Maybe one bad resistor? I've read that tin whiskering can occur even on 
relatively recent equipment, slower than lead free but still a problem.
I'd do a visual inspection just to see, shouldn't be an issue.
-A
________________________________________
From: volt-nuts <volt-nuts-boun...@febo.com> on behalf of Dr. David Kirkby 
<drkir...@kirkbymicrowave.co.uk>
Sent: 02 March 2018 03:16:24
To: Discussion of precise voltage measurement
Subject: [volt-nuts] Precision high resistance measurements / calibration of HP 
4339B high-resistance meter.

I pickup up an HP/Agilent (front says HP, rear says Agilent) 4339B
high-resistance meter from eBay. The 4339B has an internal supply up to
1000 V, an ammeter to measure current, and obviously computes resistance
from Ohms Law. It is supposed to work up to 16 Peta ohms (1.6x10^16 ohms),
which would pass a current of 62.5 fA with 1000 V.

It was sold as non-working, with no output. Even attempting to get some
output was challenging as it had an obscure 6-pin interlock connector, and
there's no information in the manual about this. It also has a trixaxial
input connector with a screw thread. I asked Keysight for some information
about the connectors. I still don't know what the triaxial connector is,
but I have the interlock sussed - a HiroseHR10A-7P-6P(73) plug fits, and
there's some information about the wiring of this in the manual for the
11617C low-noise test leads.

Keysight offered to do a free technical evaluation, so I thought I'd take
them up on it, rather than waste too much time trying myself to get it
working. It was sent to Keysight and they sent me a report showing there is
output voltage, despite it being sold as having no output, and the output
is all in spec. Unfortunately, it was out of spec on one of the current
ranges. It is reading 0.082 nA low with a test current of 10 nA, but the
specification is +/- 0.063 nA. So it is only a little bit out, but it is
still out of spec. Test currents of 100 pA, 1 nA, 10 nA, 100 nA, 1 uA, 10
uA and 100 uA were used, so the 10 nA is neither at the low nor high end of
the range of the meter.

When I received an email copy of the free technical report, I assumed the
4339B had not been subjected to a full calibration as it was only tested
with resistors up to 10^11 ohms, but it is supposed to read resistance up
to 1.6 x 10^16 ohm, so the highest value resistor used during the technical
evualation is 5 orders of magnitude lower than the what the meter can read.
Also the meter was only tested at 100 V when reading resistors, whereas I
would have thought it more sensible to test at a higher voltage, given it
can output 1 kV.

I decided to pay Keysight for a full calibration, so I got a cal
certificate, which shows the out of spec reading.

http://www.kirkbymicrowave.co.uk/tmp/1-9690444179-1-combined-file.pdf

I was rather hoping the cal certificate would have some extra readings, but
I actually received no more technical information than Keysight had given
me for nothing, although they did update the firmware as I requested.

I have two questions.

1) Is 10^11 ohms a practical limit on making resistors that can be used for
calibration purposes? I don't know what the Keysight uncertainty on their
calibration standards are, so maybe 10^11 ohms is all its practical to
make.

2) Does anyone know if the meter can be adjusted on individual ranges? I
find it a bit hard to believe that given the ammeter was tested with
currents covering 6 orders of magnitude (100 pA to 100 uA), that there's no
adjustment on individual ranges. I doubt that it is possible to design a
circuit to cover that range, without switching in some different resistors
at some point.

But I'm told it can't be adjusted, and needs a new board.

I'm wondering if anyone knows anything about this meter, just in case
Keysight are wrong. I know they should not be, but it is not a current
instrument, so it's just possible the engineer did not know it could be
adjusted.

Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford,
Essex, CM3 6DT, United Kingdom
Registered in England and Wales as company number 08914892
http://www.kirkbymicrowave.co.uk/
Tel 01621'680100 / +44 1621-680100
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