Electronics > Repair
3458A
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drhex:
Hi all,
acquired a new (to me) 3458A, looking at the date codes on the housings it is a 1997 unit, it does have the newer style A5 with one EPROM though. Haven't really done a lot with it yet, it runs ACAL ALL as well as the extended self test without errors, CAL constants seem to make sense. No RAM errors so far, will do the FRAM conversion at some point. Don't have good enough reference points to really test it properly, next step would be a calibration - before I shell out for that I'd like to know it works reasonably well though. One immediately noticable thing is it totting up voltages - up to 11V on open inputs, -.0039 mV on shorted inputs - that can't be right?
Thanks for thoughts,
Florian
Dr. Frank:
Hello Florian,
congratulations on acquiring this wonderful instrument. 1997 is a good vintage.
Please make sure to immediately extract the content of the CAL RAM to avoid calibration loss.

You will find tons of information here, how to check your instrument for sanity, especially the U180 drift problem.

If you already infected by the the volt-nuts virus, you might already own a ~ 10V voltage and a ~ 10kOhm resistance reference, for checking and maybe calibration of your 3458A.

Several well equipped volt-nuts are living in UK, watch out for them to exchange amateur grade references, otherwise.

Your 3458A has very high impedance inputs, and very low bias / leakage currents.

Leaving the input open will show a slow rise or decrease of the readings, that's completely normal, and will then cause an overflow, or automatic up-ranging.
I usually put a short on the input jacks, to avoid this self-charging effect.   3.9µV is fine, when the instrument is cold and you're using a simple short.
With a proper, low e.m.f. short and fully warmed up, zero reading should be somewhere @ several tens of nV

Frank
drhex:
Hallo Frank,
Vielen Dank für die schnelle Antwort! Will continue posting here on progress. Have ordered the usual suspects (Caps, line filter, etc.). Don’t have good references (yet) so may have to Resort to a professional calibration. Then again, I don’t have an air conditioned lab so let’s see…
Grandchuck:
One place to look: https://xdevs.com/fix/hp3458hfl/  (you were perhaps already aware of this)

coromonadalix:
do some tests to be sure the a3 board does not have drifts problems ... talk about that here ..

normally if im not mistaken and not to alarm you,     you don't have problems if all the tests are fine ...
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