HP 419A DC Null Voltmeter

The Hewlett Packard 419A DC Null Voltmeter is still popular for measuring voltages in the microvolt range. However, they have been out of production for many years, and may require servicing to keep operational and to measure according to the specifications.  Perhaps you have a Hewlett Packard 419A unit that has become unreliable or noisy? I had, and I found out that the neons for the photochopper did not fire properly. In fact, they fired very irregularly, and the instructions in the 419A service manual came up short. To overcome this I designed a simple auxiliary circuitry that would govern the alternation of the firing. The circuitry simply ensures that the neons are connected one after the other, thereby preventing double firing. The auxiliary circuitry is shown in the purple box below:

The schematic diagram was drawn with the sPlan 6.0 utility.

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So, how did the 419A and the auxiliary circuit behave much later? I am happy to report (June 2021, almost 14 years after the modification) that the 419A still works, and there are no signs of neons misbehaving. The graph below shows the input noise in the 3 µV range for 20 k measurements with the input shorted. The voltage on the back panel of the 419A was measured with an HP 34401A (set to 10 PLC integration time), and scaled to get values that refer to the input. The standard deviation is 24 nV and the peak-to-peak voltage is 178 nV, better than the specified 0.3 µV.