Keithley 262 Low Thermal Voltage Divider Teardown - Page 1
Forum section: electronics / testgear

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: September 05, 2016, 05:25:47 am
content: This is a quick teardown of a specialized voltage divider made by Keithley. This divider is called out in some performance test checks on nanovoltmeters including the 182, 2182(A).A copy of the manual can be found herehttps://doc.xdevs.com/doc/Keithley/262/262_901_01D.pdfThis is an overhead view of the divider. I purchased this from an auction to support my 2182. This should also come in handy for testing my 181's. The knobs are big and have some drag when changing ranges.This is a side view of the input and output connectors. There is provision for 4-wire connections back to the source. The output is the same connector as the 181 and 182 plus a few of the older nanovoltmeters. The 2182 series uses the same connector as the Agilent 34420A. An adapter cable will need to be made for testing the newer meters.You can see the access port for the calibration of the divider resistors.Here are some pics of the internal layout and connections to the main pcb. The resistor legs are not tinned and are specified as low thermal. It is suspected that the large output connector has cadmium based solder since it was acceptable to use at the time of manufacture.The ranges are selected using a specially designed bare copper pcb. It reminds me of the records that came with cereal boxes some decades ago. This one needed some cleaning and polishing. According to the manual, scotch-brite was recommended. A softer abrasive was used instead of the pad. I had some plastic polishing pads for another project and they did a good job. The board was cleaned with IPA. A thin film of Deoxit was added afterward to reduce oxidation.Contact with the pcb is made using copper brushes that also needed to be cleaned. A lot of care was taken by Keithley to use low thermal construction techniques throughout the divider.Here is the divider reassembled and ready for testing.Special thanks to TiN for the pics, article layout, and hosting of the original article.https://xdevs.com/fix/kei262/

author: HighVoltage
author_rank: Super Contributor
timestamp: September 05, 2016, 06:57:16 am
content: I was not aware of the Keithley instrument.Thanks for the introduction and teardown.

author: Vgkid
author_rank: Super Contributor
timestamp: September 05, 2016, 03:54:41 pm
content: Thanks for posting that teardown. Those switch contacts are pretty cool. Loving the bare copper resistor leads, beautiful.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: June 19, 2023, 02:37:42 pm
content: Any idea who manufactured the matched 'ULT 2527' resistor set? The low resistance pair have copper leads and could be wire wound with a TCR < 2 ppm/K. I wonder what the specifications of the smaller blue encapsulated resistors are.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 19, 2023, 05:00:17 pm
content: For this application the TCR of the resistors is not that important. It is more about having low thermal EMF. The resistors may even be pure copper (and this some 3900 ppm/K). Even if the wires look like copper they may also be manganin.Chances are the resistors are custom parts and ULT may be the manufacturer and not the resistor type.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 19, 2023, 08:34:35 pm
content: Quote from: leighcorrigall on June 19, 2023, 02:37:42 pmULTRAOHM most likely.

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2023, 11:18:29 pm
content: I own a Keithley 260 "Nanovolt Source" that uses a similar voltage divider for the lowest output range.It starts with a decade-resistor-controlled voltage source into a selectable chain of dividers.That divider is also all-copper: presumably the tempcos of the upper and lower resistive elements track well enough for the purpose, but copper is required to minimize thermal emfs.A teardown from 2019:https://www.eevblog.com/forum/metrology/keithley-260-teardown/(Mine has clearer knob skirts. Note the Leeds and Northrup low-emf binding posts behind the door.)

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: June 20, 2023, 04:09:44 pm
content: Quote from: doktor pyta on June 19, 2023, 08:34:35 pmI think it could also be a company called ULTRONIX.https://www.precisionresistor.com/Cross-Reference-Guide-ULTRONIX.html