Decade box maintenance

[ChristianFletcher]

I have what I believe is a high accuracy decade resistance box from Genrad. I’m guessing it’s quite old and just wondered if there is any maintenance required. The switch contact are large brass things with multiple contact fingers. I wondered if a wipe over with DeOxit may help keep things clean and in good order. I was hoping to use this to test the resistance ranges on some DVM meters I have been asked to review.

Thanks Chris

[trobbins]

I recently restored a vintage decade box - more repair than maintenance!

The 3-blade rotating switch contacts have gouged slight troughs in each of the step terminals, although for my item the main issue was the slow brass oxide coating built up over decades of no use - which I resorted to 1000 grit paper to just remove to get fairly consistent readings. For maintenance I added a light switch contact cleaner/lubricant to hopefully suppress any change in contact resistance. Even so, sometimes I measured a slight variation in resistance, so moving a step and taking a reading may need to be done a few times to confirm the reading is consistent (certainly if you down under 0.1% tolerance).

https://www.dalmura.com.au/static/GA...ance%20box.pdf

Ciao, Tim

[6AL5W-Martin]

in my Box made by Siemens is written to care the contacts of the switches with clean vaseline. It´s a laboratory class type, expensive hand work. The contacts are covered by a front sheet, not open

[Bazz4CQJ]

I've had can of DeOxit a long time, but I don't often use it. I definitely don't think that I would use it on brass. As above, a mild solvent cleaner with a lube, perhaps vaseline, sounds better to me.

B

[factory]

Similar question about cleaning decade resistor contacts on eevblog at the moment; https://www.eevblog.com/forum/repair...ve-to-deoxit)/

Looks as if the contacts are meant to be self cleaning in normal use, with vaseline or similar used to keep oxidation away from the bare copper or brass contacts. And avoid abrasive cleaning.

I did note that Genrad use silver plating on some low resistance decades & every zero contact too.
1433 RESISTOR.pdf

David

[ChristianFletcher]

Ok thanks for replying. I had heard about Vaseline but wasn’t sure as it’s such a good insulator.

[factory]

Interesting IET still make the Genrad 1433 decade boxes, but the current range have solid silver-alloy contacts. https://www.ietlabs.com/pdf/Manuals/1433_im.pdf

The instructions state the following, if the decade box hasn't been used for a while;

Operation: If switches have not been operated for an extended
period, they should be rotated a few times in both directions to
restore contact resistance to specifications.

They also mentions how to clean the contacts of the current model (with silver alloy contacts).

3. Should switch contact cleaning or lubrication be required, as may be indicated by an increase in the zero resistance, this may be done be spraying the switch contacts with a conditioning compound such as WD-40 or Deoxit from Caig Laboratories, or Super Lube with PTFE from Synco Chemical Corp.

It would be interesting to see what the older instruction say, but I haven't seen those.

Looks as if the newer ones use all silver alloy contacts & older ones use bare copper for the 1k & above contacts, except the zero contact. See about the 9 minute into the eevblog video, not sure about his choice of standard WantonDestruction-40 at the end.
https://www.youtube.com/watch?v=fKrvtYS_6fI

David

[Bazz4CQJ]

Originally Posted by factory

They also mentions how to clean the contacts of the current model (with silver alloy contacts).

David

Unusual to see WD40 mentioned right next to DeOxit; very different animals at very different prices!

B

[ChristianFletcher]

I need too open up the boxes and have a closer look at the contact material. I may even see what the cost would be for getting the calibration checked