[Test-Equipment] H-P 400 series

[Fred Olsen]

All of the H-P 400-series are related in that they fulfilled the same
function in the product line.  Originally introduced in response to the
Ballantine 300 series, the 400 model number perhaps was marketing-speak
intended to make them sound newer or better.  400s with suffixes A, AB,
B, C, D, H, and L were AC VTVMs, suffixes E, EL, F, FL, and GL were
solid state.  The latter were sold as such based on the reputation
established by the tube versions.  An L suffix on any is the 'log'
version and indicates a linear dB scale predominant with a logarithmic V
scale.

Tube versions:
Anything earlier than a D is a museum piece, or perhaps all of them
depending upon one's point of view.  The H was a premium version with a
larger meter and improved uncertainty versus the D, and would be the one
to buy if you want a BA meter.  (The L is a similar log version for dB
work.)  The best bet would be a late one in a blue case.  Note that this
is definitely not your ordinary '6AL5 plus 12AU7' VTVM.  There are nine
tubes, some hard-to-find caps, and a precision frequency compensated
voltage divider in the front end.  A manual for the correct model and
serial number is a must if you expect to make these perform as
intended.  All have a Zin of 10 Mohms, a typical Vin range of 1 mV to
300 Vrms F.S. (-70 to +50 dB), and properly calibrated will make it out
to 4 MHz.
I would strongly suggest not buying any which show evidence of damage or
heavy repair to the range switch/voltage divider area.  These meters
also have recorder outputs on the front panel, and it is not unusual to
find units which have been zapped by 'in-ing the out'.

Solid state:
The E/EL was the natural successor to the D, H, and L, but with
frequency response extended to 10 MHz.  The F/FL has additional
sensitivity down to 100 uV F.S. (-90 dB), a 100 kHz low-pass filter, and
response to 4 MHz.  The GL is a dB-optimized version having eight 20 dB
ranges from -80 to +60 (1200 Vrms max input), a 4 MHz response, and its
20 dB scale length is most suitable for communications and acoustical
work.  All have a Zin of 10 Mohms.  The same caveats regarding manuals
and repairs apply.

Note that sellers often state that these are "true RMS" meters.  They
are NOT.  All are average responding and RMS calibrated.

Fred