Thank you for teh replies.
My board is the new model (670-4126-00).
I have measured all the resistors, but without desoldering one leg and they all seem ok.
I am suspicious of S1125 (beam finder), which should be NC, connecting the -30V to the circuit.
If it is open, R1125 will be inserted in series with the -30V supply, limiting the current to both sections to keep trace in view.
It seemed that I could obtain a linear behavior of the circuit yesterday, but it was very late at night and I have to try again.
I will post my findings after more testing.
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