Hmm, this project would not let me go...
I had DMM hooked to freshly modified 3245A with updated reference, monitoring 10VDC output, and reading was going down and down. In just a hour it was already 1.5 ppm's low. Ran ACAL, back to initial level, and then started drifing down again. I suspected 3245A at first, so did next:
Connected 3245A output to K2001, using same wire as it was on 3458A.
Connected EDC MV106 10VDC to 3458A at same time.
Both meters sampled via simple LabView bodge, at NPLC10 both (NPLC100 and 200 shown same drift)
K2001 shown stable 10V output from 3245A, with noise ~1.2ppm (normal for 2001).
3458A drifted down again.
Then reversed sources (MV106 to K2001, 3245A to 3458A) , same stuff. After 5 hours it drifted down -6.5ppm's already. It's not room ambient temperature, it kept fairly constant ~27°C. I tried running colder (~22°C) and drift speed was bit faster, but value not return back when warmed up back to +27°C.
This morning switched sources back again, will see what happen when I get back home.
![Tweak :-/O](https://www.eevblog.com/forum/Smileys/default/smiliey_cal.gif)
There is
a service note 18A, describing long-term and short-term drift issue. One of parts mention A9 reference returning to "pre-aged" state if left unpowered for long time, so I hope that's the case. I have meter for a week 24/7 already, note mentions 6 weeks to return to normal condition.
Other part of note suspects drifty A3 A/D board, with only solution to replace it. Oh noes, not A3 again
![Scared :scared:](https://www.eevblog.com/forum/Smileys/default/scared.gif)
I'll hook DMM to measure actual A9 LTZ's output to see what is happening with reference voltage.