Electronics > Metrology

HP 3458a stabilty after power on?

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Oldtestgear:
I know about the problems that some 3458A's have with short term drift & how to check for this but this is not my question.

I have just bought 2 of these venerable instruments that have been in storage for some time including a short spell in a storage container. So, my question to those people who have experience of these is
 "how long should these take to stabilise?"
Both pass the self test with no indicated errors. I have not opened these as the anti tamper seals are intact.

The first 24 hours the drift was 24 -32ppm which I guess is not surprising as it was in a new environment & possibly drying out. These figures include any temp related drift of my references and the room itself. 
The second day was much lower a <1ppm for both instruments.  How long should I wait before checking the drift figures using the HP App. note?  Cosmetically these are both in good condition & have been well looked after with the displays bright & clear in both instruments. The last calibration date was 2012 for both instruments with one done by Agilent, the other by Aeroflex. I assume the latter one has come from the USA as it was set to 120V.

Any suggestions or recommendations about these two will be very welcome.

Thanks in advance

Phil

chris_11:
My experience, when you have the 3458A off for a month or more, it takes several hours to stabilise. After a good day operating in a normal lab environment, they are stable, as you found out too. My unverified assumption is, that not only the reference stabilises, that will do it probably faster, since the LTZ1000 is heated and dependent from only a few stable resistors. It will more be leakage from capacitors and humidity soaked boards, that will create the most drift on the first day. Since you have two 3458A run ACAL after a day and measure a stable voltage like a battery (there are no "stable" batteries for a 3458 resolution) with both and look for the remaining differential drift. If you have a external reference voltage, you can trust, even easier.
Seems that old references from the HP3458A are quite good. Check the vintage of the EPROMs and NVRAMs, they might need replacement. There are extensive threads here how to keep the old 3458As surviving for the next decades. The unit is fun to work on.

br
Christian

tszaboo:

--- Quote from: Oldtestgear on September 18, 2021, 07:40:21 am ---Any suggestions or recommendations about these two will be very welcome.

--- End quote ---
Leave them on full time. That's how I dealt with it, when I had one in the lab.
Power still costs less than the calibration of it, calculate it into the equipment's running cost.

Kleinstein:
AFAIK the slow stabilizing part is humidiy in the PCB that cases swelling and thus mechanical stress to the parts. Stabilization of the humidity is on the order of a week or so, though the first part can be quite fast and the relatively warm 3458 can also speed things up a bit.

Leakage should not be a major effect - that can be from a surface humidity film, that may  form with RH > 50% (the number depends on the material). With warm up the RH will be low (a 10 K temperature rise gives about a 50% reduction in RH, so usually no more film). This part will stabilize fast (e.g. minutes).

dietert1:
24 .. 32 ppm is a lot and hard to believe. Maybe you need to exercise and observe these instruments for some time, keeping them in a warm and dry place. Only then you will know what you got. Worst case they were stored due to excessive drift or calibration problems. The stories people tell when selling them may be invented.

Regards, Dieter

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