Leeds & Northrup 4737-A20 tear down - Page 1
Forum section: electronics / general

author: r6502
author_rank: Regular Contributor
timestamp: January 29, 2024, 09:32:17 pm
content: Hello all,in the mean time the L & N 4737-A20 Guarded Wheatstone Bridge has arrived.I did not power it up, but I opened it, to take some photos, see below:Frontpanel:Rear Panel:Inside view, whole device:Front terminals:Front small switches:Resistors on range selector and 1st stage:Resistors on stage 5:Resistors on stage 6:Housing, heavy aluminum die cast:Hafe fun to look at details - Guido

author: vindoline
author_rank: Unknown
timestamp: January 30, 2024, 07:07:31 pm
content: Beautiful. Thank you for the pictures!

author: Vgkid
author_rank: Super Contributor
timestamp: January 31, 2024, 09:53:18 pm
content: Thanks for the teardown. Interesting that the lowest decade is 0.01 Ohms. I figured that it would have been 0.1ohms like the 4232. It looks like your unit is newer than mine. Cool to see those smaller super accurate resistors.

author: trobbins
author_rank: Frequent Contributor
timestamp: February 02, 2024, 10:27:08 am
content: Interesting that the lowest decade is a continuous wire. I presume each segment was adjusted down by filing, with no way to add resistance back in, and then the assembly was enamelled.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2024, 07:10:31 pm
content: The vey lowest resistor part does not have to be that accurate - some +-10% may be acceptable there. No trim needed, just get a reasonable good length of the wire.The tricky part is more with the switches, to get a stable and low constact resistance.

author: TimFox
author_rank: Super Contributor
timestamp: February 02, 2024, 07:40:21 pm
content: The Keithley 260 nanovolt decade voltage source uses Leeds and Northrup low thermal emf binding posts (inside a chamber behind a front-panel door) similar to the black posts on your 4737.

author: bastl_r
author_rank: Regular Contributor
timestamp: February 02, 2024, 07:49:31 pm
content: HiVery interesting item.Does somone have a schematic or manual?I didn't found anything.Only items on ebay with crap automatic translations eg: wheatstone = Weizenstein.

author: r6502
author_rank: Regular Contributor
timestamp: February 04, 2024, 10:52:55 pm
content: Hello all,At 1st, I found a manual at ebay, located in the US for 39$ plus shipment 25$ - this is a bit expensive I think. I'm not sure if I will buy this.In the mean time, I found the time to have a closer look on the device.At 1st, I had to find out, where loose wires in power supply section have been attached. During this work, I draw the shematic for the power supply section. It was also interesting, that TR1 must have been repaced and this must have been done in the german language area, as the label text for wire colors of this transformer is written in german. It is also assembled with metric M3 screws, while moste of the remaining srews have impirial dimensions.Schematic of the power supply:TR1 left side, TR2 right side, and bridge rectifier, top view:TR2 on seperate electric insulated mounting plate with 3 teflon feed throughs:TR1 right side, TR2 left side, here the shieldig of TR1 is visible:TR3 with the small PCB for the main power supply for the bridge:The power generated from TR3 is used to supply the the bridge and the detecor circuitry.I do not have an idea, for what purpuse the voltage, generated from TR2 is used. It is rectified, but not filtered, and the the enoumous effort, for insulating TR2 is interesting. One thing here is strange: the mounting plate for TR2 is electrically floating, as it is not connected to any net of the device. Only shields of 3 cables are connected to this mounting plate. I would have expected that its is connected to guard net - what are you thinking?At the end I did measure 3 resistors with the bridge, and compared the results with my HP34401a:Nominal value: 3,75k, 1%L&N Bridge: 3.76282kHP34401a: 3.76265k ± 0.4761Nominal value 100k, 1%L&N Bridge: 99.9766kHP34401a: 99.9686k ± 10,997Nominal value 1M, 1%L&N Bridge: 1.00822MHP34401a: 1.008015M ± 110.80Looks not to bad I think. I added the uncertainty of measurement taken with the HP34401a, calculatet out of 1year spec. from datasheet.GuidoEdit: spelling, photo links inserted between image tags,

author: JohnG
author_rank: Frequent Contributor
timestamp: February 05, 2024, 01:13:57 pm
content: At this link, you will find a manual for the L&N 4735 Guarded Wheatstone bridge. You will need to scroll down the list to find it. It's not the manual you are looking for, but perhaps it has some useful information.https://www.surplussales.com/Manuals/man_l-misc_test.htmlHope this helps.John

author: Overspeed
author_rank: Frequent Contributor
timestamp: February 05, 2024, 06:00:01 pm
content: HelloOld transformer have a tendency to smoke as insulating is aged and lost its specification up to be brittle , other problem is the capacitors agingFor transformer that possible to rebuilt themThat a nice piece of equipment which need care and preventive maintenance.RegardsOS

author: Vtile
author_rank: Super Contributor
timestamp: February 08, 2024, 05:07:02 pm
content: I would not be surpised if that bridge is closer to the correct value than the multimeter.Any idea from which time frame the bridge has been manufactured. I do assume somewhere 1970s since it has both old and new L&N hardware.I have one cable fault L&N resistive bridge which all bakelite hardware (but already aluminium box) made before 1967. This looks much more modern.

author: r6502
author_rank: Regular Contributor
timestamp: February 08, 2024, 09:39:27 pm
content: Hello all,The device came without a type label, and i found no date codes on the parts. Woming Weekend will be a long weekend, and I will remove the cover of the detector and also take some photos of that part - may be that ther are some date codes hidden.As soon, as I have more material, I will post it here.Guido

author: r6502
author_rank: Regular Contributor
timestamp: February 26, 2024, 10:03:36 pm
content: Hello All,in the mean time I did some reverse engineering of the device and have drawn a schematic without the detector electronics. Please have a look on the schematics - comments are welcome. It took a bit more time, as for the same net sometimes different wire colours have been used. It was also interesting, that most of the wiring was done with solid wires. The wiring of the of the detector electronics was done with stranded wires, and it looks, as if this is not the original wiring.I will do a tear down of the detector electronics, and draw a schematics for this as well. When this is done, I will post the results here.Guido

author: r6502
author_rank: Regular Contributor
timestamp: March 03, 2024, 09:00:40 am
content: Hello All,yesterday, I found the time, to dissassemble the detector electronic box. Have a look on the photos:All parts of the detector, top to bottom, left to right:Detector outer cover;Detector frame, that hols the PCB with feedthrough for power and detector inputs on the top;Top cover for power and detector input connections;Top shield of detector PCB;Bottom shielding of detector PCB;Detector PCB with connecting cables;Fasteners for the detector assembly;Detector frame with PCB and top shielding:Detector frame with PCB and bottom shield:Detector PCB, top and bottom side:A schematic of the detector will follow.There is a black metal tube, with a socket, direct soldered to the PCB. Could this be a chopper amplifier? It is labeled with "019400 M W C". Any informations welcome. I think, as long, as the device is working, I don't disassemble this part.The age of the detector is in the time frame of 1969 to 1970, as the date codes on transistors and electrolytic capacitors are in-between late 1968 to late 1969.BR Guido

author: Vgkid
author_rank: Super Contributor
timestamp: March 03, 2024, 10:51:27 am
content: The mwc part is a transformer.I bought the manual, but i havent had a good look at it.

author: r6502
author_rank: Regular Contributor
timestamp: March 03, 2024, 08:33:37 pm
content: @Vgkid,is your bridge / the detector nor working or is the manual quality not so good?Guido

author: Vgkid
author_rank: Super Contributor
timestamp: June 26, 2024, 05:15:40 am
content: Attaching the schematic, unfortunately the detector is not included.Trying out my new toy. The manual might be in a few minutes or, later.

author: Vgkid
author_rank: Super Contributor
timestamp: June 28, 2024, 06:56:12 am
content: As promised the manual, I unfortunately had to compress it.

author: r6502
author_rank: Regular Contributor
timestamp: June 29, 2024, 01:57:27 pm
content: Hello Vgkid,thanks for uploading th manual and the original schematic.Guido

author: Vgkid
author_rank: Super Contributor
timestamp: July 15, 2024, 01:09:46 am
content: Posted the Higher resolution versions of the manual on BAMAhttps://bama.edebris.com/manuals/leads_and_northrup/4737-a2x/Unfortunately I put in the wrong Leeds.Will send them an email as I upload more.K04BB is currently pending.

author: F4
author_rank: Contributor
timestamp: February 01, 2025, 08:12:48 pm
content: Quote from: Vgkid on July 15, 2024, 01:09:46 amThanks for posting this, it helps. I have a newly acquired model 4737, no "A20", a passive unit.This unit shows a 1 to 10G ohm range, but I cannot determine how to use it.For example, across X2 and X2 or the guard, how do I adjust the dials for a 1G ohm reading? Or am I missing something about it's use?

author: Vgkid
author_rank: Super Contributor
timestamp: February 02, 2025, 06:00:54 am
content: Looking at the manual(I don't own one) it looks like you select the 1x10^9 multiplier(X-dial), then multiply it by the resistance of the adjustable arm(the decade resistance1000ohm-0.01ohm) to get the resistance. Now regarding connections connect the resistance between the X1(positive) and X2(negative) terminals. If you can do a guarded measurement do that.See page 7-8 of the manual.

author: F4
author_rank: Contributor
timestamp: February 02, 2025, 11:27:10 am
content: Quote from: Vgkid on February 02, 2025, 06:00:54 amI can never get higher than a 20k+ reading no matter what switch setup I use.The difference between units is that yours how a power supply internal, and this unit does not have one or a null sensor. Photo attached.I opened the unit up, and the largest 10x resistor on that dial is 10k. There is no way to get in the gig-ohm range with this dial set. I was mainly looking to use this for calibration and wanted a gig ohm range for that. Page 7-8 show finding the value of an unknown resistor. Also, per pic, I do not have the null sensor the A20 has.Lastly, page 9 shows a set of sample readings in table 2, I don't get the same readouts, have you are anyone else tried it? What is the process for getting a readout that matches that?