Calibration of the sweep linearity HP 8568B
 

In my HP 8568B Adjustments ( calibr. ) Manual are myriads of
adjustments.
But, surprisingly, no sweep linearity adjustment.

I have the complete schematics ( RF and IF ) and I have localized the
sweep oscillator(s).
Where and how should I start to linearize the horizontal sweep ?

I want some ( minor ) correction of the the last 25% of the sweep(s).
I found how to lengthen and shorten the sweep(s) but obviously this
does not help improve the linearity.

Thanks for any help.
Peter, PE1E.