Agilent 34401A, Error 613, (Ohms 5uA source failed), fixed !

[HighVoltage]

My Agilent 34401A showed up with an ERROR 613 during the self test.
This is a model, built in 2008, with the round knobs.

The Service Guide is listing the Error 613 like this:
"Ohms 5 uA source failed This test configures to the 10 V dc range
with the internal 10 M 100:1 divider U102A connected across the input.
The 5 ?A ohms current source is connected. The compliance limit of the
current source is measured. A 20 ms ADC measurement is performed
and the result is checked against a limit of 7.5 V ± 3 V."

The divider circuit U102-A is on the enclosed schematic (page 156 of the service guide)
It seem that the divider is functioning correctly.


Has anyone come across this problem before?
Or can someone please have a look and suggest a direction of what to look for?
The divider U102-A is also used in other ranges and they are error free.

Thanks for any help.

[VK5RC]

I can't help you directly but I found this
https://www.eevblog.com/forum/testgear/got-myself-an-agilent-34401a-with-a-label-'not-working'-on-it/
my 34401 threw up all sorts of odd error codes 1st (Error 621+) when a power supply zener died and then a bit later one of the Tantalums i another power circuit decided to short. I would check out the voltage rails as that lead me to the problem.
I recall a post about the current source transistors can be troublesome, it may have been DrFrank but don't quote me.
Another issue can be the front to rear terminal switch but yours sounds different.
Another resource is the Yahoo group Agilent/HP equipment forum, but it is a bit quiet I recall.
Wish you good hunting . Robert

[Dr. Frank]

Hi,
the error 613 description is not consistent, compare to the 500nA error description..
5µA over 10MOhm would create about 50V, therefore there is another 1.5MOhm resistor (or 1.76MOhm || 10M) involved.

The error has nothing to do with the 100:1 divider.. The relays K101,102,104, involved in this test setup, maybe suspect, see p.156.

First, forget about the test error - Does the instrument read correctly in the 2W/4W Ohms mode, associated with 5µA?
Should be 1MOhm range, pls. find table in service manual.
What about the other Ohm ranges?

Check and post result, please.

Frank

[HighVoltage]

Thank you both for the reply,

Frank,
First I discovered that also Error 613, 615, 617, and 619 are set.
These errors refer to:
613: Ohms 5 uA source failed
615: Ohms 10 uA source failed
617: Ohms 100 uA source failed
619: Ohms 1 mA source failed

The interesting part is, that the ohms reading in all ranges are perfectly right on.
Tested with a 100Ohm, 1kOhm, 10 kOhm, 1 MOhm precision resistor.
-This is true for front and rear terminals.
-This is also true for 4 and 2 Wire measurements.

The DC current during Ohm measurement also is right on.
Measured with a Fluke 87V, I am getting the following currents:
in the different ranges:
0.5 uA in 100 M range
0.5 uA in 10 M range
5.0 uA in 1 M range
10 uA in 100 k range
100 uA in 10 k range
1 mA in 1 k range
1 mA in 100 Ohm range

In other words, the ohm reading of the instruments are right on and the current through the DUT is right on.
But I still have the 4 errors in the system.

Do you have any further suggestions of what to test next.
Thanks.





 

[Dr. Frank]

Hi, and sorry,

I have written some nonsense, as I did not fully read the error description..

All of these failing tests you've reported, all check the Ohm compliance voltage, not the Ohm current sources themselves...

That means, it is checked, that in case of overload in Ohm mode (open or too high resistance per Ohm range), the output voltage does not not exceed 7V(+-3.5V).
I expect, that this compliance voltage is too high..
You may check that, by choosing a fixed (manual) 2W Ohm range, e.g. 10k range, and measure the output voltage between Hi and Lo by a multimeter (10M or infinite input resistance).

I bet it even exceeds 10.5V, where it should be limited to  7V typical.

There will be a failure either in the +18V power supply (which would endanger your whole instrument), or zener CR203 is short, and applies more than about 13V at pin 3 of Fineline resistor array U102D. See schematics page 9-10 or pdf page 157.
Zener failure is most probable, I think.

This CR203 has 4.7V or 5.1V, depending on vintage of your instrument. Therefore check with shorted Ohm input, if it zeners about 4.7..5.1V.

Your description indicates, that every other component in the Ohm current source circuitry is ok.
 
Frank


PS: On my old HP34401A, I just checked the output voltage in overload condition, with an 10MOhm voltmeter:

It ranges from 7.8V in the 2W-100 Ohm range, up to 8.4V in the 2W-1 M range.

[HighVoltage]

Hello Frank and thanks for the detailed feedback.

CR203 is a dual package zener with 5.1V but only one side used, it seems.
The Voltage across the zener is stable at 5.17 V at all 2W ohm ranges.
So, it seems the zener is working perfectly.

The output voltage at the terminals in 2W Ohm setting is wrong
As you already indicated, it is too high.
Here are tee values, measured with a 10 MOhm impedance voltage meter:

100M range = 2.5 V stable
10 M range = 5.0 V stable
1 M range = 10 to 13 V jumping around
Same for all other ranges, down to the 100 Ohm range
Sometimes the voltage is going down to 7 V but then back up to 10 or 11 or 12 or 13 V

On the OpAmp U201-A I am measuring the supply voltage as 23V, so it is 5 Volts too high.
Then I went on to the Power Supply, Page 9-15
The output of the LM317 is + 18 V (correct)
The output of the LM337 is - 17 V (bad)

Can I assume that the LM337 is bad?
Or do I need to look at some other causes that killed the regulator?
Also, isn't it strange that with such a bad supply voltage that the instrument is only giving the Ohms errors?
Also, why is the + Voltage on the U201-A at +23 V, if only the negative regulator is bad?

Thanks for any further feedback.











 

[VK5RC]

Are you measuring from the appropriate 'ground' re U201 voltage, it should be AGND? i.e. pin 4

[HighVoltage]

Are you measuring from the appropriate 'ground' re U201 voltage, it should be AGND? i.e. pin 4

Yes, I am measuring from pin 4 to pin 8 of U201
And I am getting fluctuations from 20V to 23V

The +18 output from the LM317 (U551) seems to be ok and stable.

[HighVoltage]

OK, it seems the meter is fixed.

I installed a new LM317 (U551) and a new LM337 (U552)
Output of the LM317 is now +18.83 V
Output of the LM337 is now - 18.10 V

Now the voltage between pin 4 and 8 of U201 is stable at 18.80 V
No more fluctuations.

The output voltage on the front terminal on the 2W Ohm settings are as follow:
100 Ohm = 6.477 V stable
1 kOhm = 6.477 V stable
10 kOhm = 6.718 V stable
100 kOhm = 6.934 V stable
1 MOhm = 6.942 V stable
10 MOhm = 5.010 V stable
100 MOhm = 2.503 V stable

All seems to be perfect now.

One more question:
Do I need to adjust the output of the LM317 with the feedback resistors to be closer to + 18.0 V or is 18.8 V acceptable
for the rail?

Thanks for all your help in fixing this instrument.
I really appreciate it.




 

[Dr. Frank]

Page 114:
+18V may vary from 18.0 to 20.0V
-18V may vary from -17.0 to -19.1V
So it seems to be perfect. Take an oscillogram on both voltages, if they are stable.

It's strange, that you measured +18V directly at the LM317, but 23V at U201, which is the same plane, without any filter in between. So I think, the LM317 has been oscillating before.
If C552  is a Tantalum, check it for 1uF, I suspect this to be open.

Frank

[HighVoltage]

Yes, I should have looked at the output of the LM317 with a scope.
Too bad.

I removed C552 and as you suspected, it is bad.
But not open.
Capacitance is about 89 nF instead of the 1uF .

Here is a picture.

Any suggestion what I should replace this one with?
Thanks

[SeanB]

89n means it is mostly open, or has an internal crack which isolated most of it. Replace with another, or with a 22uF 35V electrolytic if you have nothing else.

[Dr. Frank]

Use a 1µF / 25V ceramic cap instead, or a new tantalum.
Also check the twin cap C554, at -18V.

Frank

[HighVoltage]

Thanks for the suggestion to also look at C554
That one seemed to be OK, but I replaced both of them now C552 and C554

Here are two scope shots at the output of the LM317 with the new caps installed:
- The DC coupled scope picture at 5V /Div shows a straight line in normal acquisition mode.
- The AC coupled scope picture is taken at 50 mV / Div and shows a pk/pk AC of 62 mV
I would think that this is acceptable.
Would you agree?

I also went through a calibration of the OHM ranges and all seems perfect.
It looks like this was a successful repair.
Thank you all for the help and advise.

[Dr. Frank]

 

Frank

[HighVoltage]

Thank you Frank for all your great help.
The meter is working perfectly now in all ranges and all units.
Just perfect!

[coldframe]

Hello, HighVoltage
May I ask how you measure?  (I Mean about the test setup)
Thanks.

The DC current during Ohm measurement also is right on.
Measured with a Fluke 87V, I am getting the following currents:
in the different ranges:
0.5 uA in 100 M range
0.5 uA in 10 M range
5.0 uA in 1 M range
10 uA in 100 k range
100 uA in 10 k range
1 mA in 1 k range
1 mA in 100 Ohm range

Thanks for the suggestion to also look at C554
That one seemed to be OK, but I replaced both of them now C552 and C554

Here are two scope shots at the output of the LM317 with the new caps installed:
- The DC coupled scope picture at 5V /Div shows a straight line in normal acquisition mode.
- The AC coupled scope picture is taken at 50 mV / Div and shows a pk/pk AC of 62 mV

[HighVoltage]

Hi coldframe,
This is a very simple setup.
Just connect two wires from the 34401A to a multimeter that can measure uA to mA current.
Insert the two wires in to the Input HI and Input LO of the Agilent 34401A
Set the 34401A to 2W Ohms measurement, Set the Front rear switch to front terminals.
Read the current in uA or mA on the second multimeter that is set to DC current
Then go manually through the OHM ranges of the Agilent 34401A
Repeat the measurements for the rear input plugs with the switch set to REAR terminals.

[coldframe]

Thanks Hi Voltage,

Here below the  My reslult(Measured with a Fluke 189)
0.63 uA in 100 M range
0.63 uA in 10 M range
5.11uA in 1 M range
10.07 uA in 100 k range
100 uA in 10 k range
0.998 mA in 1 k range
0.998 mA in 100 Ohm range

Nearly identical, Seems to there is nothing special.   

[motocoder]

I love threads like these. It restores my faith in humanity to see people helping others to fix these wonderfully engineered products.

[coldframe]

Hi, motocoder

You are so right
Engilish is not My native language But It is only aside from the question.
We have Univesal Language, Let's say Schematic, Diagram, Graph etc., 
You know what they say, "A picture paints a thousand words after all"

I love threads like these. It restores my faith in humanity to see people helping others to fix these wonderfully engineered products.

[HighVoltage]

Hi coldframe,

it seems your first three ranges are out of tolerance.
I am not sure, but I think the current must be very precise at 5.0 uA
I can measure another 34401A to confirm this for you.