Hp 3457a AC calibration Error 256.

[Simon Spiers]

I am attempting another repair on a lovely HP 3457a multimeter that seems to work correctly other than on AC . The self test reveals a Hardware error number 256. The manual tells you to perform a diagnostic run and adjust a capacitor  capacitor A3-c332 to bring the count to zero. This adjustment has no effect and the number remains stuck at 4. Has anyone had this problem with the meter before?
To me it looks like symptoms of a faulty op amp?
Cheers Fellas!