Hi Peter, thanks for your reply and for the manual!
The fault consist in a "toothed" displayed waveform.
The Tracking Generator seem to be also malfunctioning, but I think that
this is another issue; the toothed waveform is displayed with an
external input signal.
I have both service and user manual, and I (and my colleague) have tryed
a lot of set-up with no results.
Waiting for new ideas, I will try to read more deeply the manual, thanks.

I'd check for the obvious, first: scope out all of the power-supply
busses and see if you see any kind of noise on them.

The toothed waveform, and the fairly consistent height of the tooths,
makes me suspect that the output of the log amplifier is being
corrupted... possibly by a regular variation in voltage. If the
Advantest uses a switched-mode power supply to create its analog
supplies, there might be a fault in the supply which is causing it to
dither between two different voltages... and this could throw the log
amp all out of whack.

The same fault might be affecting the tracking generator.