7613 Mainframe - Readout problem


Greetings to all,
I brought home a 7613 mainframe to insert the 7L13 (SA) plug-in, but it has a problem in readout. It currently has a 7A16A vertical plug-in and a 7B70 plug-in timebase installed.
Without input signal (solid line on CRT) the readout is correct, but by inserting for example a sinusoidal signal the characters of the readout follow the movement of the signal dancing up and down around the stable position they should have.
By inserting the plug-in 7L13 (SA) the situation worsens and the characters wander on the CRT.
I tried to replace the readout board, but without resolving.
I ask for your help in identifying the defective component or components.


-- Cheers,

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