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

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coldframe:

--- Quote from: Kjelt 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.

--- End quote ---

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

coldframe:

--- Quote from: Dr. Frank 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.

--- End quote ---

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

robrenz:
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:

--- Quote from: robrenz 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.

--- End quote ---

robrenz,
Thanks for valuable information

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

Dr. Frank:

--- Quote from: coldframe on January 26, 2014, 11:40:08 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?



--- End quote ---

Yep, I had to.
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

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