Author Topic: [SOLVED] 34401a - Cal Error 707  (Read 14891 times)

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Offline Zucca

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[SOLVED] 34401a - Cal Error 707
« on: April 15, 2014, 06:28:15 am »
It looks like there is nothing in the www regarding this error in the 34401a.

I just got an email from the laboratory I sent my 34401a (fresh from the used ebay market  ::), it passes the self test) for the calibration service.

Here the surprising new:

Quote
S/N: US361229xx
Lab Notes: Unit was unable to pass or adjust on front zero offset and Ohm tests.  Cal Error 707.  Unit drifts when in Ohm function, suspect 400K resistor and 40K resistor are either bad or starting to go bad.   Agilent reports this issue in the service notes for these units.  Unit was unable to be adjusted and failed calibration.

Since the Lab. wants extra money to fix the device, I decided to fix the problem myself, so I did my homework and here what I found in the User Manual:

Quote
707 Cal signal measurement out of range

The specified calibration value (CALibration:VALue) does not match
the signal applied to the multimeter.

In the service manual it doesn't say too much, but here the schematics (pag. 9-10) regarding the suspected two R (click on it for full resolution):



Now the good news is that I have 2 others working 34401a which I can use for reference.
The bad news is that I will start to work on it in May so this post right now is just to collect information regarding this issue and for building a reference for future similar cases.

Anyone out there who knows something more about this issue?

Ah, is the "front zero offset" related to the "Ohm tests" or should I expect more than a single problem to fix?
I don't know when but if it is possible I will fix this nice unit :box:!

Many thanks in advance,
« Last Edit: August 01, 2014, 05:02:57 am by zucca »
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Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #1 on: April 28, 2014, 01:04:01 pm »
Back in my home now, I will start soon to work on this. Here below the two pages calibration report from the lab. Looks like the 100 Ohm Range in the Ohm-meter function is not working properly (a offset is there of about 0.08 Ohm).

Regarding the failing zero offset calibration, I am not sure if those two resistors (400K resistor and 40K) can influence that too.




BTW, I understand the zero offset as the procedure to adjust the DMM in order to get 0 when 0 is applied in the input. Since I am pretty new in the calibration process then I´m trying to do little steps to make sure that I am not going in the wrong direction.

My big hope is to get some light in this dark tunnel when I open the unit and start metering the circuits up.

Keep you posted.

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Online Dr. Frank

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Re: 34401a - Cal Error 707
« Reply #2 on: April 28, 2014, 09:10:06 pm »
The cal report may not give the correct hint.
In 1st instance it shows, that some cal values are out of range, and that's what to be expected if the unit has not been calibrated for a long time.

Well, if Ohm function has problems with zero Ohm calibration, that might be the well known problem with the input front / rear selector switch.
After years of usage (>10..20 yrs.), it will be dirty by out-gaseous of the plastic and debris, and some contacts will be hi ohm, so that you get unstable and nonsense reading in 2W Ohm, especially by zeroing.
And if the zero reading is unstable, you will get unstable full scale readings, also, which will in turn prohibit a stable cal reading.

As the lowest Ohm range is out of cal, but not the higher ranges, that gives the an indication that the problem is buried here.
You may post the 4W cal report, if available what's been wrong there.

Make a first test, simply measure a 2W short, in fixed 100 Ohm range, note the reading and switch to the rear panel, repeat the reading there, switch back to the front, and I bet the zero reading will be very different.
All may be in the dozen Ohm range, instead of < 0.1 Ohm

With that unstable switch, a calibration is not possible.

We had an elaborate thread here, what was the error, and how to solve that:

https://www.eevblog.com/forum/testgear/hpagilentkeysight-34401a-2w-problem/

Frank
« Last Edit: April 29, 2014, 07:34:37 am by Dr. Frank »
 

Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #3 on: May 01, 2014, 03:35:48 am »
Thanks Dr. Frank,

It makes very sense what you are telling me. Tonight I don´t have too much time to read the thread, but I promise I will.

Regarding this:

Make a first test, simply measure a 2W short, in fixed 100 Ohm range, note the reading and switch to the rear panel, repeat the reading there, switch back to the front, and I bet the zero reading will be very different.

I liked the proposal a lot, here the results with a short 2W@100 Ohm Range:

Front: 0.013 Ohm (it starts with 0.015 Ohm or 0.014 Ohm and then it settle down to 0.013 after about 10 sec.)
Rear:  0.005-0.006 Ohm with the last digit jumping between 6 and 5

Now same test with same cable in a second 34401a unit (fresh calibrated):

Front: 0.007-0.008 Ohm
Rear:  0.002-0.003 Ohm

and again in a third 34401a unit (fresh calibrated):

Front: 0.004-0.003 Ohm
Rear:  0.000-0.001 Ohm

Unfortunately I do not have the 4W cal report, when the lab saw something was wrong they interrupt the calibration process so all what I have are the above pages.

PS: I didn´t wait to warm up the three DMMs, so the temperature could also cause a drift in the values... Also I did not check if the setup configuration was the same in all the three units.
« Last Edit: May 01, 2014, 03:41:16 am by zucca »
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Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #4 on: May 02, 2014, 03:47:51 am »
We had an elaborate thread here, what was the error, and how to solve that:

https://www.eevblog.com/forum/testgear/hpagilentkeysight-34401a-2w-problem/

Beautiful post, but it looks like none of my three 34401a has a (too) bad old front/rear switch.
Interesting enough I think I have the opposite case: 4W is not working but 2W is working.

Here my test setup:

The top one is the broken one, the other two are OK and fresh calibrated.
The middle one is used as a current meter, the other two R meter@100 Ohm fixed range.

Now this is the correct situation, using the bottom one in a 4W configuration:

5.5 Ohm as sensing current resistor and about 1 mA current to get the R value.

Let´s investigate the broken one, first in 2W:

looks good.

but in 4W:

:-DD

the funny thing is that sometime it drifts between 70 Ohm and 110 Ohm and then back to open circuit.
« Last Edit: May 02, 2014, 03:50:06 am by zucca »
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Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #5 on: May 02, 2014, 02:33:42 pm »
Just tested this morning, 4W from the back works in the bad unit.
It could be a quick fix in the front panel sense input circuits.
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Online Dr. Frank

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Re: 34401a - Cal Error 707
« Reply #6 on: May 02, 2014, 03:56:23 pm »
Just tested this morning, 4W from the back works in the bad unit.
It could be a quick fix in the front panel sense input circuits.

There is no dedicated front sense input circuit!

If rear panel works on 4W, but not on front, the probability is near 100% that the switch is faulty, also in 2W for the front.

I don't understand your test setup..

Please, simply do the test I have proposed, i.e. short circuit the front inputs, in this case all 4 ones, and switch from front to rear to front, and determine the change in readings for 2W Ohm after switching to front again.

If 4W is working on the rear, you can identify the switch section in the circuit diagram, which may be faulty, then open the instrument, and short circuit the two pins of the switch, which should be closed (but are assumed to be high ohm)

Frank
 

Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #7 on: May 02, 2014, 05:08:27 pm »
Hello Frank,

There is no dedicated front sense input circuit!
If rear panel works on 4W, but not on front, the probability is near 100% that the switch is faulty, also in 2W for the front.

Thanks for saving me time on the service manual, I did not deeply look in the schematics yet for lack of  time!

I don't understand your test setup..

This will help, surely a smart guy like you will understand immediately:

      -------------------------
      I                              I
 R Meter                     I Meter
     I                               I
     ---------------------------

it is basically a resistance meter connected to a current meter. The device under tests is the 34401A used as a R meter. The 4 Wire configuration should be obvious.

Please, simply do the test I have proposed, i.e. short circuit the front inputs, in this case all 4 ones, and switch from front to rear to front, and determine the change in readings for 2W Ohm after switching to front again.

It is exactly what I did, see above my previous post. I did not see any big difference between the front and the rear measurement value. May be it´s my English, sorry about that.

If 4W is working on the rear, you can identify the switch section in the circuit diagram, which may be faulty, then open the instrument, and short circuit the two pins of the switch, which should be closed (but are assumed to be high ohm)

Cool idea, this WE looks already overbooked I probably work on that next week. I keep you posted. I agree it´s time to open the unit.

Thanks again for helping me so much.
« Last Edit: May 02, 2014, 08:38:50 pm by zucca »
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Offline jlmoon

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Re: 34401a - Cal Error 707
« Reply #8 on: May 02, 2014, 07:02:13 pm »
Don't know about the test setup in the pictures, but I know I like and want some of those T style banana plugs .. I like those..  ;D

JLM
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Offline gaijin

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Re: 34401a - Cal Error 707
« Reply #9 on: May 02, 2014, 07:34:26 pm »
Don't know about the test setup in the pictures, but I know I like and want some of those T style banana plugs .. I like those..  ;D

looks like Pomona 1825-0 black 1825-2 red
http://www.pomonaelectronics.com/pdf/d1825-4897-4899-6545_101.pdf

They are nice but the handle/jack could be a little longer.

 

Offline jlmoon

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Re: 34401a - Cal Error 707
« Reply #10 on: May 02, 2014, 07:40:33 pm »
Thank you,  I think I am going to order some of the 4897-x series.. I like the idea of streamlined cable exits when stacking them in a complex test arrangement..

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Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #11 on: May 02, 2014, 08:04:00 pm »
Correct guys those are Pomona 1825-0 black 1825-2 red, I made those cables by myself with these wires:

http://www.mouser.com/Search/ProductDetail.aspx?R=6733-2virtualkey56520000virtualkey565-6733-2
http://www.mouser.com/Search/ProductDetail.aspx?R=6733-0virtualkey56520000virtualkey565-6733-0

I spent a lot more than china stuff, but I´m so happy. Moreover you get the length you want... I can only suggest those cool toys.
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Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #12 on: May 07, 2014, 03:56:45 am »
Quick update, I opened the unit.
The front 4W resistance measure becomes stable with correct values and stop to drift when I press down the entire plastic red front terminal block against the pcb.
I will look at the pcb tracks and do further measures. The solder joints looks good, but after I will try to resolder the terminal inputs.

Worst case the sense lines copper inside the terminal block is cracked.

It's becoming interesting now... I suspect the owner before me played basketball with this unit...
I will post pictures soon.
« Last Edit: May 07, 2014, 03:59:06 am by zucca »
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Offline gaijin

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Re: 34401a - Cal Error 707
« Reply #13 on: May 07, 2014, 05:05:03 am »
Would be nice if it was a joint, if it's the block it can be ordered through Agilent.
 

Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #14 on: May 15, 2014, 04:10:29 am »
No!

The front terminal block joints look and measure good:







mmmm...

I also reflow them but the issue was still there...

I then started to follow the sense tracks and, hello 24K resistors... why when I press you the sense line goes from open circuit back to good?
So I reflowed all those six 24K bastards. Actually it looks like three of them had a bad joint (they were invisible to me, I could spot nothing with my naked eyes).

Here after the reflow surgery:


Below now the stable measured series (3x24K) values on the sense lines front and rear (didn´t touch the six 24K on the back):
                     
rear low      72.310 [KOhm], offset 0.43%
rear high     72.319 [KOhm], offset 0.44%
front low     72.070 [KOhm], offset 0.10%                          
front high    72.279 [KOhm], offset 0.39%                            


I got the same problem like coldframe here (thanks again Dr. Frank to point me there)

https://www.eevblog.com/forum/testgear/hpagilentkeysight-34401a-2w-problem/45/

Final check:



passed  :-+

Now I will contact the lab and see if they agree this unit is now back in business.

Bastard 24K Resistors! I allow myself to declare that not only the front/rear switch but also those 24K sense resistors are a weak point of our beloved 34401a.

Finally a big thank to the new big brother on the top which helped to rescue the little one.

« Last Edit: May 15, 2014, 04:23:01 am by zucca »
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Offline sdg

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Re: 34401a - Cal Error 707
« Reply #15 on: May 15, 2014, 08:48:38 am »
Not really brilliant trace routing, there...
--
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Online Dr. Frank

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Re: 34401a - Cal Error 707
« Reply #16 on: May 15, 2014, 04:15:30 pm »
No!


passed  :-+

Now I will contact the lab and see if they agree this unit is now back in business.

Bastard 24K Resistors! I allow myself to declare that not only the front/rear switch but also those 24K sense resistors are a weak point of our beloved 34401a.



Hi, congrats that YOU found the trouble!
You should tell the lab guys, that the instrument IS fine again, although THEY pointed in the totally wrong direction.
Hopefully, they are better in calibration.

Before you send the instrument, make sure, that the switch still is ok, by measuring a short in 2W OHM mode on the front and on the rear and actuating the front / rear switch several times.

The short should give the same readings after switching.
And normally, if it's a massive Cu  short, should be below 10mOhm.
But that zero value may be affected by the failed calibration..

Frank
 

Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #17 on: May 27, 2014, 04:59:25 pm »
The lab contacted me again:

Quote
The 34401A passes all tests accept for the 100 Ohm 2 wire gain test.  I attempted to adjust it in several times, however it will not adjust into specification.  Most likely is a bad resistor somewhere on the main board.  All other tests pass and are within OEM specification.  We can issue a limited calibration on this unit.  Please advise.

 ???

See also the attached service note from Agilent.

Now the 34401a will be sent back to me, I plan to run a test over time by remote controlling the unit (why I didn't it before?  |O). I will then see how and if it drifts, it looks like the R202 is the suspected one.

Keep you posted.
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Online Dr. Frank

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Re: 34401a - Cal Error 707
« Reply #18 on: May 27, 2014, 05:17:01 pm »
Did you check the zero ohm reading in 2W mode, its stability and if the switch is oK?

As R202 is used in ALL lower Ohm ranges, it probably is ok.
Otherwise, all other ranges would also drift.
And also, your unit seems to be much younger than 1994, when this error occurred.

Oh yes, and if the instrument is still in the lab, they could try to calibrate the 100 Ohm range using the rear input jacks.

If that is stable, it's definitely the switch.

Then they could return it to you, and the calibration will be ok, as you clean the switch.

Frank
« Last Edit: May 27, 2014, 05:26:18 pm by Dr. Frank »
 

Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #19 on: May 27, 2014, 06:02:25 pm »
I tested the zero ohm reading and the switch, the stability I watched the display 1 min and it was ok. I did not set up a proper test with a graph of values over time.
Agree on the R202, I think it is used for range up to 1MOhm. Above that the R201 will kick in.

I will also cross check the serial number with the service note.

Oh yes, and if the instrument is still in the lab, they could try to calibrate the 100 Ohm range using the rear input jacks.
...

Nice idea, I feel like stupid now: why I did not have thought about that immediately? Oh yes maybe I was assuming there are 2 different sets of calibrations parameters depending on the switch position.

I proposed your idea to the lab let´s see what happen.

Thanks Frank, I need to buy you a beer.

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Online Dr. Frank

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Re: 34401a - Cal Error 707
« Reply #20 on: May 27, 2014, 08:51:28 pm »
It's a guess only, from remote.  But  it would explain the gain error.

Well, I like Italian beer, so I would like to accept your offer, if my guess turns out to be correct.
????????????????

Frank
 

Offline Zucca

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Re: 34401a - Cal Error 707
« Reply #21 on: June 11, 2014, 05:43:21 pm »
Here the reponse from the Lab:

Quote
Sorry for the delay. The lab has been slammed. Unfortunately, the rear terminals could not be calibrated either. The 100 ohm test is still the one failing and cannot be pulled into tolerance.

They will now send me the unit back as it is and I will investigate what´s going on.

It is becoming interesting now... stay tuned!
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Offline free_electron

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Re: 34401a - Cal Error 707
« Reply #22 on: June 11, 2014, 06:37:33 pm »
that string ofd resistors is troublesome. i've had a number that came loose over time.
sometimes resoldering doesnt help. you need to replace em.
The electrode plating cracks off the body. resoldering bridges the crack but they crack in another spot later...
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Offline mimmus78

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Re: 34401a - Cal Error 707
« Reply #23 on: June 11, 2014, 10:50:29 pm »
Yes replace those resistors, I had the similar problem on a K2000 ... just reflow did not worked. I'm starting thinking Keithley and HP used the same resistors parts and they just go wrong after few years.
 

Online Dr. Frank

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Re: 34401a - Cal Error 707
« Reply #24 on: June 12, 2014, 07:11:35 am »
Have they tested the 2W 100 Ohm range on the rear jacks, as proposed?

For the rear channel, there is also a different set of protection resistors involved, therefore if the problem lies there, it could also vanish on the rear.

Otherwise, the lab people are not very clever..

Frank

Btw:
Those 3x24k  resistors (R170-172, R180-182) are not at all involved in 2W Ohm mode, but in 4W Ohm only.
Follow the H, L marked lines in the schematic, p. 155 and 156:

The parts in serial with the DUT and therefore possibly affecting 100Ohm range 2W Ohm are:
- Input Jacks and its soldering
- Switch S1A, S1G (FRONT/REAR), most probable, => Label HI
- Label HI => K101 (switches OHMS_CURRENT)

The node between K101,3 and L101 is the high Kelvin Point for the Ohm function, as over L102, K103 to DCLO the DUT voltage is measured. Therefore, only the 3 mentioned parts may influence the 2W function.

If the 2W current source is not ok, (not very probable), this could be measured by monitoring the ohm current by another DMM.

« Last Edit: June 12, 2014, 09:20:42 am by Dr. Frank »
 


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