Teardown: Leeds and Northrup 4776 Decade Box - Page 1
Forum section: electronics / testgear

author: enut11
author_rank: Super Contributor
timestamp: May 09, 2020, 06:42:20 am
content: I acquired a Leeds and Northrup 4776 four decade resistance box. This is compact instrument measures 150mm x 150mm x 85mm high.Total resistance span is from 0.1ohm to 1111ohms.Inside I found some very robust twenty-position switches. The switches allow the full decade to be selected twice for a full rotation, presumably to reduce wear and tear.As you can see from the photos, the contacts are very robust but also need to be as the lowest range is in 0.1ohm steps.I did a rough check and found it to be accurate. How accurate will have to wait until I install proper low-loss banana jacks.As received, the box came with four multi-pin sockets including a centre-tapped 35.8ohm resistor. I would be happy to know the original purpose of those sockets and resistor.For my use I am going to remove the 4 sockets and replace them with two quality banana jacks or posts.Would be happy to know the resistance specs for this decade box, including tempco, if available.enut11

author: enut11
author_rank: Super Contributor
timestamp: May 09, 2020, 06:47:27 am
content: Photos of inside L&N 4776

author: AVGresponding
author_rank: Super Contributor
timestamp: May 09, 2020, 08:42:14 am
content: I wonder if that odd resistor is something to do with real time temperature compensation?You could try nudging the temperature and see if it has a noticeable swing.

author: edpalmer42
author_rank: Super Contributor
timestamp: May 09, 2020, 05:01:35 pm
content: I have a L&N Wheatstone Bridge with similar switches. Absolutely massive! Notice the triple contacts.I'm sure you've realized that those sockets are just crappy addons. I didn't have to google it to know that, but I did google it to confirm that the original unit just had a pair of terminal posts.

author: maginnovision
author_rank: Super Contributor
timestamp: May 09, 2020, 05:51:29 pm
content: Almost looks like an add-on heater. The tempco probably isn't going to be anything too fantastic but should be easy enough to get an idea.

author: enut11
author_rank: Super Contributor
timestamp: May 09, 2020, 08:26:51 pm
content: I found the specs for the L&N 4776, it is rated at 0.1% +.01ohm. Not bad.http://electroniccalibrators.tpub.com/TB-9-6625-2153-35/TB-9-6625-2153-350029.htmYes, this box deserves much better terminals and I just happen to have these...

author: Vgkid
author_rank: Super Contributor
timestamp: May 10, 2020, 04:26:37 am
content: Those connectors, and extra resistor do look like an add onI do wonder what the extra resistor is doing?Catalog listing:

author: enut11
author_rank: Super Contributor
timestamp: May 10, 2020, 05:05:30 am
content: A mystery indeed. It must have been a custom job as the original position of the input posts does not correspond with any of the 'holes' on the face-plate of my unit.The extra resistor will be going unless there is a good reason to leave it there...

author: enut11
author_rank: Super Contributor
timestamp: May 11, 2020, 04:40:48 am
content: Old sockets and mystery resistor were removed. You can see the witness marks for the original terminal posts in one of the pics.New copper terminal posts added and original leads soldered. All surplus holes were plugged. A bit of black to cover scratches.I now have a nice useful bit of gear. Good to get it back into service again.