All right,
I have applied a thin styrofoam layer underneath the reference. This helps to protect the bottom solder joints of the LTZ1000A (also inside a 34470A).
Then I attached the reference board, and a 18mm deep, 40 x 65mm plastic box over the whole PCB area. The box fits nicely into a rectangular slot around all the components on the main PCB, and also protects this main board area from air draught. The edges of the box had to be trimmed at a few positions to fit over some components, like the ceramic resistor array.
This box now gives mechanical stability to the reference PCB, like the original plastic part.
The 16h stability measurement was started 15min after power on.
It shows an initial 1h warm-up phase and then again a slower negative drift, which finally seems to stabilize in the last 2hours.
The noise now looks like the one of an original 34470A.
The 1hour noise figure between 11 ..12h is 420nV
rms, just like the 470A, but looking a bit better in terms of peak to peak noise.
That's about 2 times worse the noise figure of a 3458A, but over a shorter observation time of 1min that approaches its performance, please compare my review from 2016.
The last screen shot shows my 34465A performing as a 7-digit instrument, variation over 1min as low as 1.2µV / 0.12ppm
Conclusions so far:
1) It's possible to upgrade a 34465A to the performance of a 34470A regarding noise and stability, by simply attaching the LTZ1000A reference board.
2) The reference board needs to be equipped with stable components (at least 5ppm/K resistors), and can be set to a lower oven temperature (60°C for metrology grade performance and environment).
3) The LTZ1000A circuit is used by the main circuit as a virtual zener diode (like the LM399A)
4) Possible special 34470A procedures on this board, like use of the REF- SENSE pin, or the V
heater pin can not be observed, as probably a SW flag for 7.5 digit operation may be enabled.
This might then give rise to these unexplained ACAL problems on 34470As only, which I do not expect in this configuration, for obvious circuitry reasons.
5) The fan air flow is directed over the reference assembly area and the analogue circuitry around. This I regard as a real design flaw.
6) A protecting box around the reference is urgently required.
7) In case of the 34470A, the plastic part is mostly sufficient, although the bottom joints are unprotected.
An additional bottom protection MIGHT improve the noise, i.e. further reduce these irregular bumps in the 1h stability figures.
8 ) In case of the 34465A, this protection box assembly MIGHT also reduce the noise performance of the LM399AH, although these two connections potentially might not be as sensitive as the whole LTZ board. It has to be tested, whether the noise of the 34465A arises directly from the LM399, or from the air flow.
9) The 2% higher reference voltage affects the calibration of all modes, apart from Ohm mode. It has to be tested, if a re-calibration of the DCV modes is sufficient to bring all modes back into spec, or if a full calibration is required.
10) I will do this test, and try to find out, if ACAL will work as perfectly afterwards, as with the original 34465A.
See recent remarks from David Bogdanoff about proper operation of ACAL after a preceding CAL.
11) If my 34465A performs well/better with this LTZ reference, I will keep it inside, replace these silver wires with golden ones, and may send the instrument to KS for a full calibration.. I think it's time anyhow, after 6 years, where it's still inside the 24h specification for many modes and ranges.
Frank