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

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Dr. Frank:
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.

free_electron:
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.

Dr. Frank:

--- 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 ---

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:
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

coldframe:

--- Quote from: Dr. Frank 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

--- End quote ---

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.

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