Hello Jason,
shi(f)t happens.
You can try to restore the old value (at least for a part) by power cycling the LTZ board several times.
Power off, let the LTZ cool down and switch on again for several cycles.
I think Frank had the Idea of putting the cirquit into the freezer. (didnt try that up to now).
Of course you will not get the same voltage as before but the shift will decrease somewhat.
Up to now I had about -2 to -5 ppm after cycling.
And yes, after such a accident I usually have about double the ageing rate as before for about 6 months.
good luck
Andreas
The LTZ1000 output shows hysteresis, when temperature excursion is greater than about 30°C off from the nominal oven set temperature.
This hysteresis / offset might be permanent, or the output might slowly creep back towards its initial value, that depends on the individual LTZ1000 chip.
Slowly means on a years time scale.
There's the patent by Pickering, which might mitigate this hysteresis by temperature cycling the LTZ1000 oven temperature.. but I doubt that it works inside the DATRON / WAVETEK 7001 device, as the oven temperature is set to about 45°C, so the lower cycling temperature is room temperature only, which is too less to have an effect.
Anyhow, I run all my seven LTZ1000 at 12k/1k, or at about 45..55°C, and also encountered such oven temperature runaways.
In one case, that led to a permanent shift of -5ppm.
So I cycled the whole circuit (not powered, inside a plastic bag) to -23°C in the freezer, then let the oven run (outside the freezer) at 80°C, then back to -7°C, to 60°C, 10°C, 60°C, 21°C.
That set the output back to its initial value within less than 1 ppm.
Afterwards, the drift rate was again as high as -1.5ppm/ year for the first year, so it's obvious, that the initial ageing process starts over after that cycling process.
I think, I should publish the long term drift measurements for my references....