Hello, I have a similar plus also a worse problem with my 7081.
There are two symptoms:
1) Noise level is more or less the same as described here. At 7 digit resolution it matches the same noise level as of a new Rigol 3068 at 6 digits, 100NPL. From what I gathered here, it may even be within specs. But this is not the main problem.
2) The main problem is a sudden drop in the reading after a few minutes of warmup. This is 100% reproducible. This occurs for all measurements and already in the 10V range. There is sudden approx 80uV drop whenever the temperature crosses a certain line. By fine tuning I even got it to oscillate. I will post pics below. White trace is Rigol 3068 and red trace Solartron 7081.
I spend hours on this, to no avail. What I tried was:
a) Monitoring all sorts of PS voltages in sync with the drop - no correlation seen.
b) Exchanging IC201.
c) Using cold spray selectively on all sorts of locations, nothing systematic
d) checking the PLL voltage (2.5V) whether it jumps in sync - no correlation seen.
e) checking +/- 10V reference voltages, whether they jump in sync - no correlation seen.
f) substituting a VRE102CA voltage reference in lieu of Z-diode, but jumps continue to occur. If at a later stage this works better than the built-in Zener, I may keep it there.
3) Then there is a mysterius property which seems unrelated but puzzles me.
Probably it is a trivial mistake on my side, but so far I couldn't figure it out (without disassembling the board). It is about TP305, supposedly 0V in the reference section. It is located on the PCB between IC304 and IC307.
I measure 15Vrms 50Hz sinoidal AC between this and the 0V point TP505! TP505 itself has perfectly stable +/- 10V ref against the positive and negative reference points, TP302 and TP303. But TP 305 is per schematics the proper 0V point, and carries this high AC voltage. How come? When powered off, I measure like 2Ohms between TP305 and TP505. This doesn't make any sense to me - it appears like a transformer winding, low Ohms but a lot of AC (??)
Perhaps someone can enlighten me... and also give a tip about what to do against this voltage jump.
Thanks!!