"Remarkable simplicity of use is an outstanding feature of the hp 405BR/CR Digital Voltmeters. Just touch the probe to the voltage to be measured, and the 405BR/CR automatically zero-sets itself, chooses the proper voltage range and polarity, and displays the result in bright, clear numerals, complete even to the polarity sign and decimal point. Operator errors are reduced to the vanishing point!

For systems applications the hp 405CR is offered. This instrument is similar to hp 405BR but has provision for an external sampling command and recording outputs both in ten-line decimal code and one-line staircase code, as well as a print command for operating hp 560A/561B Digital Recorders." (Hewlett-Packard, Catalog, 1961)

BRIEF SPECS

It's the second version of Hewlett-Packard's first digital voltmeter of 1958 (405AR) and not really a big hit. Other makers at that time offered even 6-digit dvm's. But the 4-digit model 3440A which hp sold from end of 1961 (for $ 1,160) on was a great commercial success. Many of these are seen today at surplus dealers or in internet auctions.

Operation of the 405 is based on a simple ramp-comparison and an electronic counter (something hp pioneered) circuit. Today we would call it a single slope analog/digital converter. The decimal counting units - AC-4K - are the same as from hp counters of that era (521E/D, 523C, 524C). One stepping switch is used for ranging and polarity check. Three zener diodes housed in a 62°C oven are connected in series for the reference suppply. This approach differes from the usual automated Kelvin-Varley divider or DA-converter circuits of the competition.

This 1963 machine is completely restored and converted to 230 VAC mains supply. It had some functional trouble which I fixed after getting the "B/C" manual. Please see also my service note for more detailed information: hp405-svc (115k)