Repairing a 5440B/AF DC Calibrator

[Tony_G]

I acquired this unit from a DoD auction where it didn't power on - It looked like it was just missing a fuse holder so I bought the calibrator and paid more than the cost of the unit to ship it to me. I now have the unit and just replaced the fuse & fuse holder.

It powers up with a fault and no display so I thought I'd start a thread on what I found - I don't have any answers yet but I'll post here as I do stuff.

The first thing I found was that there seemed to something rattling around in the unit so I took the covers off to see what it was:



Something ceramic and it seemed to just keep coming and coming out of the unit, so I took the unit apart to clean it out. I never did find out what it was so if you have suggestions shoot them across.

This is the unit with the top cover off:



The first board I removed was the Output PCA:



Then the Output/HV Control PCA:



Sample String PCA:



Preamp PCA:



DAC/Digital PCA:



DAC/Analog PCA:



Inside Regulator PCA: 



Filter Board - Both Sides:




Guard Crossing PCA:



Memory PCA:



Controller PCA



Outside Guard PCA:



Still had some crud in the unit so I opened the bottom of the 5440:



Took the front panel off:




I was looking to see if some ceramic component had just destroyed itself and this was really the only one I could find and it looks fine:



For completeness, here are some more shots:

I/O PCA:



Outside Guard Term PCA:



Front outlets



Main board:




I think I got the all the crud cleaned out - No idea what it was as I couldn't find anything that seemed to be the cause of it. I'll start cleaning up the unit, check the power supply voltages and then clean, visually inspect and reinstall each card as I go through.

Till the next time.

TonyG

[Dr. Frank]

Hi, and welcome in the club of 544x owners..

The DAC/Digital PCA oven looks, as if it was getting too hot, judging by the color of its oven cover.
Although.. the ovens have double safety features, afair.
Open this shield, and look how it's inside.

Frank

[Tony_G]

Thanks Dr. Frank - Appreciate the welcome and tips.

When I plugged it in it just came up with nothing on the display (no zero's, no words, nothing, just dark) and the "Fault" annunciator lit. If you've seen that before, any tips on where I should look?

TonyG

[RobK_NL]

If both displays are dead, chances are the DC-DC converter on the display board is dead. Most likely cause is either C3 or C4 failed. Had that with two of these puppies.

[acbern]

First thing to do is to measure all internaly supply voltages. Let us know...

[Tony_G]

Hi All,

I put the A19 Outside Guard Term PCA, A12 & 13 Filter PCAs, A11 Guarded XFMR Term PCA and the A17 Outside Guard Regulator PCA back into the unit so I could test power levels.

Interestingly the red LED on the A17 unit wasn't lit.

Here is what I measured - For A19 the test points are:



I measured:

Test Point   Voltage
1 +29V       40.46V
3 -29V       -40.85V
4 +5V         10.54V
5 HR+5V     10.37

Example picture:



The service manual in section 3 lists the values for the regulator test points as:



I measured:

Test Point    Voltage
2 +5.15V     5.150V
3 +5V          5.021V
4 +28V        28.000V
5 +12V        11.967V
6 -12V        -12.303
8 -5V          -5.024V

I wasn't able to find a TP7 to check on the board for the -29V - See example:



Based on this (the regulated voltages are in spec even if the power supply voltages are way off from what was on the schematic) I have to believe that the power is good.

I still can't find where that ceramic came from - Next steps will be to reinstall the other boards and then open the front panel up to see if the VFD is broken or not, check on the DC-DC convert per Rob and to take the cover off the Digital/DAC PCA per Dr Frank.

Thanks for the help.

TonyG

[Tony_G]

I dropped all the cards into the unit with the exception of the actual front panel (the panel itself, not the front panel controller card that is attached to the main unit).

When I do this I can see the LEDs on the A16 Controller PCA light up and then extinguish one by one:



This seems to imply that the basic "guts" of the device is working - I'm going to take the front panel apart and see what is there.

Also I took the top off the DAC/Digital PCA as Dr. Frank asked - Here is what it looks like:



The board itself seems OK but I'm not sure why someone would write "console" on it (lower right corner).

TonyG

[Tony_G]

Interesting, plugging in the A22 Display Controller causes the power supply fault LED on A17 to turn on - When I try to read any of the test points on the board I get nothing - I Looking at the schematic and with Rob's advice I'm going to check C13 & 14:



and C3 & 4:



I removed C3, C4, C13 & C14 from the circuit and measure them with my LCR meter - Crap, while I was checking them I dropped C3 & 4 and they got mixed up but they're both supposed to be 15uF/35V caps

Cap   Value      ESR
C3     3.61uF    38.0ohms
C4     11.99uF  2.34ohms
C13   0.20uF    6.92ohms
C14   45.91uF  0.24ohms

The official values are supposed to be:

C3 & 4 - 15uF
C13 - 0.22uF
C14 - 47uF

so it looks like C3 & 4 need replacing (just like Rob was suggesting) - I'll do that after dinner.

In the meantime, I'm a complete NOOB when it comes to VFDs - I have no idea if these look good or not:




Are the black marks in the corners normal?

TonyG

[Cubdriver]

Tony -

The VFDs are basically vacuum tubes.  The black/possibly silver-ish marks in the corner are good - those are the getters.  It's a (usually) barium compound that's flashed onto the glass after the tube has been evacuated; it acts as a collector of the residual gasses that are not pumped out during manufacture.  If the vacuum seal is compromised and the display fills with air, they will turn white and flakey.

-Pat

[TiN]

Yup, VFDs seem to be ok. Doesn't mean they aren't dim from age though, but lets hope for best.

Thanks for photos, keep them coming. Can I steal them into my T&M collection (with link to this thread).

[Tony_G]

Feel free to use the pictures as you'd like.

I didn't have any 15uF caps handy so I used the closest value I had, which was 10uF - No go, it still causes the regulator to light the fault LED and shutdown the power - The circuit seems simple enough that I should just have to get close with the capacitors and not exact.

Will need to look at Q1-4 and U1 next - I can pick up some caps and a replacement U1 on my way home from work tomorrow - For the purposes of testing Q1-4 should be swappable with 2N3904/6

TonyG

[Dr. Frank]

Ok, my guess was wrong, obviously, as there is no oven on this PCB.
Your unit is even older, from about  1986, so elco capacitor defects are probable
Frank

[singapol]

I measured:

Test Point   Voltage
1 +29V       40.46V
3 -29V       -40.85V
4 +5V         10.54V
5 HR+5V     10.37

The vfd display requires a negative supply, as in post #5 it is -29V so I think it's missing.You need to restore this and the positive one for the display to work assuming the display controller and circuit is ok.

[Tony_G]

Thanks for the reply - According to the schematic the panel takes in 29V & 5V:



I measure 4.87V between pins 1 & 2 and 27.99V between pins 3 & 4. In fact the whole DC-DC converter doesn't take in a specific -29V line but just a 29V and 29V return:



The 6 main ICs on the board are socketed so I removed them and the problem persists. This seems to me to imply that the issue has to be in the DC-DC converter as that is really the only thing left connected to J4.

TonyG

[ManateeMafia]

What about CR1? Possible shorted diode in the rectifier.

[Tony_G]

Thanks for the suggestion - I checked out CR1 and it appears to work correctly (read the appropriate voltage drops and opens from the AC to the +/- pins) - I really think it has to be in U1 or one of Q1-4.

So I was just putting together a shopping list (I have to pretty much pass the store to get to work so I thought I'd drop in a grab the components regardless so they'll be on hand if needed).

The items I'm looking at are:

C3 & C4 - 15uF/35V Electrolytic Caps
U1 - LM3542N
Q1 & 2 - Fluke 272237
Q3 & 4 - 2N4403
CR 2 & 3 - TCR5315 (marked as a 2N4401 on the schematic)

In terms of replacements - I can do the following:

15uF/35V replaced with 22uF/35V (should be Ok to increase the capacitance, they don't have a 15uF electrolytic)
LM3542N replaced with a NTE1720 PWM regulator
Q3 & 4 replaced with 2N3906 PNP transistor
Q1 & 2 replaced with 2N3904 NPN transistor

The parts that I don't seem to have any information on are CR 2 & 3 - They are described as "Diode, SI, N-JFET, Current Reg, IF=5.3mA" so they must be using a JFET to protect the transistors.

I can't find anything about this part - 07910 looks like it is Teledyne Corp (Continental Devices) Hawthorne California but the closest I can come to a datasheet/more information is this page:

Military/High-Rel :  N
I(S) Nom.(A) Regulator Current :  5.2m
Z(T) Min (?) Dynamic Imped :  220k
V(L) Max. (V) limiting Voltage :  3.1
P(D) Max. (W) : 
V(P) Max.(V)Peak Oper. Voltage :  50
Semiconductor Material :  Silicon
Package Style :  TO-18
Mounting Style :  T

Any suggestions on what to use as a replacement that would be generally available? I can find several that will fit on Digikey but nothing that my local store seems to carry. I can't seem to find anything but maybe my SearchFu is just weak today...

TonyG

[picburner]

You must look for "Current Regulator Diodes".
I have found these but CR470 deliver 4.7mA only:
pe2bz.philpem.me.uk/pdf on typenumber/A-C/CR160.pdf
However before you replace them, test with a power supply if they generate a constant current of 5.2mA:
if they work is useless replace them....
Good luck.

[singapol]

My bad,  post#5 refers to main power supply of which +29 goes to display panel but strangely you measured
+40.46 and -40.85 respectively instead of +/-29 as stated.

I measure 4.87V between pins 1 & 2 and 27.99V between pins 3 & 4. In fact the whole DC-DC converter doesn't take in a specific -29V line but just a 29V and 29V return:

You are right this display uses +29 into DC-DC convertor to generate +41V (VCC2A) @ TP1 and +35V ( VCC2B) @ TP2 for . VCC2A for U4 - U7 and VCC 2B for U2 -U3 display drivers respectively. The +5 also
goes to the 6 SN75518 VFD drivers. Need to check if +41V and +35V are present at the display drivers.

[Tony_G]

Thanks Singapol,

The problem that I'm having with testing the DC-DC converter is that as soon as I plug in the Display Panel it puts the Regulator PCA into fault mode and that cuts off all the power so I just don't see anything on the test points.

I'm going to remove U1 when I get home because that should basically disable the rest of the DC-DC converter and let me know if the issue is in that area - i.e. plugging the display panel back in doesn't cause the regulator fault LED to light.

I also realized that I didn't include a full photo of the display panel - Here it is:



TonyG
TonyG

[Tony_G]

Ok I'm at a bit of a loss right now.

I replaced U1, checked CR1, CR2, CR3, Q1-4 and replaced C3 & C4. Still no go - When I hook the display panel in the regulator goes into fault mode.

Took T1 out of the circuit and unplugged all the ICs - It shouldn't be pulling the supply down.

Maybe I just need to remove all the components to see if even the bare board is OK.

Thoughts?

TonyG

[Tony_G]

And I just decided to remove absolutely everything from the DC-DC power supply:



Lo and behold! Connecting the display panel doesn't cause the regulator to fault. Now to test everything in the pile of components again....

TonyG

[Tony_G]

Well there's your problem:



C1 is being recognized as a 0.079ohm resistor. Seeing that it is a filter cap directly across the 29v supply that would generally be regarded as "bad" by the regulator PCA.

I'm going to reinstall all the components tomorrow (late here in Seattle) and see if that lets me fire up the displays.

TonyG

[Vgkid]

Best of luck, at least no fireworks.

[singapol]

Well there's your problem:



C1 is being recognized as a 0.079ohm resistor. Seeing that it is a filter cap directly across the 29v supply that would generally be regarded as "bad" by the regulator PCA.

I'm going to reinstall all the components tomorrow (late here in Seattle) and see if that lets me fire up the displays.

TonyG

Sorry I don't follow, are you testing the right pin? There are 2 supply going into SN75518  pin1 VCC2 (+41/+35) and pin40  Vcc1=+5V.  The orange tantalum reading at 79 mOhm could be correct. ( depending on capacitance value )

You replaced C3,C4 are you meaning C13 and C14?  C14 is in the +5V for pin 40.

Edit: SN75518 does not use +29V. Tantalums when new (unsoldered) can measure like what you measured.
There are different grades of tants with impedance. Try measuring a new one if you suspect it is defective which I think it is not. Afer soldering it can read higher but is expected as stated by manufacturer.
Eg. a smt 10uf tantalum states 300mOhm after soldering but before soldering it measured 78 -90 mOhm).

Are you sure NTE replacement is pin identical?
Sorry from the picture I thought you were measuing sn75518 pins when it is a tantalum after it's remove out of circuit.

[Tony_G]

Thanks Vgkid - I think the regulator PCA does a really good job of cutting the power off before anything bad can happen so I didn't see that usual burst top thing.

Sorry Singapol - I happened to be measuring the C1 capacitor and the photo was there so I thought it would make a good background.

It appears that the C1 capacitor is effectively a dead short (I wasn't showing ESR on that cap but actual resistance measurement) - When connected to the LCR meter in auto mode the meter says that it is a resistor. When I set the meter to capacitance mode it reads 'overload'. Doing the same thing with my Rigol DM3058 shows the same overload on capacitance and a slightly different but still very low resistance value. All the other tantalums measure correctly and in spec.

Based on that I believe that C1 has failed as a short and given its position in the circuit as a filter capacitor it's shorting the 29V supply.

According to the NTE 1720 Datasheet and the Fluke schematic they're identical pin outs.

I had replaced C3 & C4 with 10uF caps (which I had on hand) per Rob's suggestion that he has seen them fail - When I tested them they had indeed failed - Whether that placed undue stress on C1 and caused it to go as well or some other issue occurred I don't know. Will know more tomorrow when I put the components back in the board.

Thanks for all the feedback and help - I really appreciate everyone's willingness to help out here on the forum.

TonyG

[singapol]

Take note if you have replaced Q1-4 with 2n3904/2n3905 it will fail soon as the max. continuous collector current is only 200mA vs 600mA of 2n4401/2n4403.  6 SN75518 driving VFD is no joke.

[Tony_G]

Thanks Singapol - I was looking to use those jelly bean 390x's in case the transistors in the unit had failed. When I took them out and tested them they were fine.

I reinstalled all the components and put a 10uF electrolytic in place of the 6.8uF tantalum (will need to get a proper replacement) for testing and...drum roll...

IT LIVES!



Came up with no errors I think - some characters on the right display were it now says 'voltage mode' but then straight to this display.

They look nice and bright so I'm going to see if I can find a replacement tantalum locally and make that swap otherwise I'll order one and then reassemble the front to see if it really lives or if it's just mostly alive.

Thanks everyone for the help so far.

TonyG

[Kleinstein]

Those drop like yellow tantalum caps are known to fail quite often. So even if a few of them are still Ok, it might be a good idea to replace them - they sometimes do fireworks, not just a short.

For the filtering cap C1 and in many other positions low ESR electrolytic should be Ok too, unless you want to use the instrument at freezing temperatures.

[ManateeMafia]

Congratulations 

I hope all goes well once you get it back together. Do you have the gear to calibrate it or do you plan to send it out?

[SeanB]

Looking at the ceramic that came out it looks a lot like drywall gypsum crumbs, and as well the inlet by the fan shows dust, which might be that it was in an area where a remodel occurred and the dust got inside, along with a few larger clumps of drywall interior. Then it was packed with this still inside.

Leave the electrolytics you put in as C3,4 and up the voltage to 63V, they are being used as DC isolation to prevent the core saturating as duty cycle changes, and this is a very bad place to use a tantalum, so an electrolytic capacitor ( or even better a ceramic capacitor, though there the voltage drop of capacitance might need 200uF of capacitance to get an effective 10uF capacitor at 30VAC) is a lot better, and a higher voltage as well as the cathode foils are formed to a higher voltage so will last longer. slightly bigger, but there is plaent of room to lay them down sideways and stake to the board with a drop of superglue.

[Tony_G]

Thanks everyone for the support and suggestions - Appreciate it - I checked my local store's web page and they don't have the same spec tantalums (or electrolytic) so I put the unit back together to give it a try out.

Kleinstein - Thanks for the comments - There are only a few caps on the board so I'll follow your suggestion and replace them all - I assume that Fluke went with the tantalum cap for the filter because of the low ESR and the temperature stability - Maybe they just had a bunch of them already.

ManateeMafia - This, along with a 5200A are my first forays into owning an actual calibrator. I got this 5440 for $210 or so from a DoD auction (cost me $263 to ship it too me) and bought the 5200A based on what I've read here on the board and some hints/guidance from Dr. Frank. I live just down the road from the Fluke Cal home base so I was going to get them up and running to the point of my gear here and then drop them off for an official Fluke cal - As I can drop off and pick up, I can save a lot in shipping these monsters around.

SeanB - I never did find where that stuff came from - It really reminded of the ceramic core of an old wire wound resistor or an old ceramic isolator but I can't find anything like that anywhere in the system so your theory is better than anything I had. I'll up the voltage on C3 & C4 when I order the tantalums from DigiKey and replace them on the board.

So far I have the unit warming up - The Service manual says that it needs an hour to do the analog self test so that the ovens are warm - Will check back in with the results when that is done.

I did plug it into my DMM to see if it generated output and it does - No faults so far - Here is a photo of the front panel:



The VFD is OK - The right one seems flawless, the left one doesn't have that gap as the photo shows (shutter speed etc) but it is a little dimmer than the right - The 3rd last 0 is actually quite bright - It's more than good enough to work with though.

TonyG

[Dr. Frank]

Great. You got one of the best DC calibrators for a real bargain.

Use the left/right arrow to move the selected digit.. i.e. the highlighted one.
That's no fault, either. Use up/down to change value.

Unit needs 4h to fully stabilize. Then do an internal cal... autocal.. and check 11, 22, 250 and 1000V range with DMM.

In fact, you will instead check the range calibration of your DMM.

Frank

[RobK_NL]

In fact, you will instead check the range calibration of your DMM.

Unless that DMM is a freshly calibrated 3458, of course 

There's two 5440B's coming up for sale in the very near future ...

[Tony_G]

Thanks Dr. Frank - Just tried using the arrows and it's exactly as you say.

The service manual said give it two hours warm up so it was in the middle of the internal cal when I saw your post - I'm going to put the covers back on and then let it warm up for 4 hours and redo the internal cal.

Where it sits right now, with my Keithley 2015 - 6 1/2 digits but no idea on complete accuracy yet:

Set          DMM
0V           -000.0009mV
11V         11.00037V
22V         21.9998V
250V       249.993V
1000V     999.962V

Seems reasonable to me - Will have to see what it shows when everything has warmed up.

A problem did leap up, the 2 button just fell off:



If you look closely you can see some epoxy on the connector so it has broken some time in the past and was glued on - I'll have to redo that.

TonyG

[Dr. Frank]

Well., your DMM is probably out of cal, as its ranges do not fit, and your DMM is of the 20..30 ppm class.
Use 1.9 V, 10V / 19V, 100V / 190V and 1000V as Cardinal Points, instead. 1000V needs 30sec settling time, at least.
Downranging from 1000V the same

Frank

[Tony_G]

So close but yet so far, while I was waiting for the 5440 to warm up for 4 hours the switch I had repaired has now become stuck down:



Even though I was very careful and only applied a small amount of glue, the only thing I can think of is that some of it still made it's way into the side of the switch.

According to the parts list the switch is:

    Switch, Pushbutton, SPST, Keyboard FLUKE STOCK NO 513473 MFRS SPLY CODE 89536 MANUFACTURERS PART NUMBER 513473

This is a Fluke part - Probably unobtanium - I tried spraying a bit of IPA into it to clean it out and I can move the switch but it just doesn't spring back when pressed in.

[Tony_G]

The switch is heat staked together at the corners - Using an exacto knife I cut through the heat stake and here is what is inside:



You can see the cruft on the left side of the upper switch casing - I'm going to see if I can clean that out and then super glue the assembly back together.

That said, if anyone has any of these they'd be willing to part with please let me know.

TonyG

[Vgkid]

Nice job so far. Good luck fixing the switch.

[Tony_G]

Thanks - I have the switch fitted back together for testing:



It works like the others do - Not quite a smooth but almost. Took a nail file, Exacto knife and small screwdriver to it and cleaned it out I think.

Plan to take a little relax before gluing it back together as this has been somewhat stressful (my own fault but still stressful).

TonyG

[nidlaX]

So close but yet so far, while I was waiting for the 5440 to warm up for 4 hours the switch I had repaired has now become stuck down:



Even though I was very careful and only applied a small amount of glue, the only thing I can think of is that some of it still made it's way into the side of the switch.

According to the parts list the switch is:

    Switch, Pushbutton, SPST, Keyboard FLUKE STOCK NO 513473 MFRS SPLY CODE 89536 MANUFACTURERS PART NUMBER 513473

This is a Fluke part - Probably unobtanium - I tried spraying a bit of IPA into it to clean it out and I can move the switch but it just doesn't spring back when pressed in.

Here's your keyswitch: https://deskthority.net/wiki/Datanetics_DC-60_series. If you are in need of replacements, you might go hunting for some Fluke Y1700 keyboards: http://sandy55.fc2web.com/keyboard/y1700.html. I see that they've been sold for cheap on eBay recently. On the Deskthority wiki page, there's a link to an AliExpress auction for some switches standalone, so those may pop up every once in a while.

Sadly, keyboard / keyswitch knowledge is about all I can contribute to this thread.

[Tony_G]

That is awesome - thanks nidlaX - I'll put a search into eBay to get some "backups".

TonyG

[Tony_G]

And finally back to where I started this morning at:



Goals for tomorrow:

1. Warm up 5440 for 4 hours
2. Don't screw it up
3. Run Internal Cals

Thanks everyone for the support and information.

TonyG

[Kleinstein]

If I see it right from the plan C3 and C4 are used back to back for DC isolation of the transformer. Not a really good solution to use tantalum caps here. The way better solution would be a single non-polarized cap, like those used for audio. Two 15 µF in series is only 7,5 µF - this is still possible with film caps (though larger size) and modern MLCC ceramics (e.g. 2 times 4.7 µF in parallel), as the voltage should not be that high and the size of the caps is not critical.

As the current is limited fireworks are not that likely. The one failure was a non smoker so far.

[TiN]

Those key caps and binding posts just beg for some hydrogen peroxide action

Nice work, now you only have 3458A missing... Oh, I said only? What a mistake..

[Tony_G]

I turned the machine on 3 hours ago so I have another hour to wait till I'll run the internal cal routine again but I thought I'd try out the Error Display with my Keithley 2015:



Working out the values for error mode from the 1 year specs for the 2015THD:

Range (V)	Input (ppm)	Range (ppm)	Input (%)	Floor (uV)
0.1	50	35	0.005	350
1	30	7	0.003	700
10	30	5	0.003	5000
100	45	6	0.0045	60000
1000	45	6	0.0045	600000

The actual results I get are:

Range (V)	ERR (ppm)	Pass/Fail
0.1	-37	Pass
1	-33	Pass
10	34	Pass
100	-6	Pass
1000	-34	Pass

Looks like things are working and will only get better with the cals.

TonyG

[Tony_G]

Just finished all the internal self tests (digital, analog & high voltage) and the ran the internal calibration.

It seemed to make a big difference w.r.t the divider (I turned it on in the previous measurement to get more resolution in 100mV & 1V range). As this applies only to those ranges here is the change:

Range (V)	ERR (ppm)	Pass/Fail	Previous ERR (ppm)
0.1	-62	Pass	-37
1	-35	Pass	-33

so it seemed to make a big difference to the 100mV range.

Without using the divider I see this:

Range (V)	ERR (ppm)	Pass/Fail	Previous ERR (ppm)
0.1	40	Pass	-37 (divider)
1	34	Pass	-33 (divider)
10	34	Pass	34
100	-69	Pass	-6
1000	-21.7	Pass	-34

I'll need to go away and read the theory of operation in more detail to see what it says about the divider (you can't use the external sense with it. Also the 2015THD inputs in 100mV and 1V ranges are rated at >10Gohms) to understand why the difference but as you can see it passed my Keithley in either case - Now is the Keithley anywhere near spec? I don't know but once I have Fluke run the 5440 through an external cal I should be able to tell for DC at least.

Thanks everyone for the comments, suggestions and guidance (especially the guidance).

I'll be swapping the 5440B/AF from the workbench and moving onto the next item:



TonyG

[Dr. Frank]

The divider is quite unstable, and needs to be calibrated externally, quite often.

If you generate 0.1 and 1V on the 11V, you will have the non-linearity problem. The DAC is linear to 0.2 (measured) .. 0.5 (spec.) ppm, so 1V may be precise to about 2..5ppm, 0.1 V to about 20..50ppm..
Make sure to do a good zero on the Keithley (i.e. output 0V from the 5440A, and zero the 2015, then output 0.1 and 1V).

I thought, the 2015 would have 1.9x "overrange", for each range.. compared to 1.2x of HP DMMs.

Then you could test with 1.9V for the 1V range, 10V for the 10V range, both are same range on the 5440A, so only the linearity uncertainty applies.


Anyhow, the Keithley may be inside its 1yr. specification, but the 5440A reveals, that it has been drifting a lot since the last calibration.

A sign of that is that the 10V range drifted + 34ppm, compared to the others, which drifted in the negative direction.. Even if the 5440 would be outside its spec (maybe a few ppm), then all of these 10:1 ratios of the 2015 can be checked precisely.
The 5440 makes very accurate 10:1 ratio calibrations for those 4 ranges, I assume, even better than the 57x0As.

Frank