I've done that and reported this first about 10 years ago on the
time-nuts list.
By adding a 100k, < 10ppm/K resistor in parallel to R411, 15k, on
alternative assembly place X411 (see CLIP), oven temperature is lowered
from about 95°C to 65°C, improving annual drift to typ. < 2ppm/year. See
appended picture of my changed reference board.
You need to re-calibrate your instrument because this will change the
reference voltage by about +50ppm/K *(-30K) ~ -0.15%
Afterwards, you also have to limit the ambient temperature to a normal
cal lab temperature (18..28°C), i.e. max. ambient temperature 35°C, keep
the fan always clean (dT = 13..15K), and avoid using the instrument
inside a closed rack.
In total, you need to monitor the internal temperature, TEMP?, which
must not rise above ~55°C any more.
I use my instrument in constant environment in our basement, that is
mostly 21..23°C over the whole year, TEMP? is 34..38°C.
Precise measurements are not possible without having such stable
conditions, btw.
Recently, I did a new DCV calibration of my 3458A, which is running
intermittently only.
The absolute drift of the LTZ 1000A after 7 years was +1.8ppm, compared
to specification of 8ppm/year (probably specified for running 24/365).
Frank
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