### Author Topic: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem  (Read 23866 times)

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#### coldframe

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##### [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« on: January 24, 2014, 12:01:13 am »
When I measure 2-wire ohms I get an offset in the reading.

For example.
I will get readings from 31 Ohms in 2-wire mode when measuring a 10 ohm resistor.
* In 4-wire ohms mode,  an accurate stable reading of 10.0 ohms.
* This is my 10 Ohm resistor Information  http://www.te.com/catalog/pn/en/1624323-4

This resistor measures a stable 10.0 ohms on a known good meter.
Any Idea?

Best regards.
« Last Edit: March 08, 2014, 04:24:28 pm by coldframe »

#### free_electron

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #1 on: January 24, 2014, 01:48:43 am »
most likely the output protection cascade for the meter has taken a hit from overvoltage.

in 4 wire mode this cascade is bypassed and the current sources feed the 4terminal output directly.
the cascade is a chain of 6 or 7 transistors. most likely one of em is fried.

look in the schematic it is very easy to find the cascade
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#### coldframe

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #2 on: January 24, 2014, 02:44:23 am »
most likely the output protection cascade for the meter has taken a hit from overvoltage.

in 4 wire mode this cascade is bypassed and the current sources feed the 4terminal output directly.
the cascade is a chain of 6 or 7 transistors. most likely one of em is fried.

look in the schematic it is very easy to find the cascade

free_electron..

I have often heard your name., I'm honored to meet you.

I'll get it checked soon.
Thank you for the tip.

*  most likely one of em is fried. , What does the "em" stand for?

#### Fluxed Matter

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##### HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #3 on: January 24, 2014, 04:41:52 am »
That's slang in the US for them.
Have a Great Day!
Fluxed Matter

#### SeanB

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #4 on: January 24, 2014, 05:00:51 am »
"em" means them. Replace all the transistors with new from the same batch and you will only have to test them to see which was faulty. They are cheap.

#### coldframe

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #5 on: January 24, 2014, 05:21:35 am »
That's slang in the US for them.

Ah.. I Got it. Thanks

#### coldframe

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #6 on: January 24, 2014, 05:22:24 am »
"em" means them. Replace all the transistors with new from the same batch and you will only have to test them to see which was faulty. They are cheap.

Okay, I'll do that.
SeanB Thanks for the help.

#### free_electron

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #7 on: January 24, 2014, 07:58:35 am »
yep , you got the correct block in the schematic
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#### Dr. Frank

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #8 on: January 24, 2014, 11:28:15 am »
Most likely, the front/rear switch S1 on p.9-8 (pdf-page 150) of the service manual is faulty, i.e. either S1-A or S1-G is high ohm (20Ohm?). In 4 Ohm mode, any additional serial resistors in the current path are eliminated (that's the purpose of 4W ohm), so the reading is correct again.

You can make a very simple check, by switching from front to rear and using the rear jacks in 2W Ohm mode.

I predict, the reading will be okay.
In this case the switch needs to be replaced, or desoldered and cleaned internally - if you dare to make such a repair.

You can measure this contact resistance of 20 Ohm with an additional DVM, if you open the case, switch to front panel again and measure the resistance across the named switches. Normally, they have to be < 100mOhm.
Or perhaps a solder joint at the switch is faulty, that can be detected by measuring resistance from H or L input to the outputs of those switches, behind the switch, on the PCB, see schematics.

Similar errors already have been reported by several other users.
And my own 34401A shows the same behaviour, if I use the rear jacks.

Remark:
It is not true, that the high voltage protection transistor-bank Q203-Q210 is not active in 4W Ohm mode.
Wouldn't make sense, as the current source in 4W Ohm mode has to be protected the same way as in 2W mode
And the schematic clearly shows, that there is no switch to bypass it.

Conclusion: If 4WOhm is working correctly, the protection circuitry definitely is working properly.

Frank
« Last Edit: January 24, 2014, 01:23:04 pm by Dr. Frank »

#### coldframe

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #9 on: January 24, 2014, 04:33:40 pm »
Most likely, the front/rear switch S1 on p.9-8 (pdf-page 150) of the service manual is faulty, i.e. either S1-A or S1-G is high ohm (20Ohm?). In 4 Ohm mode, any additional serial resistors in the current path are eliminated (that's the purpose of 4W ohm), so the reading is correct again.

You can make a very simple check, by switching from front to rear and using the rear jacks in 2W Ohm mode.

I predict, the reading will be okay.
In this case the switch needs to be replaced, or desoldered and cleaned internally - if you dare to make such a repair.

You can measure this contact resistance of 20 Ohm with an additional DVM, if you open the case, switch to front panel again and measure the resistance across the named switches. Normally, they have to be < 100mOhm.
Or perhaps a solder joint at the switch is faulty, that can be detected by measuring resistance from H or L input to the outputs of those switches, behind the switch, on the PCB, see schematics.

Similar errors already have been reported by several other users.
And my own 34401A shows the same behaviour, if I use the rear jacks.

Remark:
It is not true, that the high voltage protection transistor-bank Q203-Q210 is not active in 4W Ohm mode.
Wouldn't make sense, as the current source in 4W Ohm mode has to be protected the same way as in 2W mode
And the schematic clearly shows, that there is no switch to bypass it.

Conclusion: If 4WOhm is working correctly, the protection circuitry definitely is working properly.

Frank

Dr. Frank
Oh, I see. Let me check.
Thanks for the update.
Thanks for a detailed explanation.

P.S
Yes. 4WOhm is working correctly.
Vielen Dank

#### Dr. Frank

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #10 on: January 24, 2014, 06:16:09 pm »
Just checked my 24 years old instrument.
Simply shorted Hi/Lo on front and rear.
Initially measured 0.15 Ohm on the front.

Still has this 2W Ohm problem on the rear, when I actuated the REAR switch..
Reading is unstable, jumping between 1..40 Ohm.

Now, FRONT reading is unstable also... same error.. definitely switch.. oxydized contacts perhaps..

Next construction site for me, also...

Welcome in the club..

Frank

Update: Have opened my 34401A, and found root cause, as expected: Switch S1 , Hi contacts S1A are the "stinkers".
If I short those pins directly on the topside of the switch, namely pins #1-#2 for front or #2-#3 for rear, the readings become rock stable again at around 0.5 Ohm rear, 0.1 Ohm front. Otherwise several ten Ohms and jumping around.

Switch looks as if it can be opened, disassembled  and cleaned.

Follow-up soon.
« Last Edit: January 24, 2014, 06:54:40 pm by Dr. Frank »

#### free_electron

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #11 on: January 24, 2014, 06:52:18 pm »
oops. brainfart on my part. DrFrank is right.

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.
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#### Dr. Frank

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #12 on: January 24, 2014, 07:03:49 pm »
oops. brainfart on my part. DrFrank is right.

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.

You mean those 2 x 6 x 24k ones?
You mean cracks in the solder joint?
Mine still look very well under magnification, but I'll check also.

Thanks!

#### Dr. Frank

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #13 on: January 24, 2014, 10:20:37 pm »
Ok, here's an intermediate result and an obvious root cause found.

I desoldered the switch completely, (quite easily done) and measured all contacts.
Found 2 out of 8 switches which had higher resistance of several Ohms.

Then I carefully pulled out the lever inside a small box, because I ended up with 8 very tiny springs and 8 tiny wipers.

During pulling out and testing different wipers on the faulty contacts, I identified, that 2 of the wipers were the problem, not those 6 out of the 24 stator pins.

The stator pins were glued to the case, at its bottom and at the top  plate, with some sort of epoxy, which already showed signs of decomposition.
Inside the plastic case, small pieces of debris could be seen.
The debris originates quite obviously from the epoxy.
The surfaces of the stators seemed clean, as far I was able to look inside.
I could also see some abrasive dust (correct term?) originating from the lever.
So I also cleaned the inside with a cotton bud.

When I looked closer at the gold plated wipers, I noticed a faint film on the wipers, which can be carefully rubbed of with a dry, soft cotton bud.
This film probably caused the bad contact.
Otherwise, there were absolutely no signs of oxidation or abrasion of the gold surfaces.

My explanation for that film is evaporation of plasticiser from the case plastics, or from the epoxy.
A normal process for 25 years old stuff like that..

(I have seen Buzz Aldrins lunar space suit in the Smithsonian's Air & Space Museum, and all the plastics of that historic piece were in the process of disintegration, without any chance of holding that up, as it was mentioned there, I think)

Tomorrow morning, I will clean all wipers, and will try to reassemble those small puzzle pieces. Break-Before-Make wipers and Make-Before-Break wipers.... oh Dear!
Sometimes, small springs spring far away...

If I am successful, I will post a few pictures.

Good night - Frank
« Last Edit: January 24, 2014, 10:31:54 pm by Dr. Frank »

#### coldframe

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #14 on: January 25, 2014, 01:47:24 am »
Ok, here's an intermediate result and an obvious root cause found.

I desoldered the switch completely, (quite easily done) and measured all contacts.
Found 2 out of 8 switches which had higher resistance of several Ohms.

Then I carefully pulled out the lever inside a small box, because I ended up with 8 very tiny springs and 8 tiny wipers.

During pulling out and testing different wipers on the faulty contacts, I identified, that 2 of the wipers were the problem, not those 6 out of the 24 stator pins.

The stator pins were glued to the case, at its bottom and at the top  plate, with some sort of epoxy, which already showed signs of decomposition.
Inside the plastic case, small pieces of debris could be seen.
The debris originates quite obviously from the epoxy.
The surfaces of the stators seemed clean, as far I was able to look inside.
I could also see some abrasive dust (correct term?) originating from the lever.
So I also cleaned the inside with a cotton bud.

When I looked closer at the gold plated wipers, I noticed a faint film on the wipers, which can be carefully rubbed of with a dry, soft cotton bud.
This film probably caused the bad contact.
Otherwise, there were absolutely no signs of oxidation or abrasion of the gold surfaces.

My explanation for that film is evaporation of plasticiser from the case plastics, or from the epoxy.
A normal process for 25 years old stuff like that..

(I have seen Buzz Aldrins lunar space suit in the Smithsonian's Air & Space Museum, and all the plastics of that historic piece were in the process of disintegration, without any chance of holding that up, as it was mentioned there, I think)

Tomorrow morning, I will clean all wipers, and will try to reassemble those small puzzle pieces. Break-Before-Make wipers and Make-Before-Break wipers.... oh Dear!
Sometimes, small springs spring far away...

If I am successful, I will post a few pictures.

Good night - Frank

Problem SOLVED!!, All is back to its normal state

Be careful.  wipers!
They are not all the same, 2 wipers little bit difference
Check out Attachment (Bottom 2  pieces)

Well, all right. I guess we both need some rest
I wish you all the best

Good night
Danke Dr. Frank

PS.
I checked  the PNP Transistors, P,N J-Fet one by one, There is nothing wrong
This is mechanical faults.
« Last Edit: January 25, 2014, 03:46:28 am by coldframe »

#### coldframe

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##### Re: HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #15 on: January 25, 2014, 03:07:51 am »
oops. brainfart on my part. DrFrank is right.

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.

Problem Solved! , All is back to its normal state

I'm very honored to meet you.
Thank you free_electron.
« Last Edit: January 25, 2014, 03:47:41 am by coldframe »

#### Dr. Frank

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #16 on: January 26, 2014, 08:19:54 pm »
Well, I was also successful repairing my old 34401A.
And here are some more photos, as I assume, that this is a typical error of vintage 34401A instruments.
Many owners will have to do that easy repair.

The first photo shows, how to disassemble and reassemble the switch, taking care not to lose any of those tiny springs.
You can also see the epoxy bead, with signs of disintegration
All parts are displayed, in the box.
In the 3rd photo, the faint film is visible, contrasting the shining gold plated surface.
The surface has to be cleaned with cotton sticks and alcohol to avoid any scratches.
I also cleaned the interior of the switch assembly with alcohol and a thinned cotton stick

After reassembly, all contacts had less than 15 mOhm again. Before, 2 of them had up to 30 Ohm (unstable).

The best contacts now have 3 mOhm.

Afterwards, I had to recalibrate the instrument, especially the Zero calibration.

The polluted wipers had spoiled  a foregoing calibration totally.

Now, in 2W Ohm mode, Hi/Lo-inputs shorted, front and rear measurements show 3 mOhm maximum resistance; that is the variance of the switch contacts after actuating the front/rear switch.

« Last Edit: January 26, 2014, 08:31:54 pm by Dr. Frank »

#### Kjelt

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #17 on: January 26, 2014, 09:30:09 pm »
Would an IPA bath not have taken care of it also without disassembling the switch?
Saves a lot of time but still nice to see the craftmanship at work.

#### GreyWoolfe

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #18 on: January 26, 2014, 10:29:13 pm »
"Heaven has been described as the place that once you get there all the dogs you ever loved run up to greet you."

#### Dr. Frank

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #19 on: January 26, 2014, 10:34:58 pm »
Would an IPA bath not have taken care of it also without disassembling the switch?
Saves a lot of time but still nice to see the craftmanship at work.

IPA = iso propyl alcohol.

Well, I doubt that.

The film sticks very badly on the contacts, in a way that multiple actuation of the front/rear switch does not heal the problem at all.

The film can be mechanically rubbed off completely by a cotton swab already.

I could not determine, if the film dissolves in alcohol; I used that only to be sure to remove any grease like stuff, and to have a wet interface between the cotton and the surfaces, which would evaporate easily.

No, it's really safer to disassemble and clean mechanically.

Frank

#### coldframe

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #20 on: January 26, 2014, 11:20:05 pm »
Would an IPA bath not have taken care of it also without disassembling the switch?
Saves a lot of time but still nice to see the craftmanship at work.

Hi, Kjelt
You right, this is time-consumer, but I enjoy this time of the year
in my case,This is mechanical faults
The best thing to do would be to disassemble/clean(polish)/assembly or replaced worn-out switch
without disassembling the switch?, I'm not sure it worked.
take the Instrument to the Local Service Center, Then you can save time and be more productive
I say this out of kindness
This is Not lawnmower, Do not add WD-40 Something
« Last Edit: January 26, 2014, 11:51:37 pm by coldframe »

#### coldframe

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #21 on: January 26, 2014, 11:40:08 pm »
Well, I was also successful repairing my old 34401A.
And here are some more photos, as I assume, that this is a typical error of vintage 34401A instruments.
Many owners will have to do that easy repair.

The first photo shows, how to disassemble and reassemble the switch, taking care not to lose any of those tiny springs.
You can also see the epoxy bead, with signs of disintegration
All parts are displayed, in the box.
In the 3rd photo, the faint film is visible, contrasting the shining gold plated surface.
The surface has to be cleaned with cotton sticks and alcohol to avoid any scratches.
I also cleaned the interior of the switch assembly with alcohol and a thinned cotton stick

After reassembly, all contacts had less than 15 mOhm again. Before, 2 of them had up to 30 Ohm (unstable).

The best contacts now have 3 mOhm.

Afterwards, I had to recalibrate the instrument, especially the Zero calibration.

The polluted wipers had spoiled  a foregoing calibration totally.

Now, in 2W Ohm mode, Hi/Lo-inputs shorted, front and rear measurements show 3 mOhm maximum resistance; that is the variance of the switch contacts after actuating the front/rear switch.

Congratulations!!
Did you Zero calibration?
according to Service manual (Page 77)  "4-wire short (copper) across"
What are you using to Zero calibration the short-bar?

Writing about a topic helps you integrate new knowledge with what you already
And this, obviously, is what Most 34401A users really want
You are proper person for the work ^^;
Dr. Frank, Post New Article Please

Congratulations again
« Last Edit: January 26, 2014, 11:47:56 pm by coldframe »

#### robrenz

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #22 on: January 26, 2014, 11:48:05 pm »
Excellent place for Deoxit G100L a very good gold lube. I rub it in well and then wipe liquid off with a clean Kimwipe.

#### coldframe

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #23 on: January 26, 2014, 11:58:55 pm »
Excellent place for Deoxit G100L a very good gold lube. I rub it in well and then wipe liquid off with a clean Kimwipe.

robrenz,
Thanks for valuable information

This is my solution ^^
I rub superfine fibres and wipe cotton

#### Dr. Frank

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##### Re: [SOLVED]HP/AGILENT/KEYSIGHT 34401A 2W Ohm Problem
« Reply #24 on: January 27, 2014, 08:21:22 pm »

Congratulations!!
Did you Zero calibration?
according to Service manual (Page 77)  "4-wire short (copper) across"
What are you using to Zero calibration the short-bar?

After repair, zero reading on the rear panel was 0.59 ohm, caused by the faulty switch and in turn wrong zero cal.

I soldered a real low ohm (<< 1 mOhm) and low thermal voltage (<< 1 µV) zero ohm bridge, see photo.

The bent copper wire on the left serves the same purpose, but for my HP3458A.

Frank
« Last Edit: January 27, 2014, 08:23:23 pm by Dr. Frank »

Smf