used 3458A - what to be aware of?

[dr3hl3rt]

Hey all,

I’m planning to pick up a used HP/Agilent/Keysight 3458A from a dealer who has quite a few of them in stock. I’m dropping by in person soon and was wondering what I should be checking while I’m there.

What tests can realistically be done onsite without fancy lab gear? Are there common failure points I should be aware of, or specific serial ranges to avoid?

Some units are HP, others are Agilent or Keysight — are there any practical differences between them aside from branding/age?

And for those wondering: no, I don’t need 8.5 digits — but hey, who doesn’t dream of having one of these beasts on their bench?

Appreciate any tips you might have!

Cheers,

Henrik

[Bill158]

Henrik:
About 15 years ago I decided I "needed" a HP 3458A.  I downloaded the manuals and read up on self tests and how to do them.   I decided to only look at those in my area so that I could see them and run the self tests.  I found one surplus dealer who had two for sale at around US$3,500.  He was willing to let me pickup in person and inspect before paying.  I took a Fluke 731B with me along with a 10K resistor that I was sure was accurate.  I was new to this whole 3458A issue.  One of the DVMs gave an error which I learned later on was the NVRAM batteries had gone dead.  The other passed all tests just fine.  When applying the 10 volts the reading was at least 100 ppm or more low.  I just figured that it needed a good CAL. I had already done an ACAL on it.   NOT TRUE!  It had the dreaded A3-U180 drift issue even though it didn't fit the S/N range shown in service note 18A.  So after months of troubleshooting I had to get an exchange A3 PCB ( another US$ 850 ) which fixed the problem.  Since then I have learned that the U180 issue can happen in ANY S/N!!!
So my advice is:
1.   Using some accurate ( 10ppm or better ) voltage reference, check what is displayed.  In my case the U180 had drifted downwards during the extended period of the DVM being off, excessively.  After I powered it up the DVM kept drifting upwards day after day after day.  Of course if the NVRAM has gone dead then all bets are off on this issue.
2.   If you can get the dealer to agree to a 30 day right of return then you are probably ok.  I had no way to check the AC or current ranges.
3.   Of course check the VFD for missing "dots" or low brightness.  While you can now get a new VFD from China, this has 68 pins and can take hours and hours to replace IF you are good at desoldering.  Getting the new VFD in can also be a real task.  Mine had a spacing between the rows of 1.4 inches.  The replacement I got from HP/A/K was 1.5 inches.  So there is a difference between the early production and later on.  My S/N was around US280-12xxx and was probably manufactured around 1992 or 93.
Good luck,
Bill

[srb1954]

Run a TEST command from the front panel and note if any errors are reported. If any errors are reported these can be decoded with the ERR? and ERRSTR? commands.

When the DVM has warmed up sufficiently run an ACAL ALL command so the that meter runs through a complete self-calibration and check any errors reported. Note: the ACAL ALL command takes about 11mins to run.

These DVMs use a battery-backed RAM chip to store critical calibration constants.  If the battery runs low then the unit needs an expensive trip to a cal lab for full recalibration. Loss of battery power will usually be indicated by a RAM checksum error message during power-up.

If the units are more than 10 years old a battery replacement is advisable. Earlier units used a Dallas chip with internal battery, which is long out of production and unobtainium. Most users opt to replace this chip with alternative NVRAM chips; there is plenty of info on this forum about substituting for the Dallas chip. Later units used an ST chip with a easily replaceable snap-on battery hat.

Other useful information is contained in the non-volatile memory and can be accessed using front panel commands:
   CALNUM?  records the number of calibration cycles the unit has been through
   CALSTR?   is a stored text string that may contain useful info regarding when the unit was last calibrated, by whom and under what conditions.
   REV?        will report the S/W version number
   OPT?        will report any installed options
 
You can check the serial numbers against the HPAK website:
   https://support.keysight.com/s/service-and-warranty-status

If the units were serviced/calibrated by HPAK there may be useful info on this site, particularly regarding whether the A3 PCB has been replaced to fix the dreaded U180 drift problem. To check for this drift problem you need to run the procedure listed in 3458A service note SN18A. Unfortunately this needs about a week of continuous running to get usable results and if you discover the unit is drifty the only option is to return to HPAK for service as the replacement A3 PCBs are no longer sold by HPAK.

There is plenty of other info available on the web about maintenance work required on older 3458As e.g. replacement of RIFA caps and Schaffner mains filters, general recapping, replacement fans etc.

[coromonadalix]

unless calibrating it yourself, sending it to a cal lab may cost you $$$