Strange cal issue with 34401A

[chilternview]

I've come across a strange issue when trying to calibrate a HP34401A.

The DCV/DCRES/DCI cal is straightforward, but on the AC 100mV range there is a cal point at 10mV (10% of full range) and at 100mV.
Now the full range cal works as expected, but when I try to do the 10mV one it seems to have no effect. These are the steps taken:
1) Apply 10mV at 1kHz from calibrator (datron 4200) to HP34401A and HP3458A (Keysight calibrated)
2) 3458A (setacv sync, res .001, lfilter on) reads e.g. 10.0142mV, 34401A (filter slow/3hz, resolution 6 digit slow) reads 9.8654mV
3) Enter calibration menu, cal displays 100.0000mV
4) Set cal display to 10.0142mV and enter
5) 34401A displays cal 10mV AC
6) Display returns to measurement 9.8654mV (last digit changing due to noise as normal)

Now I'd expect after entering the cal value the 34401A would display, within the error margin of 45uV, the entered value but it does not, it's as if the calibration has no effect. Which is in contrast with other calibration steps, but all of them are at full range or near full range.

Am I missing something? The manual does not indicate anything different about this cal step.

[J-R]

Why does it appear that your calibration steps are out of order? 100mV then 10mV?  Service guide has them as: 10mV 1kHz -> 100mV 1kHz -> 100mV 50kHz

It appears that you're supplying 10mV to the 100mV step and then skipping the 10mV step completely.  At least the way it's written.

[chilternview]

Sorry if its not clear. The 34401A does not have a 10mV ACV range - the lowest range is 100mV. The cal steps for ACV are, according to the service manual:
- 10mV on 100mV range @ 1kHz
- 100mV on 100mV range @ 1kHz
- 100mV on 100mV range @ 50kHz
- 1V on 1V range @ 1kHz
etc

So you apply as the first step, which is 10% of the (100mV) range i.e. 10mV. And the 34401A displays 100mV, which you then set to the applied voltage of 10mV. The 34401A realises, from the applied voltage, that you're carrying out the 10mV cal and notifys you. It presumably adjusts the offset on this range to improve low reading accuracy before going on to adjust the full range gainin the next step when 100mV is applied. However, the 10mV cal step does not appear to actually do anything.

[chilternview]

So I looked at the report of another 34401A that I had calibrated by a reputable UK company a couple of years ago (calibrated, not adjusted - even though it had not been calibrated for 10 years it still did not need any adjustment; 34401A's seem to hold their cal very well if not abused).

The ACV calibration report did not include the 10mV, 1kHz step at all; they only tested at 100mV full range. Maybe the 34401A cannot be calibrated at this point, despite what the service manual says.

[J-R]

My HP 34401A calibration data report does have all three steps listed.  In my case:
9.9992mV
99.9932mV
99.9854mV

Could you try sending that calibration step command via GPIB or serial instead of using the front panel?

[chilternview]

Could you try sending that calibration step command via GPIB or serial instead of using the front panel?

Yes, tried that - same result. Also tried another 34401A - similar behaviour, at least it's consistent. The second 34401A at least does give a much closer result at 10.0133mV (3458A read 10.00824mV)