[SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem

[coldframe]

a problem comes up

Hi,

It's all fine, except it's got a fault when measuring ohm with 2-wires again

For example.
I will get readings from 9.5 Ohms Value when Power On
But, Over time, Increase Ohm Value 10.7 Ohm
See attached.

*  When using 4-wires-measurement everything's alright.

Did anyone have that problem before?
Thank you

Coldframe

[coldframe]

Hi, Dr. Frank

How about you?
A problem has been encountered while measuring the Ohm
(increasing steadily Ohm Value)
Where do you think the main problems occur?

I'd be happy to hear what your thoughts are on this matter.

Regards, coldFrame

[robrenz]

a problem comes up

Hi,

It's all fine, except it's got a fault when measuring ohm with 2-wires again

For example.
I will get readings from 9.5 Ohms Value when Power On
But, Over time, Increase Ohm Value 10.7 Ohm
See attached.

*  When using 4-wires-measurement everything's alright.

Did anyone have that problem before?
Thank you

Coldframe

You may just be seeing the TC of the resistor as it warms up from the drive current of the meter.

[coldframe]

You may just be seeing the TC of the resistor as it warms up from the drive current of the meter.

Hi, robrenz
What does the "TC" stand for?
Could you be more specific?

Thanx!

[robrenz]

Temperature Coefficient.  In this case I implied TCR for Temperature Coefficient of resistance which is ppm change in resistance per degree change of resistor temperature.

[coldframe]

Temperature Coefficient.  In this case I implied TCR for Temperature Coefficient of resistance which is ppm change in resistance per degree change of resistor temperature.

Oh.. I see
Thank you for the explanation
So, This resistor measures a stable ohms on a known good meter.
* Temperature Coefficient (ppm/°C) ±5

[Dr. Frank]

a problem comes up

Hi,

It's all fine, except it's got a fault when measuring ohm with 2-wires again

For example.
I will get readings from 9.5 Ohms Value when Power On
But, Over time, Increase Ohm Value 10.7 Ohm
See attached.

*  When using 4-wires-measurement everything's alright.

Did anyone have that problem before?
Thank you

Coldframe

Don't understand your new problem, sorry. And my instrument is now rock stable in Ohm again, right from the beginning.


Zero Ohm reading is now ok, in 2W Ohm, in the beginning, and a short also reads << 1Ohm after warm-up / later?

Then you measure a 9..10 Ohm resistor, which reads always the same over time, when you use 4W Ohm?

And only if you measure the same resistor in 2W mode, then the reading will go up over time?

What kind of resistor do you measure?

Frank

[robrenz]

Here is a thought,  In 2 wire mode even if you zero out the lead resistance you are seeing the TCR of the resistor and the copper leads. Also since the connection points are carrying current and measuring voltage drop there will probably be a greater thermal emf generated. Reversing the connection and seeing the difference in readings will give a clue as to how much thermal emf is involved.

I am not saying that there isn't something wrong with the meter but you have to consider the above points.

[coldframe]

Don't understand your new problem, sorry. And my instrument is now rock stable in Ohm again, right from the beginning.


Zero Ohm reading is now ok, in 2W Ohm, in the beginning, and a short also reads << 1Ohm after warm-up / later?

Then you measure a 9..10 Ohm resistor, which reads always the same over time, when you use 4W Ohm?

And only if you measure the same resistor in 2W mode, then the reading will go up over time?

What kind of resistor do you measure?

Frank

Hi, Dr. Frank

Yes, When using 4-wires-measurement everything's alright.
Have you seen the Attatched Picture?
Only 2-Wires Over time, continuous increase

This is Resistor Specification.
http://www.te.com/catalog/pn/en/1624323-4

Do you have any good ideas?

Thank you
coldframe

[coldframe]

Here is a thought,  In 2 wire mode even if you zero out the lead resistance you are seeing the TCR of the resistor and the copper leads. Also since the connection points are carrying current and measuring voltage drop there will probably be a greater thermal emf generated. Reversing the connection and seeing the difference in readings will give a clue as to how much thermal emf is involved.

I am not saying that there isn't something wrong with the meter but you have to consider the above points.

Please understand,  I'm Not native English speaker

2-Wire
Attachment #1 Small Resistor Box
Attachment #2 Start
Attachment #3 10 Minute Later

4-Wire
Attachment #4 Stable

even now, on the increase....
what seems to be the trouble?   

Thanx

[Dr. Frank]

Well, please make a cross check over time, with a solid short in 2W mode.
Should read <0.2 Ohm or less, and be stable over time.
That's what I expect strongly, as your instrument should be ok again, after repair.

The resistor has got 5ppm/K, so no chance that the drift of 0.2Ohm in your picture comes from heating up.. that would require 4000°C.

You obviously simply wrapped the leads of the resistor around the posts- that makes no good contact at all.
This is the most probable root cause.
Please solder that resistor to the posts.

Perhaps the jacks themselves do not make contact very well.
That could be the reason, why the 4W measurement gives stable results... looks strange,, anyhow.

4W measurements give always smaller values than 2W measurements.. So test your instrument with a short first..

As you have a low Ohm resistor, you should use 4 jacks for the resistor box anyhow, i.e. 4 pole Kelvin connection.
All the contact resistances kill the accuracy of your resistor.

Frank

[coldframe]

Well, please make a cross check over time, with a solid short in 2W mode.
Should read <0.2 Ohm or less, and be stable over time.
That's what I expect strongly, as your instrument should be ok again, after repair.

The resistor has got 5ppm/K, so no chance that the drift of 0.2Ohm in your picture comes from heating up.. that would require 4000°C.

You obviously simply wrapped the leads of the resistor around the posts- that makes no good contact at all.
This is the most probable root cause.
Please solder that resistor to the posts.

Perhaps the jacks themselves do not make contact very well.
That could be the reason, why the 4W measurement gives stable results... looks strange,, anyhow.

4W measurements give always smaller values than 2W measurements.. So test your instrument with a short first..

As you have a low Ohm resistor, you should use 4 jacks for the resistor box anyhow, i.e. 4 pole Kelvin connection.
All the contact resistances kill the accuracy of your resistor.

Frank

All right, I will try immediately!
Thank you for the tip.

coldframe

[Dr. Frank]

Ups.. don't you own a simple, short lab cable with decent plugs, to short Hi and low in 2W mode???

Frank

[coldframe]

Ups.. don't you own a simple, short lab cable with decent plugs, to short Hi and low in 2W mode???

Frank

Dr. Frank
My apologies. I don't understand what you mean.

[Dr. Frank]

Ups.. don't you own a simple, short lab cable with decent plugs, to short Hi and low in 2W mode???

Frank

Dr. Frank
My apologies. I don't understand what you mean.

I asked you to make a test in 2W Ohm, shorting High and Low with a simple laboratory cable with good plugs.

What you are showing, looks not so good.

If you simply do the test, you can see, if the 2W mode is working correctly and if the zero is properly calobrated, after your repair.

Frank

[coldframe]

I asked you to make a test in 2W Ohm, shorting High and Low with a simple laboratory cable with good plugs.

What you are showing, looks not so good.

If you simply do the test, you can see, if the 2W mode is working correctly and if the zero is properly calobrated, after your repair.

Frank

I am sorry for the delay in my response.
Crash my Laptop, I have sent my Laptop be mended.

Would you Read The Personal Message?

Thanx

[Dr. Frank]

Hello Coldframe,

sorry, it took me a while to shoot "speaking" photos (I hope so).

Just repeat your measurements, like I show you in the pictures.
Use 6 digit, slow mode.

Pic 1: in 2W mode, short input with a decent (good) cable. Check, if reading is a few milli - Ohm.

(See my 34401A, it still has 22mOhm, although it has been calibrated after the repair.
The switch is clean now, but worn-out. That still leads to changing readings, when using front/rear switch,  but now several mOhm only, and that does not disturb the readings)

Pic 2: Use Null function to remove the offset.

Pic 3: The 10 Ohm resistor is now measured correctly in 2W mode

Pic 4: 4W mode does not need Null math. The reading is even smaller compared to 2W Ohm mode with Null.

Pic 5: The resistor is soldered, that is better than clamping. Small resistor values always need 4 pole connection.

Please tell me, what you have measured.
Good luck.

Frank

[coldframe]

Hello Coldframe,

Pic 1: in 2W mode, short input with a decent (good) cable. Check, if reading is a few milli - Ohm.

(See my 34401A, it still has 22mOhm, although it has been calibrated after the repair.
The switch is clean now, but worn-out. That still leads to changing readings, when using front/rear switch,  but now several mOhm only, and that does not disturb the readings)

Pic 2: Use Null function to remove the offset.

Pic 3: The 10 Ohm resistor is now measured correctly in 2W mode

Pic 4: 4W mode does not need Null math. The reading is even smaller compared to 2W Ohm mode with Null.

Pic 5: The resistor is soldered, that is better than clamping. Small resistor values always need 4 pole connection.

Please tell me, what you have measured.
Good luck.

Frank

Hi Dr Frank,

This is My Test Result, the Order are the same.

First of all,  press the button for the 2W Ohm and slow 6 Digit Mode Setting (A: MEAS MENU)

Pic 1: 0.09Ohm, Sometime 0.2Ohm... variable

Pic 6: When I Measure 2W Ohm Reading Value Increse  as time passes, This is My Problem
* This is Not Agilent, old HP
 
Thank you for your consideration.
Danke schön!

[Dr. Frank]

Ok, then your instrument still has a BIG problem with the front/rear switch.

You can test that, if you short the input in 2W mode, and engage (press) the front / rear switch.
After returning to "front", the reading will increase strongly, perhaps several ohm already, I assume.

Then try pressing this button a little bit, not completely, you will see the zero-reading change a lot.

I have the same problem with my cleaned switch, but not so strong.

If nothing helps, order a spare switch from agilent.

PS: The "old" HP 34401A are still good, but switches are always a problem.

The HP3458A has a similar switch, same supplier, same construction, it's about 13 years old, and  it shows also increased and unstable resistance on the 2W Ohm mode, but not yet so strong, also.

Frank

[coldframe]

Ok, then your instrument still has a BIG problem with the front/rear switch.

You can test that, if you short the input in 2W mode, and engage (press) the front rear switch.
After returning to "front", the reading will increase strongly, perhaps several oohm already, I assume.

Then try pressing this button a little be, not completely, you will see the zero-reading change a lot.

I have the same problem with my cleaned switch, but not so strong.

If nothing helps, order a spare switch from agilent.

PS: The "old" HP 34401A are still good, but switches are always a problem.

The HP3458A has a similar switch, same supplier, same construction, it's about 13 years old, and  it shows also increased and unstable resistance on the 2W Ohm mode, but not yet so strong, also.

Frank

Yeah, I guess so.
After change a part, I'll let you know the result

Thank you Sir.

P.S
I have not one but seven 34401A
This is My Fisrt, Oldest, Favorite, Still Good Condition Except 2W Ohm

[coldframe]

Problem has been solved

This is a complicated problem, Quick summary about it

First : Front/Rear Select Push Switch mechanical Falut (ie, oxydized contacts..)
Pic 1. New Part (C&K Components Silver Plate Push Switch)
Alternate Part# 51902 (F8UEE)
http://www.mouser.com/ProductDetail/CK-Components/51902-F8UEE/?qs=%2fha2pyFadujTidtN1Llg9KzbODfiX9eD%252bMABDo%2f5rXk%3d

Agilent Part# 3101-3162
http://www.home.agilent.com/myagilent/faces/partDetail.jspx?partNumber=3101-3162&imageStatus=YES&_afrLoop=688445535652000&_afrWindowMode=0&_afrWindowId=null#%40%3F_afrWindowId%3Dnull%26_afrLoop%3D688445535652000%26imageStatus%3DYES%26partNumber%3D3101-3162%26_afrWindowMode%3D0%26_adf.ctrl-state%3D1220ztkopw_13

If nothing helps, order a spare switch from agilent.
PS: The "old" HP 34401A are still good, but switches are always a problem.

Second : H/L Sense Seriese Resistor Fault
Pic 2. I can't see the problem with the naked eye
Pic 3. Desolder them and Check one by one(3 defect of the 12 resistors,  25 percent )
Pic 4. Soldered the circuit boards (72Kohm Seriese resistor temporarily)

Test : Function : 2W, Resolution : 6 Digit Slow Mode, Hi/Lo Shot and NULL function
Pic 5. Benchvue
Pic 6. Multimeter


CRCW251224K0JNEG   24K 1W 2512 Chip Resistor
http://www.mouser.com/Search/Refine.aspx?Keyword=CRCW251224K0JNEG

Agilent Part# 0699-3406
http://www.home.agilent.com/myagilent/faces/partDetail.jspx?partNumber=0699-3406&imageStatus=YES&_afrLoop=52685776462000&_afrWindowMode=0&_afrWindowId=2irg93od1_19#%40%3F_afrWindowId%3D2irg93od1_19%26_afrLoop%3D52685776462000%26imageStatus%3DYES%26partNumber%3D0699-3406%26_afrWindowMode%3D0%26_adf.ctrl-state%3D2irg93od1_31

another thing to check while you got the machine open is the big 2512 resistors close to the big switch. i have seen those develop cracks on the electrodes. simply resolder them.

After many complications the matter was settled But,
At the moment still has corroded solder joint problem but expected that

To Do.
Find Potential Problems in PCB(ie, Corroded solder joint, Cracked Joint, Pad Contamination..)
Ordering new parts
Calibration

Thank you again to Dr. Frank, free_electron for a work knowledge , and everyone who helped me out on this forum.

coldframe

[razberik]

Hello. I am writing to contribute to repair logs of 34401A.
I received cheap, broken 34401A with missing mains switch push rod, that plastic part holding front terminal in place and rear plastic frame.

1. problem - not booting
Multimeter was not starting properly. When switched on, it illuminated all display segments, then two beeps and then nothing. Shift was working.
Exact behaviour like this:

Turned out to be bad connection between front panel and mainboard. Here is what I found. Broken connector. Somebody torn off that plastic part holding ribbon cable in place. Temporary fixed with wires extension. Not very clean solution, but it works until I receive proper replacement.
After this repair, multimeter starts properly.

2. problem - measuring wrong numbers on some ranges
After ribbon cable fix, multimeter was showing something, but nonsense numbers. When switched to 100V, 1000V range, everything works. All DC amperes OK. 4W Ohms OK. Random errors from selftests.
Then I realized that 2W sometimes work innacurately, sometimes dont work at all.
So I started to trace the signal path when some little voltage applied to input. And then it suddenly started to work.
Turn out to be bad connection on R104 2512 13K resistor. When I was touching that place, results were changing randomly. Resolved by applying flux and reflowing solder joints on R104.

Therefore I am confirming this:

another thing to check while you got the machine open is the big 2512 resistors close to the big switch. i have seen those develop cracks on the electrodes. simply resolder them.

and indirectly this (from this thread):

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).

After repair, multimeter passes all tests and seems to be working properly. Compared to another two 34401As.
I hope it might help somebody.

Now I should sketch up missing plastic parts for 3D printer. We will see...

[bitseeker]

Nice hunting to a successful repair. Having 3D models for replacement plastic parts would be great as more and more of them become unavailable from Keysight's parts inventory.

[coldframe]

hello razberik,

I congratulate you on your success 
I'm sure great help to someone.
coldframe.

[HighVoltage]

Nice repair and good price!
I had a broken 2512 resistor before as well and it is an easy fix.
This seems to happen more often than expected.