD-700 using BMF resistors. Remember reading that somewhere.

Swapped wiring +1 position, it's now 80K on 1.0 input and 90K on 1.1. We getting there, lol.

[TiN]

It also have now fixed 40k on output, does not matter which dials i set.
Fixed 1:2 divider! 

[Dr. Frank]

All parts seem to be originally from FLUKE.

Decade B (and maybe also C) seem to be replaced in 1998 or later, but the technician failed to do it right and randomly soldered wires in the wrong places.
(And he made horrible solder joints, btw.)

As far as I can see in the pictures, the basic circuitry is unchanged..

Well he only changed a part of the wires of DEC A, left (seen from back of switch) of the brown wire, seemed to be ok., so maybe 3 or four pairs to the right were wrong, only


Frank

[ManateeMafia]

It also have now fixed 40k on output, does not matter which dials i set.
Fixed 1:2 divider! 

I thought someone was trying to build a random voltage divider. My bad.

[Jay_Diddy_B]

TiN,

would any more pictures help?

Jay_Diddy_B

[TiN]

Measured resistances of oiltank.
25-26 is 250 ohm. If i switch S1 to CAL and S8 to CK B (opening 25-26 R313-R314) they read 24.78K.

14-13, 12-11, 10-9 are each 103.7 ohm.
Rest of resistors are expected 9.896K

[Jay_Diddy_B]

Measured resistances of oiltank.
25-26 is 250 ohm.
14-13, 12-11, 10-9 are each 103.7 ohm.
Rest of resistors are expected 9.896K

Is this with the wire disconnected from the oil tank?

You might want to disconnect the wires.

You don't want the problem to be in TiN can. (sorry)

Jay_Diddy_B

[TiN]

All wires are intact. And it should read correct value, as it's just series resistor chain.

I updated previous post, R313-314 are ok.

[TiN]

Positions on decade A switch from .6 (blue wire) to .4 (orange) are dead short.

[Jay_Diddy_B]

TiN,

I am suggesting that you remove the wires from the oil can to check the resistors. This to make sure that wiring errors by the previous owner aren't messing with the readings.

Jay_Diddy_B

[TiN]

I think it's safe to say problem on the switch. I don't want to apply any heat on oil can. It's not necessary

I checked my spare switch from Dec.A bought before - no shorts on those pins. So I'll just replace switch next time.

[Macbeth]

I highly doubt the oil tank resistors would fail by going low. Resistors tend to go high or open circuit when they fail. Definitely think this is the bodge wiring from the previous "repair"

[TiN]

Here's good switch photo. I'll install this one and take apart faulty to see what's wrong with it. I think the brush finger got damaged and likely one of brush leaves is just stuck across three adjacent contacts.
This is what my crystal ball shows, at least.



As you can see, no shorts across three positions should be there in any way. So switch is shorting oil tank resistors via trimmer resistors. Hence the 103.x ohm reading across oil 9K resistor.

Also acrylic holders for rotary knob shafts are cracked and damaged on decades D and E, so i'll have to replace those too. I'll have to find where did I toss them.

[Vgkid]

I recently picked up a second "720A" from an auction recently. I grabbed it for parts so wasn't expecting much.

I wanted to make some comparison measurements for TiN but something is definitely wrong here.... FYI, all resistors have 82 datecodes. No mica ww in this beast. Anybody recognize the sig on the backside of the pcb's?

The pics I uploaded are from 720A #1.

interesting. Those resistors appear to be kelvin hermetic WW resistors. I believe that either the high end datron/Solartrons used them.

[lowimpedance]

Looking at the oil tank filling port 'plug' , the cap screw seems out of place as a plug that fluke would have used  not sure without peeking inside another one !.

TiN has showed us his oil plug, here is a picture of mine:
I think they look pretty similar.

Regards,
Jay_Diddy_B

Ah yes I also had a look at an old 720a at work and indeed the cap screw is normal ! (was wondering before if the oil tank had been tampered with given the 'other stuff' thats been done.)

Anyway while I was looking I took some more pictures for reference if needed.
One interesting mechanical failure which I suspect most of these oldies suffer from is the acrylic standoffs craze then crack and fall apart. see pic.

[doktor pyta]

I just found pictures of my 720A.

[TiN]

I've got original switch out. It immediately fallen apart in front section.



I think it's not supposed to be like that, and shaft supposed to be one long plastic driver. Perhaps someone tried to glue it together before?

[TiN]

Meh, anyone can help to confirm where thick green/white wire goes on Dec.A switch? Is it same as thick red from Dec.B ?

[zlymex]

I think it's not supposed to be like that, and shaft supposed to be one long plastic driver. Perhaps someone tried to glue it together before?

It broke at exactly the same position as mine. Apparently, they use several brushes and more force on the brushes to minimize the contact resistance, therefore easy to break the shaft joint because more torque is needed to operate the switch.

[TiN]

I guessed wiring correctly



Measured input current, ~100uA. Hooked input to LTZ1000 (3458A). No adjustments performed yet.

Also excuse the inproper wiring...

[WN1X]

Winner winner chicken dinner! 

[Vgkid]

Nice. Assuming that both meters were cal'd identically/ reading the same. That kvd is only out by 1 counts.
.7006129
vs.
.700612797

[Macbeth]

Well done! The adjustments and characterisation will be interesting.

Unfortunately my 6 decade esi RV622A doesn't have adjustments. Some very specific settings are as high as 10ppm out, but most appear to be within 1-2ppm. (Well with my utterly crude setup anyway)



But at least it has metal shafts! 



Ok, I know - not anywhere in the same league... 

[TiN]

Taking of shafts.

I'm replacing acrylic bad ones with plastic from my 720A parts box. Took few quick measurements to make 3D CAD in case some poor voltnut want to 3D-print some, as I don't have access to this advanced technology.

PDF-drawing
3D STEP214 model file.

.

Next step - assembly and full calibration against 3458A. 

[VintageNut]

Measuring the output of the KVD with your DMM introduces an error. At a KVD setting of 0.7XXXXXX the output impedance is about 70,000. Thats about 7ppm loading with a 10G DMM. If the DMM is 10M input, the loading is 7000 ppm.

I use my DMM as a null detector and compare the output of a KVD to an identical voltage. Adjust one or the other voltage until the difference is as close to zero as you can get. In my office, the air conditioning and the unshielded cables make the individual microvolts not extremely stable.

[zlymex]

Linear had to select the buffer amp LTC1152 to meet <10pA Ib in AN86(page 9), otherwise the 100pA maximum Ib will produce possible 0.7ppm error at 70k output impedance of the KVD.

I was wondering why Linear did not choose LTC1052 over LTC1152, the former is better in almost every aspects. Datron used many pieces of LTC1052 in 4910 DC reference.

[Kleinstein]

The LTC1052 is an old type, still using external capacitors. This allows using a lower chopper frequency and thus less bias current due to charge injection. But the external caps are also a little more difficult to handle and can pick up "noise" if the layout is not good.

The LTC1152 does not look like especially high performance (except low noise in the high frequency range) - more like a first try to have the caps inside. There should be better suited AZ OPs with internal caps now. Worst case bias current could be corrected: at least it is reasonably constant with AZ OPs. If time is not that critical one could even chose one with more noise to get lower bias.

[EmmanuelFaure]

@ TiN : I love your box

[TiN]

Lol, I have no idea whatever that says.

I'll be working on F845AB restore to get it working again, perhaps it would be a better choice for KVD output?
3458A would be then reading the input voltage from MFC.

[VintageNut]

The Fluke 845 is a very good choice for the null meter.  Any good microvolt meter will work. When you have null, there are at most a few uV across 1M ohm and so the current drawn is a few pA.

If the uV meter is adjusted well, you can look at individual ppm or sub-ppm and know that the current in the KVD is not causing large error.

In my equipment stash, I have qty 2 of the Keithley 147 that were not expensive to buy. The fluke 845 is crazy expensive in the USA on eBay. I paid $20 for one of my KE147. The seller did not realize that you have to install a short to the input to center the meter needle. The meters drift to one side quickly with an open input. When I received it, I installed the short and then turned it on and it works perfectly. The KE147 has 30nV full scale range up to 100mV full scale in 1-3-10-30.... range increments. It is easy to calibrate against any good DMM. The input cable is expensive but Keihtley still produces the cable for this nV meter. Other obsolete digital nV meters use the same cable. The cable is solid copper, low thermal and is good for the null meter application.

In my office, playing with individual ppm measurements with the KVD and the null meter, it is difficult to get perfectly stable null measurements. The air conditioning and the unshielded cables induce microvolts of drift, noise and offset. I am in the process of buying shielded twisted pair cable with solid copper single-wire for each of the two signals in the cable.

Slicing 1uV into 10 parts riding on a 10.00000V signal is a challenge but it can be done with very a old KVD and a very old uV null meter. The tricks are stable temperature, air flow, low thermal cables with good shielding.

[EmmanuelFaure]

@ TiN : Something like "My jewellery box . The things I love bring back many memories."

[Macbeth]

@ TiN : Something like "My jewellery box . The things I love brings back many memories."

Nooo! Don't tell me TiN has got a new girlfriend, devoting time to her instead of EEVBlog or xdevs?! "TiN and GF sitting in a tree, Kay Eye Ess Ess Eye En Gee"    

[VintageNut]

Hello TiN

If you want to "read" the KVD , you should use a buffer with low pA input bias current. The Jim William appnote is a perfect example.

Normally, the KVD creates a voltage to compare to an unknown voltage.

I insert a picture below to show using the KVD. I have a Fluke 731B driving the 721A Lead Compensator which in turn drives my KVD and it drives my home-brew bridge of two 10k resistors.

I balance the bridge with a piece of wire. I tap the wire to find the balance point. The polarity of the bridge is flipped to look at offset. Moving the wire tap and adjusting the KVD for a "perfect" balance with no offset lets you know that the bridge is balanced and when you are finished the reading on the KVD is exactly 1/2.

My KVD is low by 5 out of 5,000,000 so it is low 1ppm. I paid $300 for this KVD and I am very pleased with it.

The DMM7510 is the null meter.

My plan is to use the bridge inside a DIY cascade divider to adjust every 10V in the bottom 10 resistors to be exactly the same as my 731B 10V.

[TiN]

VintageNut

Yes, I plan to use buffer amp to try first.
Perhaps you can you plot a schematic of your setup?

I don't have 721A Lead Compensator, so thinking ways how I can get away without using it. I have multiple battery powered LTZ sources instead and 845AB (which need to get fixed first), so perhaps if I can get away without 721A if keep isolation high?

[VintageNut]

Lead compensation is mandatory if you want to compare the KVD output to another voltage.

The KVD input draws current which induces voltages into all of the cables/wires between the voltage source that drives the KVD and the KVD input.

You have to balance the voltages of the wiring so that there is no voltage difference between the KVD HI and the HI of your divider under test.

The same for LO. You have to balance the voltages in wiring so that there is no voltage difference between the KVD LO and the LO of the divider under test.

I will post the Fluke diagram for the test setup. It is in the KVD manual and the 721A manual.

[VintageNut]

Here you go. This is the basic idea. Variations of the basic idea are used to compare an unknown resistance against a known standard resistor.

Current is always going to flow in the KVD input.  You just do not want current flowing out of the KVD output. If the two sides are balanced and equal, the null meter has 0V across it and there is no current flowing out of the KVD output.

[TiN]

I get that, however my current task is not exactly comparison to unknown divider but calibration of 720 itself, using 3458A as linearity reference. I'll follow initial procedure from manual first.

Alternative idea I have instead of getting 721 might be use of SMU to inject compensating current perhaps to get null..

[VintageNut]

What will you be using to drive the KVD? You have to account for the voltage drop in the cables between the driving voltage and the KVD and the voltage drop inside of your voltage source because of the loading of the KVD on your voltage source.

For example, I drive my KVD with a Fluke 731B. The 731B manual discusses how much error is introduced by loading the 731B. Driving a 100K KVD introduces an error of 0.5ppm. This probably assumes that there is no voltage drop in the cables between the voltage source and the KVD. Any voltage drop in the cables is more error.

You may be able to use your 3458A measure the voltage drop in the cables between the voltage source and the KVD.

If you build a buffer amp to isolate the KVD output, you have to characterize all of the uV of error in the wiring of your buffer amp and in the cabling between the KVD and the amp and between the amp and your 3458A.

Its not a simple plug and play situation.

A null meter with a 1 uV full-scale range or even lower is a good instrument to show you where uV are induced. An analog nanovoltmeter is even more sensitive.   

If you have the Keihtley 1801 clone working, that is the best digital nanovoltmeter on the planet.

[TiN]

HP 3245A 10V will be used to drive KVD. I already confirmed it to be stable within 0.5ppm over 24 hours after pimp-up with my LTZ module. Current compliance of this source 100mA, 0.5 ohm output impedance.

Measurement of input voltage will be done by KVD via 3458A, output by buffered 2002.
I'm not looking for plug and pray solution, so it's alright to go round way for the goal.

My A10-nV setup not ready yet, so this 720A project is another checkpoint to get nV-setup going. I need to build LNA and nV-divider to complete the cycle.
Fluke 845AB and Keithley 182M will act as extra meters for sanity checks.

[VintageNut]

Sounds like you have everything well planned. I look forward to seeing results.

For nV divider, the Keithey 260 claims to use all copper components. I have not performed a teardown of the divider since it is enclosed and it works well. What are you planning for the divider?

[TiN]

For divider something DIY will be used. Like Keithley 262, but without switches.

[Echo88]

I recently aquired a Keithley 720A, sold as used but working. Turns out its was defective. 

Failures were as following:
The input resistance in the 1.0 and 1.1 range were 110 and 120kR instead of 100 and 110kR as stated in the manual, the output resistance didnt follow Figure 2-3 (Dial-Setting determines output resistance) and the function switch S8 was very stubborn and would only switch smoothly between about 3-functions instead of all functions. The optical inspection revealed nothing, everything was nice and without obvious errors.

After making a drawing of the function switch S8 and comparing it to the schematic, measuring the whole stuff and so on i concluded that the function switch S8 doesnt work, because it doesnt shunt the decade resistors (for example R301 and R302 shunted by Shunt S2). Thats why the input resistance was 10kR high on both input-ranges. After bypassing the S8-switch between R1051 and S1C (so that it will shunt at least the A-decade correctly) i could get the A-decade to work halfway properly and the input resistance was according to the manual again.

Now it was time to disassemble the S8-switch and see if it was really the culprit: Indeed, the plastic-clutch was broken and the remaining plastic crumbs led to the stubbornness of the switch, while the wipers of the switch didnt move. I think it maybe repairable, but wanted to ask here first what would be the best stuff to glue it back together? I thought of cleaning the metal-shaft on the switch-cover and the remaining plastic shaft in the switch and then gluing it together with epoxy. Since theres also a thread on the metal-shaft and in the plastic-shaft it might have enough surface to properly transmit the torque. Or maybe someone here has such a switch lying around and wants to sell it to me?   

Also: After drawing the connections of the S8-switch and measuring the connections in the A-decade and the S2-shunt it appears that the schematic is not fully correct i think (but i might be very well wrong). For example: (look at the lower left side on the schematic) between the Resistor R1051 and Pin 26 of the R314 in the oil tank i measure 0R instead of about 250R (R1048 + R1049 || R1050), so it seems R1051 and the trimmer-combo are interchanged in position on the schematic, while working fine of course.

[Conrad Hoffman]

Comparing KVDs, I've used a "poor man's compensator", a balancer actually. It's just a length of bare copper wire between the two units, with a copper alligator clip attached at the right point between them. Or soldered. Allows a few ppm of adjustment with minimal thermals or added resistance.

[plesa]

I recently aquired a Keithley 720A, sold as used but working. Turns out its was defective. 

Failures were as following:
The input resistance in the 1.0 and 1.1 range were 110 and 120kR instead of 100 and 110kR as stated in the manual, the output resistance didnt follow Figure 2-3 (Dial-Setting determines output resistance) and the function switch S8 was very stubborn and would only switch smoothly between about 3-functions instead of all functions. The optical inspection revealed nothing, everything was nice and without obvious errors.

After making a drawing of the function switch S8 and comparing it to the schematic, measuring the whole stuff and so on i concluded that the function switch S8 doesnt work, because it doesnt shunt the decade resistors (for example R301 and R302 shunted by Shunt S2). Thats why the input resistance was 10kR high on both input-ranges. After bypassing the S8-switch between R1051 and S1C (so that it will shunt at least the A-decade correctly) i could get the A-decade to work halfway properly and the input resistance was according to the manual again.

Now it was time to disassemble the S8-switch and see if it was really the culprit: Indeed, the plastic-clutch was broken and the remaining plastic crumbs led to the stubbornness of the switch, while the wipers of the switch didnt move. I think it maybe repairable, but wanted to ask here first what would be the best stuff to glue it back together? I thought of cleaning the metal-shaft on the switch-cover and the remaining plastic shaft in the switch and then gluing it together with epoxy. Since theres also a thread on the metal-shaft and in the plastic-shaft it might have enough surface to properly transmit the torque. Or maybe someone here has such a switch lying around and wants to sell it to me?   

Also: After drawing the connections of the S8-switch and measuring the connections in the A-decade and the S2-shunt it appears that the schematic is not fully correct i think (but i might be very well wrong). For example: (look at the lower left side on the schematic) between the Resistor R1051 and Pin 26 of the R314 in the oil tank i measure 0R instead of about 250R (R1048 + R1049 || R1050), so it seems R1051 and the trimmer-combo are interchanged in position on the schematic, while working fine of course.

I have unit with same symptoms. But I was unable to find the spare part switch. Is this switch still available on market?

[Vgkid]

I have unit with same symptoms. But I was unable to find the spare part switch. Is this switch still available on market?

Switch info here:
https://www.eevblog.com/forum/metrology/4-wire-switch-selectable-resistance-standard/msg1193999/#msg1193999

[Echo88]

Thanks for the info. I will try to repair the switch next week with epoxy and hoping for the best, while searching on ebay for a similar one.

Also: Got the KVD running again after carefully rotating the shaft with the brushes in the S8-function switch to the OPR-function with long nose pliers and soldering it back in for a sanity check. It performs beautifully, apart from the 7. decade which seems to be partly corroded on the switch and therefore gives false readings in some positions.

[Echo88]

I could successfully repair the S8-switch by cleaning and then glueing (epoxy, "Uhu Endfest 300") the metal-thread-shaft from the switch-front to the remains of the plastic-shaft, which rotate the bushings. The switch now performs really nice and hopefully lasts as long as i use the KVD (fingers crossed).

Since the plastic shaft was badly damaged and very short, i didnt see any possibilty to repair the switch by means of a shaft coupler, therefore i used the glue as a last resort. If the switch breaks again at some point, it might be not repairable anymore since one cant open it due to the glueing (if it doesnt break at the glued shaft-transition).

[TiN]

Time to come back to this poor abandoned project.

What is better than fixing Fluke 720A?? 



Fixing two of them! Second one got delivered today.

Somebody vandalized poor KVD, and gutted kelvin-varley potentiometer for bridge balance (lucky for me, I have one spare!) and Fluke factory trim low ohm (4-6 ohm) wirewounds for decade A calibration. 



Perhaps after some time, previous owner got hit by regrets and remorse, so they included a bag with removed wirewounds in the package. All but missing one 



Lucky for me again, i have 10+pcs of these too.

I guess idea is to play some tetris to find the proper ones to go proper spots... 



TBD...

[Echo88]

And after that you can use both KVDs to rebuild the setup from the Jim Williams AN86 with 3x 3458A and "spend
many delightful hours “surfing the Kelvin”". 

[TiN]

I'm missing 3rd 3458.... 

[ManateeMafia]

Put a 2002 in 3458A emulation mode. No one will ever know.

[e61_phil]

Put a 2002 in 3458A emulation mode. No one will ever know.

I doubt that
The 2002 isn't nearly as linear as the 3458A. It is more in the league of a 34401A (talking about linearity only)

[TiN]

The 2002 isn't nearly as linear as the 3458A. It is more in the league of a 34401A (talking about linearity only)

Maybe 2002 isn't as linear as HP, but saying K2002 is in league of 34401a is understatement at least
Here's what I got from today's test on one of the meters.



If we stay over 15% of the FS, then total system (MFC+DMM) linearity is <0.2ppm.

[e61_phil]

Hi Tin,

to what value are your ppm's referenced to? 20V?

I attached the meaurement of my HP 34401A after ADC adjustment. Deviation is referenced to 10V. The measurement attached is measured against my Fluke 5440B. A measurement against a 3458A looks quite similar.

This kind of linearity is what I measured over and over again on many 34401A's. Dr. Frank showed similar results here in the forum.
It seems to me that the Multislope III is much better in linearity than the (faster) Multislope IV (beginning with 34410A to the actual units).


Edit: Here I measured the Keithley 2002 against two 3458As. For me it looks like the K2002 is in the league of a 34401A (we are talking about linearity only!)
https://www.eevblog.com/forum/metrology/dmm-linearity-comparison/msg1354870/#msg1354870

[TiN]

Reference is to calibrator output value, not range. I can try your script once I have meters free (this test was part of my calibration check report procedure, its not targeted to pure INL tests anyway).

[meandeev]

I bought a 720a from a legacy in unknown condition and the shafts are broken, too. Also ALL plastic standoffs are broken, as lowimpedance wrote.

...
One interesting mechanical failure which I suspect most of these oldies suffer from is the acrylic standoffs craze then crack and fall apart. see pic.
...

What material are these plastic standoffs? Can I use PMMA/Acryl tubes for this?

@TiN:
Which material and technology did you use for the black shaft? (It seems not the "normal" FDM-3D printing. It looks like ABS with injection molding technique...)
I can see, that your standoffs of the faulty wired switch are also made of a different white material. Do you know, what it is?

Taking of shafts.

I'm replacing acrylic bad ones with plastic from my 720A parts box. Took few quick measurements to make 3D CAD in case some poor voltnut want to 3D-print some, as I don't have access to this advanced technology.

....

[branadic]

Looks like some thermoset material. FDM printed thermoplastic material would probably be to soft and could break pretty easy. Maybe you can use extruded PMMA, if you plan to machine them. If you plan to print them, you can use SLS parts that have been infiltrated  or use DLP printed resin parts.

-branadic-

[TiN]

Replacement black shaft is not 3D printed , it is spare part from other 720A.

[meandeev]

Now I used 4/10mm PMMA (acrylic plasic) tube for the standoffs (see the picture). The original screw sockets fit very well 
For the broken shafts: a few drops of dichlormethane make them strong again (so it seems it´s really PMMA!)

But then I measured the input resistance: 80k-ohm. I´m afraid of a broken decade A switch 

to be continued...

[LKM]

Hello all - I've been watching this thread with interest as I have a sick 720A here with a broken shaft coupler. I bought some replacement couplers from Fluke recently - I will post a picture and P/N in a follow-up later.

I was wondering - can anyone recommend a procedure for removing/replacing the couplers for example in decade B or C? Is everyone taking the dials off? I'd like to minimize how far I have to disassemble the unit.

[LKM]

OK, much delayed but as promised current Fluke P/N for these shafts is 701979. I've attached a photo of the order envelope.

Also attached is a photo showing the 720A mid-replacement on one of the decade switches. I marked the dial position with some nail polish but a sharpie would work as well. I then loosened the two dial set-screws and unscrewed the metal coupler thing mounted to the bulkhead. This let me slide the metal shaft out the front just enough to play Operation and get the old broken acrylic shaft out and get the new black plastic (Nylon?!) one in. Buttoned it all up and the switch works great.

One caution is that some of the new black shafts have molding artifacts and may not slide freely into the multideck switches. I'll probably have to trim the worst ones before installation.

Hope it helps someone. 

[essele]

Bumping an old but very helpful thread ... I am now also an owner of a non functional 720A.

I'm seeing very slightly high input resistance (100.007k) when set to all zeros, but it jumps up to 100.033k when decade C is set to anything other than zero.

Output is "roughly ok" (a bit low on higher ratios) with decade A and B, but decade C on anything other than zero causes a jump about 12x what it should be and then no variability between settings, the remaining decades seem to be twice as sensitive as they should be (e.g. 20uV instead of 10uV.)

I need to run through a proper check, but from my quick dabbling late last night I think I was mostly inline with the output resistance table from the first page, but I saw around 40k output resistance once decade C was non-zero (with all else on 0.)

So my feeling at this point is that S3 is faulty, probably going partially open circuit when non-zero (and then the 40k would be the shunt?)

It does actually look like this decade has been removed or replaced before, it has similar paper labels on all of the wires (although these are printed rather than hand written) -- I'll post some photos this evening.

I'm a bit daunted by the prospect of removing this and trying to fix the wafers ... any advice on how to go about this?

Thanks.

[Kleinstein]

An open contract at the switch sound like a reasonable failure mode. However, with an open contact the lower decades should not have any effect other than changing the resistance to the output a little.
Anyway an open contract should be measurable with a DMM even in circuit, at least in one of the first decades  (could be tricky with very low resistance ones at the end).

If the decade was replaced before, there is a slight chance this replacement was a failed repair and the connections may be wrong around this decade.

[essele]

Thanks Kleinstein ... on further investigation I think you are completely right. The switch does look to be functional.

It looks like they have swapped over the two red/white wires with the black one... TiN's original and a photo of mine attached.

There is a lot of slack on the black wire, and on TiN's one it actually goes around the back which would make more sense.

I'll have a thorough look for other wiring problems first, then I'll see if that does the trick.

EDIT -- yep, that fixed the problem. All seems absolutely spot on now!

Thanks for the pointers, and I'm really pleased I didn't start trying to take that switch out!

[essele]

Just for completeness...

I had actually asked both Fluke (UK) and a reseller for pricing and availability for the C deck switch when I thought that might be the problem.

Fluke have been incredibly responsive and helpful which I thought was worth highlighting since seems to conflict with some other reports on this blog, that said, the cost of a new assembly (they didn't provide just the wafer set cost) was £4,165 presumably plus VAT!

They also suggested that a calibration of a 720A would be £1500.

Thought it might be of interest.

[dl2ocb]

Hello to all Voltnuts  !!

I am a very happy owner of a Fluke 720a KVD since some Days too.

Faulty A Decade Switch seams to be very common Problem, so my KVD also got that Problem.

On my KVD i measured 103 Ohms between Pins 19 and 20 of the Oilcan. I checked the Resistor inside the Oilcan by removing the Wire going to the Rotary Switch (Decade A). The Resistor shows the expected 9,9... KOhm and i found out, that the Rotary Switch of Decade A has a short between two Pins (grey and whites Wires).

All other Decades are OK.

Ill test the Switch today and i'll post some more Details soon. Hope that i can help others to repair this nice little Poti too.


Does any body know how to replace the Decade A Switch with a new Switch ? Which Brand of Switch will fit in there ? Maybe some Grayhill Stuff ?

I found one Provider named "https://www.multi-tech-industries.com" on the Web. On that Homepage i saw rotary Switches (Model 820) that looked like the ones in the Fluke 720a.


CU
Stefan