Hello again
We have many 7A26, the issues you mentioned are common.
LF /DC cal is the first priority. Afterwards HF comp and finally transients response.
Be sure of your calibration sources.
1/ RE attenuator cleaning...very important to follow precautions in manuals. PCB and contacts are easily damaged.
2/ replacement of carbon resistance is not trivial as the self capacitance and inductance of a film or deposited resistance may affect the HF response.
3/ Most such problems on 7A24, 7A26 are resolved by exercising of every control and trimmer, and carefully cleaning switch contacts.
Accuracy of 5..10% is OK on these, its not a precision calibration.
Bon courage
Jon
Jean-Paul