After cleaning the contacts of the lower manual, the first real problem becomes clear.
The lower C# (keynr. 9 of the lower manual) of the sustain registration group hangs, and sounds always when the 'lower manual sustains' is switched on.
First I suspected a spring of the key contacts always being closed. That prooved not to be the case.
Studying the schematics pointed me towards the divider print. Measuring with a voltmeter and an oscilloscope I narrowed the problem down to three components, a malfunctioning resistance, transistor or capacitor.
Turned out to be the capacitor shown in the picture, marked with the yellow arrow.
Exchanging the capacitor of 33 µF - 40V solved the problem.
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