EEVblog #933 - Keysight U1272A EMC Issue

[wraper]

IMO, it is pretty big as compared to the U1272A.  Its size actually made me realize I don't have a compact DMM on my bench yet.  I like the removable holster though.

It's mostly a visual perception thing. If you check actual dimensions, the difference is very small.

[bernroth]

I finished checking the modded DMM against DMM Check Plus reference standard.
Current and voltage readings are within spec, only on resistance measurement I noticed that a reference 100 Ohm resistor reads 200mOhm less than on the "new" Keysight U1272A
My meter is quite old (PCB tells Rev004) and maybe the error is due to aging of the meter itself.

The two series inductors inside the device which have been replaced have each a DC resistance of 220 mOhm.
If they would have been compensated in software, the reading would be more off than by only 200 mOhm.

Maybe I should have checked the readings before and after modding the DMM

[Tom45]

My meter is quite old (PCB tells Rev004) and maybe the error is due to aging of the meter itself.

Have you noticed what is different between your Rev004 and the Rev005 or Rev006 in my photos?

Tom

[hayatepilot]

I've modded my U1272A and everything is still within spec (compared to the replacement  U1282A).

But if I short out the probes on resistance measurement, I get about -0.45 Ohm.
So the resistance is offset by that amount. Of course if I null the meter everything is ok and it shows the same resistances as the U1282A. 

Does anybody know how I can do a null calibration? So that my meter is more acurate without nulling everytime (specifically for sub 100 Ohm measurements)

Greetings

[TheSteve]

0.22 ohm smt resistors are easy to get from Digikey etc. Maybe instead of a zero ohm jumper you could add in resistors to replace the inductors. Or perform the zero ohm calibration if you can do only that step.

[hayatepilot]

I found the Service Guide for the Keysight U1270 Series.

But I don't know if I can calibrate just the short resistance and don't have to touch the other values. I REALLY don't want to loose the factory calibration.
A step by step guide or a video of someone who is braver than me would be very much appreciated. 

Greetings

[bernroth]

@Tom45:
I have not yet checked the differences of REV004, 005 and 006.
Attached you can find a picture of my fixed REV004 (so ignore BD4, BD5 and R55 )

UPDATE:

Instead of 0 Ohm resistors I have now replaced the inductors BD4 and BD5 with 0.2 Ohm resistors (P/N: CRL0805-FW-R200ELF).
R55 is 2k4 (P/N MC1W080512K4)
EMC immunity is still OK and the measurement error in resistance mode is gone.

A short circuit with a short measurement lead was first measured by the instrument at 0.2 Ohms, then it went slowly down to 14mOhm.
After a power cycle (even with no batteries while taking the picture) the correct value is immediately spot-on.

[TheSteve]

Firmware version 1.05 can now be downloaded from the Keysight website for the u1282a. They posted it a while back but the download itself was broken.

[lowimpedance]

Okay My replacement U1272a has arrived. (anyone else in Aust. received theirs yet?). So just a quick test at 10Mhz/100mV straight into each A input. See pic.
If I place my hand near the Agilent meter the reading jumps all over while the Keysight just sits with no fluctuation of the LSD. 
A very big thanks to Keysight from me too. 

[Dwaine]

@Tom45:
I have not yet checked the differences of REV004, 005 and 006.
Attached you can find a picture of my fixed REV004 (so ignore BD4, BD5 and R55 )

UPDATE:

Instead of 0 Ohm resistors I have now replaced the inductors BD4 and BD5 with 0.2 Ohm resistors (P/N: CRL0805-FW-R200ELF).
R55 is 2k4 (P/N MC1W080512K4)
EMC immunity is still OK and the measurement error in resistance mode is gone.

A short circuit with a short measurement lead was first measured by the instrument at 0.2 Ohms, then it went slowly down to 14mOhm.
After a power cycle (even with no batteries while taking the picture) the correct value is immediately spot-on.

So we have to use 0.2 ohm because we are missing the calibration after the repair.

[bernroth]

So we have to use 0.2 ohm because we are missing the calibration after the repair.

The value may depend on your specific model but on mine (REV004) 2x 0.2 Ohm were fine.
Afterwards there is definitely still a bit of error due to the fact the calibration was done with inductors. In my case the error was around 10-50 mOhms.
Anyhow, when measuring these low values with 2-wire measurement, it is still recommended to zero the error out.

I think this will not have a big influence on normal measurement tasks.

[lowimpedance]

Some further internal checks for modifications compared to the original Agilent meter.
Agilent PCB is a REV 4.
New Keysight is a REV 6 with the mods shown in the attached pics. otherwise it appears to be the same.
Noted that the input divider and fuse holder clips have been soldered in after the main PCB had been loaded and soldered on the REV 6 , but were done at the same time as the whole PCB on the REV 4 (or at least the REV 4 had a better clean!).
On my REV 6 the noted parts changed (or appeared to have been) are BD4, BD5, C53 and R55 which was not as obvious as a hand solder and lastly, as Zbig asked, on the top side BD7 has been changed to the same value as R55.
Sorry for the pics , had only phone camera , but they still convey the required info.


EDIT: I just removed C53 from each PCB and tested on a fluke 87V capacitance mode and both measure 3.3uF, so no need to bother with this component.
Question as to why the later REV PCB looks like it was hand soldered  .

[lowimpedance]

And BD7 pic since I could not attach in prev. post due to size.

[lowimpedance]

One other thing that caught my attention is the shield over the input divider on the older REV 4 Agilent is showing quite notable tarnishing,  degradation !, all be it not actually rust at this stage  . And no this meter has lived mostly in air con lab environments nothing harsh at all.
Still I don't think I'll lose any sleep over it.

edit: Also forgot to mention that of course the CAL certificate envelope was not sealed and an updated CAL certificate with a date of Jan 16 was inside, replacing any previous one.

[bernroth]

@lowimpedance: Thank you for your photos!

I replaced the BD7 inductor (was 1,5µH/0.2Ohm) with 2k4 on my meter and I did not notice any changes.
The trace from the resistor looks like going to the range selector switch.

The shield on my divider looks good even though it's an old meter as well.

[Dwaine]

My meter just showed up.  Test fine.  Now I just need to fix the old meter.

[Zbig]

My replacement U1273A (the OLED version) has just arrived. Thank you, Keysight! Any thoughts what was the possible reason for Keysight to include the inductors early in the signal patch in the first place? And what could be the potential (negative?) side-effect of removing them? Could they originally be there to somehow cancel out the parasitics of the rest of the circuit, I wonder?

[lowimpedance]

Original Agilent mods complete , for now!.
Indeed it would have been interesting to know the design choice for the original specified parts, but I think that will be only speculated on. What we do have is what changes have been considered necessary to harden the spec for  EMI resistance.

 Out of curiosity I changed only BD4 and 5 first to a resistor value of 0.2 as per what values bernoth used and tested the EMI immunity with the result being now only a few milliamps indication on the display was shown when a 10Mhz 100mV signal was applied to the Amp terminal and this reading did not seem to change with activation of the LPF. So a considerable improvement in de sensitizing the current mode to external EMI but not quite there.
 So next was to change BD7 and R55 to both 2K4 . And now testing this complete mod shows no obvious EMI effect at the applied RF signal (or any stray effects when a hand is placed around and on the meter). Now its quite possible that more signal amplitude or different frequency may have a different outcome but for my uses I'm more than happy to leave it there with now two 1272's.
 I did test the ohms ranges and noted the CAL was still good with an input short. Both meters showed identical behavior when presented with a short in the ohms mode. I did note that the LSD on the old Agilent '72' in Current mode was sitting between 2 and 3 counts so perhaps a RE CAL of that function may be needed. Looking at the CAL. procedure it looks as if it maybe possible to do an individual function as opposed to the whole meter. Perhaps I'll give it a go sometime if I get motivated enough and will post here whether successful or not!.

[wraper]

Looking at the CAL. procedure it looks as if it maybe possible to do an individual function as opposed to the whole meter. Perhaps I'll give it a go sometime if I get motivated enough and will post here whether successful or not!.

Could you please point to the CAL procedure mentioned?

[Zbig]

Looking at the CAL. procedure it looks as if it maybe possible to do an individual function as opposed to the whole meter. Perhaps I'll give it a go sometime if I get motivated enough and will post here whether successful or not!.

Could you please point to the CAL procedure mentioned?

It's a part of the "Service guide" available for download in the Technical Support section of the Keysight's webpage (you might need a Keysight account).

[lowimpedance]

hayatepilot posted a direct link a short time back and no account etc needed. click and download.

[Dwaine]

If we can do the calibration ourselves , then we could just stick with the zero ohm resistors.  Instead of using the 0.2 ohm resistors. 

[hayatepilot]

If we can do the calibration ourselves , then we could just stick with the zero ohm resistors.  Instead of using the 0.2 ohm resistors.

Yes, but we don't know if it is possible to calibrate only the short values or if all the resistance values have to be calibrated.
Nobody (me included) seems to be brave enough to risk the factory calibration. 

[128er]

My new U1273A arrived yesterday. When I compared the new Keysight side by side to my old Agilent, I noticed the huge different in display brightnes and density of the pixels. I mean it's the same amount of pixels on the display, but you realy can see the gaps between the pixels on my old Agilent. Now I wonder if the OLED display on my old meter faded out so much or did Keysight anything with the display?

Maybe I will take a picture later.

P.S.:
Thanks to Keysight for the great service!

[M8A4X]

Hi to all!

Yesterday my replacement U1273A arrived out of the blue . Thanks to Keysight for the excellent customer support!!

I also compared the screens of the Agilent and Keysight U1273A.
It seems as if the Keysight screen brightness is a litte bit higher than the Agilent one.
However I think it is either an updated hardware version of the screen or the general brightness is set to a higher value because the Agilent U1273A is only about half a year old and has merely seen one hour of on-time (don´t ask why ), so Oled ageing would not be my first thought.