HP 3440A Digital Voltmeter

The 3440A is a solid-state digital voltmeter mainframe with a four digit nixie display and a leading nixie +/- polarity indicator. Decimal points are formed by frosted NE-2 style lamps positioned end-on below and between the display tubes. Its measurement capabilities are determined by the plug in (required for operation) installed in the instrument, and it was introduced in the 1963 catalog (listed as POR; pricing first appeared in the 1964 short-form catalog).

Five plug-ins were available in 1965:
3441A Range Selector: DC, three manually selected ranges - 10, 100, and 1000 V full scale
3442A Automatic Range Selector: Same ranges as the 3441A, but adds an autoranging and remote range setting ability
3443A High Gain/Auto Range Unit: Adds 100 and 1000 mV low ranges to the capabilities of the 3442A above
3444A DC Multi-Function Unit: Allows DC voltage, DC current or resistance to be measured. Manual mode and range selection - DC in five ranges 100 mV to 1000 V or 100 µA to 1000 mA (mA ranges on instrument incorrectly labeled as MA), or resistance in five ranges from 100 kΩ to 10 MΩ full scale by decade
3445A AC/DC Range Unit: AC or DC voltage measurement, manually selected in three ranges from 10 to 1000 V full scale

Its design was interesting in that by constructing the inner chassis platform with gaps between the front and rear panels, the designers were able to alleviate the need for a cooling fan and keep the cabinet relatively sealed up; the instrument was able to passively cool itself due to the convection currents circulating within the chassis spreading the heat sufficiently to keep it in a safe operating range.

HP 3440A Digital voltmeter (with 3444A DC Multi Function Unit):

HP 3440A Decimal point detail:

HP 3440A Decade counter PCB:

HP 3440A Polarity and gate count PCB:

HP 3440A Ramp generator PCB: