Quickmill 05000A - Motor of the mill hums constantly

Hello all!

First of all, I think it's great that this forum exists and also exotic machines like the Quickmill fully automatic machines are treated here by you. I am currently in the process of (hopefully) restoring a Quickmill Acquaviva 05000A from 1994.
Mechanically, after a gasket replacement and various new valves, the machine is almost ready for the next few years.

However, there is one problem that I have not been able to solve so far:
When switching on the machine, the motor of the mill is constantly under a slight voltage (measured around 5V), which causes a humming noise
.
Because of this, the old connection cables were already badly worn and the sheathing was brittle.Even with new wiring, the problem persists.

In the course of troubleshooting I replaced all optocouplers, triacs and thyristors on the board, unfortunately without the hoped for success. The diodes for the motor control still work. Would you have an idea where I could look further?
Unfortunately I didn't find a schematic for this board, but maybe someone knows this version of the control electronics.

I would be very happy about your answers.

David