David,
that instrument on ebay is definitely from the agilent area, i.e. built
after 2000.
It is really ok, and does not need any repair.
Obviously, this AC range is a bit outside calibration @ 3V, 4MHz, but
that is normal after some years w/o adjustment, and not a defect at all.
Probably you know the different kinds of calibration at HPAK, obviously
they chose the cal w/o adjustment option.
Currently, the KS server is down, otherwise one could download the full
calibration document, and check that, also the date of manufacturing .
If you'd ever need these high frequency measurements, it's possible to
do that by means of an appropriate signal generator and a thermocouple.
The internal LTZ1000A circuit might stabilize with age, also the
internal 40k VHP100 reference resistor might get better.
The annual drift of the LTZ circuit is nominally 8ppm, but that is not
fully specified.
In theory, that applies only if the instrument is powered on continuosly.
If it's switched off most of the time, the LTZ chip should not age at
all, but it may show hysteresis (see AN-18, first item)
So to my opinion, in this case it makes no difference, if you have an
older or a younger instrument.
The rest of the circuit is either (short term) stable, or not, also
mostly independant from vintage.
Due to the regular AutoCal process, this is of no real concern, as all
these other drifts will be cancelled by the machine.
The only component which determines the short term stability, is the ADC
ASIC.
It might have a good T.C., from zero to max. 0.5ppm/°C, and it might
have a low timely drift in its calibration parameter CAL? 72.
If latter is not the case, as described in AN-18, 2nd item, you really
have a monetary problem.
But in any case, this stability risk is the same for every vintage of
instruments.
Anyhow, I would prefer a newer unit like this one, as some other
components degenerate over time, like the un-obtainium fast comparators.
In the end, this instrument is not cheap, especially w/o proper
adjustment. 3000$/€ would be a more reasonable price.
Frank
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