Electronics > Metrology

Keysights reaction on modified 3458a (lower ltz temp, FRAM,etc) at calibration?

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aronake:
Hi,

I am thinking of sending one of my 3458a for service (classic swap of A3 board), calibration and adjustment.

I currently have it running with 100K modification for lower LTZ1000 temp and use another A3 board.

Ideally I would want to keep the 100K modification, but could see a small risk that Keysight have some reaction to it.

Anyone with experience sending in a somehow modified 3458a for service, calibration or adjustment?

aronake:
My own guess is that they would not react or notice 100K LTZ temp modification, if it isn't that the LTZ board is faulty. For other more visible modification like FRAM memories they may have some comments. And probably replaced with fresh Dallas memories.

maat:
I had an HP3458A with the FRAM modification a few years back. It went to calibration a few times and they didn't mind back then. They also replaced the front panel switch at one time and did not complain. The DMM finally died with a

114,"SYSTEM ERROR -- balanced rundown convergence" error and a
202,"HARDWARE FAILURE -- "SLAVE TEST: CONVERGENCE" self test error,

the famous ADC drift bug.

At that time, the DMM was under extended warranty. This sudden death came about a few months after they had released the new black edition and they decided to gut the old device and replace all PCBs except the power supply and the reference during repair. So this DMM is now essentially a black edition inside. At any rate, they never complained about any modification I made to the device.

aronake:

--- Quote from: maat on March 25, 2024, 07:57:53 am ---I had an HP3458A with the FRAM modification a few years back. It went to calibration a few times and they didn't mind back then. They also replaced the front panel switch at one time and did not complain. The DMM finally died with a

114,"SYSTEM ERROR -- balanced rundown convergence" error and a
202,"HARDWARE FAILURE -- "SLAVE TEST: CONVERGENCE" self test error,

the famous ADC drift bug.

At that time, the DMM was under extended warranty. This sudden death came about a few months after they had released the new black edition and they decided to gut the old device and replace all PCBs except the power supply and the reference during repair. So this DMM is now essentially a black edition inside. At any rate, they never complained about any modification I made to the device.

--- End quote ---

Thanks! Thats a pretty generous approach on all areas! Maybe they feel they need to compensate for being extremely ungenerous by stop selling bare A3 cards and charging 3500 USD for a repair...

Did you ever do the A3 drift test in the beginning of your ownership or any time later? If so how did it come out? My suspicion is that most or all 3458a that eventually get the "HARDWARE FAILURE -- "SLAVE TEST: CONVERGENCE" error are fualty and have A3 drift already from shipment.

maat:
I guess you are talking about Service Note 18 (https://xdevs.com/doc/HP_Agilent_Keysight/3458A/service/3458A-18A.pdf). IMHO, this service note was probably written for people unable or unwilling to write up a quick script to continuously poll that CAL 72 constant and plot/fit it. Its meaningfulness is therefore limited. Using only two points to do the drift calculation needs to account for other environmental factors that affect the CAL 72 constant (like temperature). Therefore the 0.43 ppm/d figure given in the SN is extremely high. My 3458A drifted -0.027 ppm/d before being called to its creator 4 months later.

I recommend continuously collecting the CAL 72 constant along with the internal temperature of the device over a few days, then doing a linear regression against both temperature and time. If you see any noticeable drift over time in either direction and a fairly large tempco of the CAL constant your device is likely gonna need a new A3 board in the not too distant future.

I have attached my fits for both the tempco and the drift as an example. The plot shows the 10 V range. The 1 V range drifted in the opposite direction at +0.014 ppm/d. The drift plot also explains the necessity of the huge error margin HP factored in. The data fairy noisy. So giving a number like "worse than 0.01 ppm/d" would trigger a lot of false positives. To measure the temperature I stuffed a 10k thermistor into the case.

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