HP 3458a stabilty after power on?

[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]

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

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

[Oldtestgear]

Thanks for the helpful comments. Storage was not ideal at all as even in the British summer, a container can be damp.  The initial drift was from shortly after power on, so will include some LTZ1000 conditioning until up to temperature.  Both are now reading within 0.5 ppm over 24 hours which is within the limits for the two Cropico standards I am using for the tests types ESC1 & VS10.  After a week to fully stabilise to the new environment I will start a proper test in accordance with the HP app note. This will be the real test for stability & show whether I have bought 2 lemons or not.

Phil

[Dr. Frank]

Hello,
my 3458A, which has a modified oven to 60°C instead of the nominal 95°C, starts at +9ppm after turn on, and settles to < 1ppm deviation after 10 minutes.
Maybe it then undershoots a bit , but after 2h it should be stable to < 0.2ppm, w/o ACAL.
Hard to tell, if your units have a problem, as an ACAL after warmup might give another big deviation, depending on the ACAL temperature done before storage and now, in your location.
Account for another 0.3ppm/K difference.

The Service Note 18A describes  "Long term Drift", max. 15ppm, which is a hysteresis effect of the LTZ1000A itself, after a longer period of storage.
This is caused by the high oven temperature used, as far as we know.. at lower oven temperatures this effect usually does not occur. I further assume, that might be a basic problem for all 3458As, not for a certain batch of their references only.
Anyhow, it can be cured inside your instrument by six weeks of letting it run 24/7.
After it has stabilized to account for this drift, you may check the "Short Term Drift" of U180.

If you test the CAL72? gain parameter, don't forget to measure the internal temperature of the 3458A for each test point, as this gain is also depending on the ~ 0.25ppm/K T.C. of the U180 ADC chip.
This is not mentioned in the SN 18A, but might give a distorted picture, when your environmental temperature changes more than 1..2 °C.


Frank

[Oldtestgear]

A quick update.

One instrument is definitely working properly with a calculated drift ( 7 days) of 0.14ppm. This includes a temperature change of 2C in ambient temperature. The second  gives a rather less impressive change of 0.5ppm with the same change in ambient temperature. This is the older machine that came with a note that it was to be calibrated using a 110V 60Hz mains input. Now running on 230/240V 50Hz although I doubt that this makes much difference to the results. As this one came from the USA originally (Telogy Inc sticker) it may have been out of use for much longer than the first one & this may be taking longet to stabilise.  It has been switched on for less than 2 weeks.  This instrument also has Rev 5.2 firmware & the one with lower drift has Rev 7.2. Nothing to do with drift but possibly it has not been used for longer as both came from the same company site.

I am beginning to remember why I sold my first 3458A a few years ago. These are fabulous meters but very susceptible to environmental changes. I live in an old, large, draughty house which is exactly the environment these instruments do not like. 

I plan to let the unstable instrument continue "warming up"/ stabilising (with luck) & start updating the one I bought first.

Phil
 

[Oldtestgear]

Final comment & update.

Both 3458a's are now performing well within the worst case drift specification of 0.43ppm. The one that was drifting the most is now down to 0.10ppm/day over 7 days. I re-capped the power supplies, changed the Dallas RAMs, plus the 2 Rifa caps & the mains filter. I updated the firmware to V9.1 and fitted extra SRAM for Option 001.  The change was noticable from day 2 as a temperature change had a clear effect on the direction of any change in calibration constant.
My first one was also re-capped etc and the drift was further reduced to <0.1ppm/day.

One will be off to Keysight for calibration later this week with a bit of luck.

I plan to sell the other one at a later date.

Phil