[volt-nuts] Two 4338B high resistance meters fail on the same range - Keysight can't adjust EEPROM of either.

[David C. Partridge]

My bet is that they've lost the "secret sauce" for updating the EEPROM :(  So it's measuring as best it can with no calibration adjustments stored.

Dave
-----Original Message-----
From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Dr. David Kirkby
Sent: 02 May 2018 00:47
To: hp_agilent_equipment at yahoogroups.com; Discussion of precise voltage measurement; RSGBTechnical at groups.io
Subject: [volt-nuts] Two 4338B high resistance meters fail on the same range - Keysight can't adjust EEPROM of either.

I"m pulling my hair out over an identical problem on two instruments. I am
hoping someone might think of a possible cause I have not thought of. Sorry
the post is a bit long, but this is not any easy problem to describe.

I have an Agilent 4339B high resistance meter. This instrument essentially
consists of a variable power supply (0.1 V to 1000 V) and a very sensitive
ammeter. It works out resistance using Ohms Law. It can display resistance,
current, surface resistively and volume resistively.

The service manual states there are no adjustable components in this - all
calibration is performed using software that updates an EEPROM. The EEPROM
is on the CPU board.

This was sent to Keysight in the UK for a firmware upgrade and calibration.
They updated the firmware (stored in a ROM), but the 4339B failed
calibration.

* All the output voltages from the internal PSU were within specification.

* The 10nA current range was slightly out of specification. It was reading
about 0.8℅ high, but the specification is about +/- 0.6%, so it was only
slightly out of specification. (All other ranges were within specification,
but some were not far from the limits. One range might have been 0.5% off)

I think the full scale of the current ranges are 10 pA to 100 uA, so 10 nA
is not at either extreme.

* All resistance measurements were within specification. (Keysight test up
to 1e11 ohms,   but it can read up to 1.6e16 ohms. I guess they simply
can't get accurate resistors above 1e11 ohms).

Calibration at Keysight includes any firmware upgrades if you request
updates.  It also includes the cost of any adjustments needed - unlike most
calibration labs.

Keysight said the EEPROM could not be adjusted to bring the 10 nA range
within specification, so it needed a new CPU board. I never received a
formal quote for repair, but I was told about £2000 (GBP), which seemed a
lot considering the CPU board is about $600 (USD) from Keysight.

These meters sell for around $3000, but other instruments available for far
less use the same CPU.  I was intending repairing my 4339B  by swapping CPU
boards from a cheaper instrument, and using a new  EEPROM, just in case it
was the EEPROM faulty, as that goes in a socket on the CPU board.

However, I  managed to find another 4339B at a good price, so that was
purchased and plans to repair the first instrument were put on hold.


I asked for a quote for calibration based on it having a blank EEPROM. I
thought this would be advantageous, as Keysight could put each range "spot
on". I expected the cost to be a bit higher but it was not.

Much to my surprise, the instrument worked and seemed reasonably accurate
even with the blank EEPROM.

I sent this second 4338B to Keysight for calibration. A couple of days
later i received an email from Keysight telling me the second  instrument
has a fault. The fault is on the 10 nA range (as the first instrument) and
the EEPROM can't be adjusted (like the first instrument). This time it is
reading about 0.7% low, which is not much out considering the specification
is about +/- 0.6%.

So I now have two 4339Bs, both being within specification on all ranges
except 10 nA, and neither being adjustable! So naturally I queried why both
instruments appear to have the same fault.

I then received an email from someone st Keysight who had noticed I said
the EEPROM was blank. He asked where did I get the EEPROM from. LUCKILY I
had bought the EEPROM directly from Keysight,  despite I could have got s
very similar one from Mouser for a tenth of the price or a supposedly
identical one for even less from China on eBay.

I am hoping to speak to someone at Keysight  tomorrow,,  but does anyone
have any ideas what could cause two instruments to be slightly out of
specification on the same range, but neither instrument will allow them to
update the EEPROM?

Note one instrument reads high and the other low. I can understand that
perhaps the resistors used in the current to voltage converter on the 10 nA
range might be a bit less stable than used on other ranges, but I can't
understand why Keysight can't bring the meters in spec just by updating the
EEPROM.



Dave
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