[Test-Equipment] HP 3478A - Bench meter recommendation?

[Fred Olsen]

Several here have mentioned the 3478A as a good choice.  While I agree 
that it is a good instrument there are some caveats to be noted.  Most 
important, the rated maximum non-destructive input is 303V, 101% of 
F.S., for both DV and AV.  Many might want to use the meter for work on 
Boatanchors or other tube gear and will be accustomed to the usual 
1000DV and 750AV input ranges of other instruments.  Try that and you 
WILL nuke the 3478A.  Even a common 350V B+ is too much.  The result is 
dumpster food.  The 3478A is out of support by Agilent since 2001 and 
too complex for most to repair anyway.  For what it's worth Agilent does 
have both ops and service manuals posted.

The 3478A uses "closed-case calibration".  Cal constants are stored 
electronically.  There are no internal adjustments.  While cal can be 
performed manually without the instrument and standards running under 
GPIB control, the equipment required for either method is decidedly lab 
grade.  Make a mistake, diddle with the calibration lock and lose the 
constants, you're done dancin'.  The meter will work but you'll have no 
clue as to its uncertainty.

By the way, ohmmeter calibration is done in either two-wire or four-wire 
mode but not both.  I don't recall any way to tell which was used on a 
particular instrument at its last cal.  Also note that this is one of 
those instruments with very high input resistance on the lower DV 
ranges, something like >10^10 ohms up through the 3DV range.  As such 
the meter will be highly subject to stray voltage readings if ones 
technique is sloppy.

The 3478A is also from the period when HP was using the un-loved LCD 
displays on many instruments.  If you don't mind that, go for it.  The 
meters do work well and are reliable.  If you really need a 5&1/2-6&1/2 
digit instrument though I might suggest the Agilent 34401A instead. 
(The older ones are branded HP since they predate Fiorina's Folly.) 
Used ones are still a little pricey but are coming down now since 
they're being superseded.  It has customary maximum ranges, a display 
which can actually be seen, and works very well.  It's no more 
self-serviceable than the 3478A but should still be in full support for 
quite a while and is at least as reliable.

Fred