Leeds & Northrup 8690 Millivolt Potentiometer

[trobbins]

I'm starting to check out a 101 mVdc FS null potentiometer - originally for thermocouple measurements, but I'll likely use it for millivolt measurement comparisons as it has 20uV minor tabs, so down to and below 10uV resolution.

The 8690 model is identified in the L&N 1972 catalog (on xdevs.com), but not much else is on-line of similar relevance afaik.  The 8690 has a base 0.05% spec, with switched 10mV steps using 0.01% divider resistors, and a nice 2 turn slider with a robust cam for 11mV FS dial, from what would be a 100mV FS internal reference (derived from what was a corroded 3x D-cell battery pack). 

Null measurement on my unit appears to use the L&N 324050 null detector kit (from 1972 catalog) - but it's not obvious if this was a retrofitted kit, or came original.  This detector appears to be used in the 8695-2 temperature potentiometer.  The detector uses 2x 4.05V batteries (each battery made up from 3x MR50 mercury oxide cells in a 'AA' style cylindrical pack).  Interestingly, the detector uses a pcb with a single 8-pin can IC made by National Semi that has an unknown (as yet) part number "S:/H8340  193207  SL60015".  Any clues on this part (presumably an opamp)?  I'll have a go at reverse engineering the pcb schematic.

The internal reference is an L&N Weston cell, which so far seems to be displaying reasonable stability, and with a voltage indicating maybe more than a decade of service life still to go (using HP 34401A hi-z loading).

The instrument didn't come with a manual, so I'd appreciate anyone able to make a scan (even the schematic would be useful), as that would make it easier to confirm the internal divider resistors haven't been stressed and to confirm calibration with the present Weston cell voltage.

Ciao, Tim

[trobbins]

The null detector appears to be based on a 100x gain 741 opamp, given the vintage, and IC pin assignments on the pcb.

Given a 2x 4.05V HgO battery feed, the opamp may well have been a factory selected 741 part with confirmed operation down to at least +/-4V, rather than the datasheet cutoff of +/-5V for performance.  That indicates a simple replacement of two 4x AA (or AAA) packs should be fine, and should fit in the plastic battery housing provided for the main 1.5V battery.

The 10mV switched step dividers use 5 ohm 0.01% L&N resistors.  The 2-turn slider is about 11.5mV span for 720deg rotation, so about 6 deg for 100uV, and 0.6 deg for 10uV resolution (half a minor tab) of the ww resistance wrapped on the support wire.  Rotation of half a minor tab is also about the available turn resolution of ww resistance turns and the wiper contact.