Thank you for your reply Ozan.
I have measure the resistance to ground on the power rails at Z-axis boards and they were similar to the ones described on the famous article from Servicescope.
From that measurement I am deducing that there are no "active shorts" t o grounds, although there might be some shorted caps where the series protection has opened.
I am also confused about the need to connect the light common to the dummy load.
It is P1482 pin 10.
It seems to be connected to ground, but it goes to main interface and I got lost.
Regards,
Roger
