Products > Test Equipment

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

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coldframe:

--- Quote from: free_electron 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.

--- End quote ---

Problem Solved! , All is back to its normal state

I'm very honored to meet you.
Thank you free_electron.

Dr. Frank:
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.
 

Kjelt:
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:
Congratulations on your repair.

Dr. Frank:

--- 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 ---

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

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