Hi Simon, thanks for the feedback and the additional hint. Found the indicators (they have been hidden behind the label of the lowest PCB) and no one is triggered:
But probably discovered the no-go under that panel as there seem to be two boards missing here: Slot BY (2nd from top is empty) and there
is a open plug/empty slot between CU and CV (3rd from bottom):
A pinout of the "workshop-plug" J3 would be quite helpful as this surely has access to all supply voltages - so it would be easy checking for shorted caps there. Thanks also for the hint on heat philisophy in the LTN72. Of course I am aware from my experienct that slip rings and bearings to not like dust at all ;-)