Unable To Calibrate 34465A 10VDC Range

[EE54]

Hi everyone, I recently had a friend of my repair a broken 34465A that I bought. He manage to repair it but then it also had a problem where all DC ranges would drift over a few minutes. Problem was traced to a bad ADC buffer amp (OPA827). Now the unit will hold cal and not drift.
But another problem arise. For some reason, all DCV ranges will accept new values except for the 10 VDC range. Even when supplying 10 VDC and entering 10.000000VDC exactly, it will still show a 7mV error that just can not be calibrated away.
Could it be the 10VDC range has drift so much that it's outside the adjustable range? Could the new replacement buffer IC plays a role?
It also has a Cal count of 271, which seems like a lot.

[laichh]

Hi EE54,

Did you perform [ADC and Zero Calibration] as mentioned in the Service Guide page 556 prior to the DC voltage gain calibration adjustment?

[EE54]

My friend told me he performed all the calibration steps, tho I'm not sure if he used a proper shorting block or just use his 5700A to do the zero cal.

[ITArchitect]

Has this issue been resolved?  After the zero cal, did you run the auto-cal to compensate for any temperature drift?  Curious on what was found or if it's still an issue.

[EE54]

Hi, this whole thing was put on hold for a bit as I am busy with other projects. I suspects that my friend diy calibration was not adequate, I doubt he used a proper 4-wire short for the zero cal.
I’m planning on having it sent to a local cal lab for a proper cal to see if the problem persists. I’ll update in 1-2 weeks.