How to clean contacts of a precision resistor decade (0.01%)?

[Kjelt]

It even gets weirder, just cleaned my drawer and when placing back the Kontakt 61 and Kontakt WL they felt empty and they are empty.
Never used in three years and then poof gone with the wind. Unbelievable.
My Kontakt Dustoff, PCC and IPA are ok though. 

[Dr. Frank]

Hi,

Years ago, I also cleaned a Burster resistance box, same problem, first with Q-Tips and alcohol, but that was not enough, so I did it mechanically, using fine polishing emulsion I use for our CERAN cooking field.

Take a bit of this emulsion on a Q-Tip, and polish the contacts until the copper contacts look bright again. The Q-Tip will get really black.
Afterwards, wipe the emulsion off with another Q-Tip.

This procedure lasts for many years already, zero resistance is still about < 20mOhm.

I can confirm the rusty Kontakt-60 can-
I'll never use that stuff again.

Frank

[carl_lab]

Hi Frank!

Years ago, I also cleaned a Burster resistance box, same problem, first with Q-Tips and alcohol, but that was not enough, so I did it mechanically, using fine polishing emulsion I use for our CERAN cooking field.

Take a bit of this emulsion on a Q-Tip, and polish the contacts until the copper contacts look bright again. The Q-Tip will get really black.
Afterwards, wipe the emulsion off with another Q-Tip.

Are you sure, that were copper contacts?
The 1422 contacts looks more like gold or silver plated to my eyes... 
Maybe your decade is 1406 or 1407 (7-digits)?

You had the problem in "0-position" or in "10-position"?
I think "0" and "10" are prone to contact problems, because there is no possible "mechanical overshoot" when dialing these positions (the mechanical end stops are very tight tolerated).
If I wiggle the dial very little bit back and forth (barely noticeable), contact resistance becomes immediately good.

This procedure lasts for many years already, zero resistance is still about < 20mOhm.

That's not enough for the 1422 spec. (5 mOhm for each 3-digit "half").

My decade is in spec. regarding "zero". Only problem is the 10x1 Ohm position.

I can confirm the rusty Kontakt-60 can-
I'll never use that stuff again.

Déjà-vu... 

I also never used Kontact-60 again, and if I used Kontakt-61 I double checked it is not K-60 (with a bad feeling in my stomach remaining).

[Dr. Frank]

Hi Frank!


Are you sure, that were copper contacts?
The 1422 contacts looks more like gold or silver plated to my eyes... 
Maybe your decade is 1406 or 1407 (7-digits)?

You had the problem in "0-position" or in "10-position"?
I think "0" and "10" are prone to contact problems, because there is no possible "mechanical overshoot" when dialing these positions (the machanical end stops are very tight tolerated).
If I wiggle the dial very little bit back and forth (barely noticeable), contact resistance becomes immediately good.


That's not enough for the 1422 spec. (5 mOhm for each 3-digit "half").

My decade is in spec. regarding "zero". Only problem is the 10x1 Ohm position.

You're right, it's an old 4107, similar to 1407.
It should be < 10mOhm, and has about 13mOhm at the moment. (added: 9.6 mOhm with 4W and OCOMP)
I'd guess, that after 40 years, the spring force of the wiper contacts have decreased a bit.
Maybe, that's the problem with your 1Ohm decade.

For determining the contact material I had to open my unit, see picture.

It's hard to tell, what Burster has used.
It's definitely no pure copper, but I also doubt that it's gold plated, as these strong spring contacts would easily rip that off.
Also, gold plated contacts would be easy to clean with alcohol, as gold does not oxidize.
Seems to be something like a brass alloy, or phosphorous bronze.

I remember, that most of these contacts had been looking really black, and the self-cleaning mechanism, as advertised by Burster, did not do the job any more.
So I had to do this mechanical polishing.

Frank

PS: does yours really have these stepper motors inside, with IEEE488 interface?
Would like to see and hear this resistor box in action..   

[carl_lab]

No, it's not the motorized option.

Thanks for posting the photo of your switches.
Yes, determining contact material by appearance is very difficult. The color of your contacts looks very similar to mine, see my first 3 photos. (My last 3 microscope photos were highly colour degraded because of poor white balance or wrong camera colour profile - I had to use that unfamiliar microscope of our neighbour department...)

Have you relubricated contacts after polishing?

What do you think about the "mechanical overshoot" point?
I'm not sure if I could make clear enough what I meant by "mechanical overshoot"...

[Vgkid]

Those switches look rather nice. If chemical means dont work, go with some fine sandpaper/emory cloth. I actually prefer that one.

[carl_lab]

I ordered "Oszillin T6", again.
I hope Buerklin can deliver this time...

[Kjelt]

No sandpaper that is too coarse, use normal printing paper, takes a bit more time but it is just as effective but does not take off the plating.

[Smith]

I normally use Kontakt 60 for cleaning really dirty contacts, then clean/rinse very well with IPA and cotton sticks. Normally fudging some white paper soaked in IPA between the contacts works well enough. For bigger contacts I even use wet grid 2000 sandpaper or metal polish (and clean with IPA afterwards).

I have used Kontakt 61 (for protecting instead of cleaning contacts) on some heavily corroded but cleaned contacts. It still seems to work well, much better than without the Kontakt 61.

[Krampmeier]

I have a similar resistor decade, but with 4 digits only. Very nice ww manganin resistors inside, got it for just 15 euros at a ham radio flea market...
The unit had unstable connections at all the contacts, and cleaning did not help, so I took the thing apart and bent the spring contacts a bit to increase the pressure. That solved the problem and the decade is still very close to it's spec.
As this method was not suggested yet, I wonder if bending the contacts (possibly made of beryllium copper alloy) by hand is something so terribly bad that no one should ever even just consider it? Will it harm the contacts on the long run?

I attached two photos of the box, unfortunately I did not take any pics of the contacts...

By the way, those Burster decades are really nice. They sometimes drift and go out of spec after 30 years or so, but usually they are very reliable. There are lots of them at my workplace, including some with motor and GPIB connection. They are annoyingly loud when changing the value though. Unfortunately, Burster does not have any spare parts for the motors any more, so if one of those fails, you have a big problem...

[babysitter]

My father who was caring for and feeding thousands of relay contacts (post office) had wooden sticks with paper and/or leather on the end in his workbox, I have to ask him if they were used immediately on the contacts.

[Smith]

The unit had unstable connections at all the contacts, and cleaning did not help, so I took the thing apart and bent the spring contacts a bit to increase the pressure. That solved the problem and the decade is still very close to it's spec.

I would only use this as a last resort. I have seen people use this method for bad contacts, but you are possibly creating a weakspot in the metal you are bending. Depending on how much the spring contact is moving and its thickness, the problem can come back quite soon. Of course, the second time you bend it, it gets worse and worse.

Bending contacts to overcome dirty connections can also wear out the connections prematurely due to the connections scraping eachother with more force. I have seen this happen a few times.

[SeanB]

My father who was caring for and feeding thousands of relay contacts (post office) had wooden sticks with paper and/or leather on the end in his workbox, I have to ask him if they were used immediately on the contacts.

Yes, those were used to burnish contacts. the leather strips were dipped in a little alcohol and rubbed on the german silver contacts to clean them, and the paper ends are used with the relay contacts held closed to burnish the mating surfaces to a even fit. The leather was reusable, and the paper could be cleaned a few times in alcohol before you tossed the stick out. The paper used is a very high clay art paper, and the clay is the abrasive material there.

You can also use some plain high clay art paper ( at the craft store, used for watercolours as it does not wrinkle) cut into strips to burnish the contact surfaces, simply cut into strips and draw between the contacts a few times, and toss after each contact.