Teardown: Early Leeds & Northrup 4776 Decade Resistance Box

[fivesixzero]

Hi Friends,

A Leeds & Northrup 4776 decade resistance box recently came into my life and I thought some here may find photos of it interesting. This is my first resistance box and I'm slowly working on building out a proper bench so its likely to see some use even with its relatively limited sub-1k resistance ranges.

I did find another post about this box, but the device documented there was of a much newer vintage - SN 16xxxxxx vs 6xxxxx on this box - and had some apparent aftermarket mods. This one looks to be stock and was built a bit differently.

As part of my research on this little guy I dug up some old ads. The earliest I could find was from the 'front matter' (ads section) of the journal Science dated May 9, 1941. Back then it was advertised at $35, or about $641 in 2021 dollars (if my math is correct). Pretty neat.

Based on the batch of stuff it was in at my local surplus dealer I'm guessing it came from the student labs at a local university. Minnesota has some pretty old public and private universities so its possible that this thing was purchased sometime between the 40's and the 60's (given the SN range) and ended up being used quite a bit.

Its in pretty great condition, all age considered. The only odd thing I found was a soft rattling sound inside which ended up being a loose bit of wax. The hand-wound resistors on it were sealed with what looks like parafin or bees wax and I'm guessing an inattentive student probably pumped too much current through a few of the 10x and 0.1x resistors, leading to a minor meltdown.

The knobs turn smoothly and the wipers on the bottom are in good shape, although I still took some time to give the wiper surfaces a wipe down with some contact cleaner. Given the thick layers of gunk that came off (and the improved linearity in resistance changes) its possible that I could've been the first person to give it a bit of TLC, or at least the first in a long long while.

I took resistance measurements at all ranges before and after disassembly and contact cleaning, but they're not really interesting. If anyone's actually curious I can take the time to format them into a table and post em up. The TL;DR on the data is that its within 0.1% of its full-range values across the board, even after all these years.

The only modification I've considered doing would be to replace the old binding posts with some proper banana jacks, or something fancy like the copper posts/jacks like the ones on the µCurrent. It seems like a shame to disrupt this long-lived old timer though so its likely going to be kept as-is.

[TimFox]

I don’t know about the binding posts on their resistance boxes, but other manufacturers, e.g. Keithley, used L&N binding posts where low thermal emf was required.  You might want to keep them.

[fivesixzero]

I don’t know about the binding posts on their resistance boxes, but other manufacturers, e.g. Keithley, used L&N binding posts where low thermal emf was required.  You might want to keep them.

Thanks for mentioning that. Another good reason not to tamper with this beauty.

I haven't taken a super close look at the posts yet. Will have to dig in a bit more, maybe dig up the old DSLR and macro lens beforehand...