One of my pride and joy finds -- HP made some really beautiful test equipment 50 some years ago.
I came across this HP412A DC Vacuum Tube Volt-Meter (VTVM) a few years ago. As the name implies, a VTVM has vacuum tubes (hence it needs external power) that are used as an amplifier. This design makes this instrument much more sensitive than a traditional Volt/Ohm Meter (VOM) when diagnosing very low level resistance and voltage readings in a circuit. For working on my old Sun scopes --- these things are the "bomb".
One of the most interesting aspects of this meter is the the AC powered photoelectric "chopper" circuit that cleverly synchronizes the modulation and demodulation of a DC signal to provide a super accurate reading of VERY low voltages.
Today we can do much of this with cheap Integrated Circuits, but in 1965 HP did it with awesome engineering and very expensive modular components.
The pics show the small array of 4 light bulbs that are positioned over the AC powered photoelectric chopper circuit
I bought this thing at a HAM swap meet for a few bucks. It's pretty sophisticated for my limited knowledge, but I managed to get it back in working order and refurbish the exterior as well. My father had one of these in the 1970's and it was his "pride and joy" -- in 1965 it cost as much as a color TV. Here's to you dad!!!
Finished it up and painted --