HP 400 GL manual dated September, 1969: Except for minor cosmetic changes (binding posts, switches, etc.) and component upgrades such as plastic sealed trim pots for older metal frame potentiometers, it appears to match later model units.
HP 400 in the 1983 hp catalog
Surplus 400 series AC Voltmeter buying Tips:
In general we find it desirable to buy a later model year 400 series meter. The latest we have seen so far is from 1983, as identifed by date codes on the parts, such as the polarized capacitors and the metal frame resistor calibration trimmers on the main board. There are always exceptions such as a beat up and/or damaged late model meter compared to a well cared for older unit. We notice that later models can be identified as follows:
Grey binding posts (with a black or red ring), as opposed to solid colored binding posts
Black plastic power switch toggle, as opposed to silver metal power switch
A clear or milk colored power lamp cover as opposed to a red power lamp
A modern instrument line cord jack as opposed to the older 3 pin style
and, the latest units appear to have a grey colored fuse cover (compared to all black)
We also recommend avoiding bent or severly dented frames and broken plastic meter cases as well as broken knobs. These seemingly obvious items might require very close study of a picture to notice them.
Most cabinets will have a collection of old labels, dirt, and sticky sections from typical hp yellow information and warning labels. The meter comes apart relatively easily. Organic solvents such as "simple green"TM work quite well on the disassembled side frames, side panels, and top and bottom panels, but not the electronics. You might want to work outdoors or in a well ventilated area or at a basement utility sink. I found that making a mist with a scrub brush while working well to clean the metal frame parts, left me with a stinging sensation in my lungs for a while. Use a respirator if you are sensitive to cleaners and solvents. "Soft Scrub"TM works well to take off most ink markings. We are still looking for the mildest available solvent or cleanser to remove the sticky areas without damaging the surface (or our health). Please let us know if you have a suggestion for this job.
We use Caig Deoxit D5TM followed by application of Caig P5TM on the rotary switch and binding posts. Also, try not to get these chemicals on the main board and don't leave damp areas on the chassis walls. Spraying in a well ventilated area, such as a garage or basement, on a section of corrugated cardboard works well. Remove excess chemicals with a paper towel. Then use some ispropyl alchohol on a paper towel or cleaning cloth to remove residual contact cleaner and lubricant from the resistors mounted on the rotory switch.
Some HP 400 GL have an option "OPT C01" that might be related to "leveling" (or not). Much appreciated if someone could tell us more about this option. This option does not appear to be in the catalogs or in the 1969 manual. It might have been specially offered under a military contract? The meter scale and boards superficially appear to be the same on all 400 GL models from similar time frames.
Other hp 400 series voltmeters:
The 400 F, 400 FL, and 400 GL have the most sensitive ranges of 0.1 mV (100 µV) full scale. The “L” indicates the presence or emphasis of the log scale on the display. These meters also have amplified AC V output terminals and can be used as preamps for a digital scope, less sensitive AC voltmeter, or a chart recorder with an AC voltage range. They also have front panel switch selectable 100 kHz filters. The 400 E, and 400 EL have BNC input connectors and optional outputs, but not the 100 kHz filters. Among the optional outputs on some of the 400 E / EL meters is a DC output representative of the AC levels. The 403B is a smaller package (shorter depth) and has no AC output. The 403B also can have batteries that almost certainly will need to be changed and may have leaked. Note that the hp 400 series are all “average responding” AC voltmeters. Average responding means that a meter is calibrated for AC sine wave waveforms in volts RMS (root mean squared). For noise measurements the reading will be low by a factor of 1.13, thus hp 400 noise values will read low and should be multiplied by a correction factor of 1.13.
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