Decade Box maintenance and servicing (L&N 4775)

[Rax]

In the spirit of "I'll soon have to move my bench just to the other side of the garage door, because the garage is choke full of stuff," I just grabbed a cheap L&N decade box. BTW - does anyone have a manual for this, operation and/or servicing?

The 10 ohm module had a slipping shaft, and so I have to secure it, and also bring it to the correct indication of the dial. Other than that, it seems fully functional, and at least at a first round of measurements, close enough to indicated values.

Are there some recommended maintenance things to do to this? Some absolute don'ts?

For instance, I'm tempted to clean all contacts with pure alcohol and then apply just a bit of deoxit on the copper plates for contact conditioning.

Thank you for your input.

[Vgkid]

That sounds reasonable for a cleaning regiment . Though you may find that you don't need to do anything . Especially after rotating the dials back and forth a few times.

[mendip_discovery]

I have cleaned up the odd Croppico unit for a customer. I would prefer not to but I was getting some rather un repeatable zero readings even after a fair bit of exercise of the contacts it was still giving up to 0.5 ohm error and respectability of 0.3 ohm. I tend to do a check on the zero before measuring the decade and after just to see if it wanders.

I cleaned with deoxit and a cotton swab. I would like to know if there is a lube that is recommended to prevent the contacts oxidizing and to allow the contact to move smoothly.

I was lucky as I stripped them for cleaning I was able to check all the screws for the stoppers were tight, several had rattled loose over the years.

[jonpaul]

restoration of many precision decade resistors, dividers and capacitors.

isopropyl alcohol for cleaning case and removing dirt, old lube on switches

contact cleaner on switch contacts, Deoxit is also fine but crazy expensive.

very slight lube of rotating switch shafts, drop of mineral oil or Vaseline

Exercise every switch and contact.

very corroded contacts may need fine emery paper to burnish

Bon courage

Jon

[Rax]

That sounds reasonable for a cleaning regiment . Though you may find that you don't need to do anything . Especially after rotating the dials back and forth a few times.

Thank you very much for the attachment. Interestingly, it says "[limit of error] statement assumes that box is in regular use, or that dials will be rotated before box is put into use after a period of idleness." I guess my conundrum exactly.

[TimFox]

The style of rotary switch used by L&N and other contemporary manufacturers clean the contacts by wiping when rotated.
If it sits around too long without rotation, then crud can form on the contact until you clean it by rotating a few times.

[trobbins]

A significant difference can be the duration of inactivity and the environment during that time.  Restoring an inactive box from pre 1940-50's may need a different approach compared to a box that has been well cared for and intermittently operated (even if last operation was a decade or 2 ago).  Contact oxidation may have grown over many decades in time to a level that not even repeated use will give a consistent contact resistance, as jonpaul recounts.

[Rax]

The style of rotary switch used by L&N and other contemporary manufacturers clean the contacts by wiping when rotated.

That's a really pertinent point, I was wondering why the particular mechanical choice for the switching assembly.

The unit I have doesn't have particularly aged or oxidized etc. switches. I just like to recondition and care for my instruments (while, critically important, not damaging anything). In that sense, it sounds like cleaning with IPA and applying a bit of D5 will be fine.

The fact that one switch was mechanically compromised (like I said, the shaft was not secured in place and so the switching arm was slipping) is another matter, and I hope my repair is good. I did fiddle with the main nut holding the switch shaft - which I'm not sure I needed to - which has some type of "thread-locker" applied.

So I guess my concern with that is whether the tension on the switching copper leafs ("arm") - which I assume this nut would contribute to setting - is part of calibration of this box, or I'm overthinking it and the "thread-locker" is just there to secure the nut long-term. I can't imagine this being so (first scenario), but I thought I'd ask the question here to collect some input.

[mendip_discovery]

The style of rotary switch used by L&N and other contemporary manufacturers clean the contacts by wiping when rotated.

So I guess my concern with that is whether the tension on the switching copper leafs ("arm") - which I assume this nut would contribute to setting - is part of calibration of this box, or I'm overthinking it and the "thread-locker" is just there to secure the nut long-term. I can't imagine this being so (first scenario), but I thought I'd ask the question here to collect some input.

I doubt it. The use of torque on these kinds of devices I feel would be rare, I have yet to see service manuals for things that state torque settings for stuff. The thread lock is mostly just to avoid the nut coming loose.