Leeds and Northrup KS-10376 Null Voltage Test Set

The KS-10376 is a Bell Telephone test set. It can measure voltage ratios, resistance ratios, or absolute resistance value to five digits of precision.

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The null voltage part of the instrument is a sensitive zero-center galvanometer. 

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Unfortunately my galvanometer had an open coil, probably from being overloaded.

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The U shaped piece is the magnet for the galvanometer.

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I guess it is steel rather than Alnico.

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To make it useful, I replaced the galvanometer with a pair of binding posts so I can use an external null meter.


The schematic is pretty straightforward. It uses a Kelvin-Varley divider network for the three higher decades, and a precision potentiometer calibrated from 0 to 100 for the lowest two decades. The galvanometer has four resistors in series with it to cut down its sensistivity and limit the current to prevent burnout of the galvanometer coil. There are three buttons, each selective progessively more sensistive ranges for the galvanometer. You are supposed to start with the least sensitive range and set up a rough null on the bridge. Then you move on to the next more sensitive range. The previous owner probably neglected this precaution and consequently blew the coil.

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The interior is beautifully constructed, with a high-quality precision potentiometer, fully-enclosed Leeds and Northrup precision tap switches, ...

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… and precision wire-wound Leeds and Northrup resistors.

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It is packaged in a portable case with a removeable cover.

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The cover contains the instructions.


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And the interior of the case holds a copy of the schematic.

I’ve combined the schematic and the instructions into one PDF and put it up for download in the Liberated Manuals section of this site.

© Steve Byan 2011-2019